Mapping the functional surface of insulin by design: structure and function of a novel A-chain analogue

The three-dimensional structure of insulin and its receptor

Insulin is a peptide hormone essential for maintaining normal blood glucose levels. Individuals unable to secrete sufficient insulin or not able to respond properly to insulin develop diabetes. Since the discovery of insulin its structure and function has been intensively studied with the aim to develop effective diabetes treatments. The three-dimensional crystal structure of this 51 amino acid peptide paved the way for discoveries, outlined in this review, of determinants important for receptor binding and hormone stability that have been instrumental in development of insulin analogs used in the clinic today. Important for the future development of effective diabetes treatments will be a detailed understanding of the insulin receptor structure and function. Determination of the three-dimensional structure of the insulin receptor, a receptor tyrosine kinase, proved challenging but with the recent advent of high-resolution cryo-electron microscopy significant progress has been made. There are now >40 structures of the insulin:insulin receptor complex deposited in the Protein Data Bank. From these structures we have a detailed picture of how insulin binds and activates the receptor. Still lacking are details of the initial binding events and the exact sequence of structural changes within the receptor and insulin. In this review, the focus will be on the most recent structural studies of insulin:insulin receptor complexes and how they have contributed to the current understanding of insulin receptor activation and signaling outcome. Molecular mechanisms underlying insulin receptor signaling bias emerging from the latest structures are described.

Single-Shot Solid-Phase Synthesis of Full-Length H2 Relaxin Disulfide Surrogates

Chemical synthesis of insulin superfamily proteins (ISPs) has recently been widely studied to develop next-generation drugs. Separate synthesis of multiple peptide fragments and tedious chain-to-chain folding are usually encountered in these studies, limiting accessibility to ISP derivatives. Here we report the finding that insulin superfamily proteins (e.g. H2 relaxin, insulin itself, and H3 relaxin) incorporating a pre-made diaminodiacid bridge at A-B chain terminal disulfide can be easily and rapidly synthesized by a single-shot automated solid-phase synthesis and expedient one-step folding. Our new H2 relaxin analogues exhibit almost identical structures and activities when compared to their natural counterparts. This new synthetic strategy will expediate production of new ISP analogues for pharmaceutical studies.

New Horizons: Next-Generation Insulin Analogues: Structural Principles and Clinical Goals

Design of "first-generation" insulin analogues over the past 3 decades has provided pharmaceutical formulations with tailored pharmacokinetic (PK) and pharmacodynamic (PD) properties. Application of a molecular tool kit-integrating protein sequence, chemical modification, and formulation-has thus led to improved prandial and basal formulations for the treatment of diabetes mellitus. Although PK/PD changes were modest in relation to prior formulations of human and animal insulins, significant clinical advantages in efficacy (mean glycemia) and safety (rates of hypoglycemia) were obtained. Continuing innovation is providing further improvements to achieve ultrarapid and ultrabasal analogue formulations in an effort to reduce glycemic variability and optimize time in range. Beyond such PK/PD metrics, next-generation insulin analogues seek to exploit therapeutic mechanisms: glucose-responsive ("smart") analogues, pathway-specific ("biased") analogues, and organ-targeted analogues. Smart insulin analogues and delivery systems promise to mitigate hypoglycemic risk, a critical barrier to glycemic control, whereas biased and organ-targeted insulin analogues may better recapitulate physiologic hormonal regulation. In each therapeutic class considerations of cost and stability will affect use and global distribution. This review highlights structural principles underlying next-generation design efforts, their respective biological rationale, and potential clinical applications.

Peptide Model of the Mutant Proinsulin Syndrome. II. Nascent Structure and Biological Implications

Toxic misfolding of proinsulin variants in β-cells defines a monogenic diabetes syndrome, designated mutant INS-gene induced diabetes of the young (MIDY). In our first study (previous article in this issue), we described a one-disulfide peptide model of a proinsulin folding intermediate and its use to study such variants. The mutations (LeuB15→Pro, LeuA16→Pro, and PheB24→Ser) probe residues conserved among vertebrate insulins. In this companion study, we describe 1H and 1H-13C NMR studies of the peptides; key NMR resonance assignments were verified by synthetic 13C-labeling. Parent spectra retain nativelike features in the neighborhood of the single disulfide bridge (cystine B19-A20), including secondary NMR chemical shifts and nonlocal nuclear Overhauser effects. This partial fold engages wild-type side chains LeuB15, LeuA16 and PheB24 at the nexus of nativelike α-helices α1 and α3 (as defined in native proinsulin) and flanking β-strand (residues B24-B26). The variant peptides exhibit successive structural perturbations in order: parent (most organized) > SerB24 >> ProA16 > ProB15 (least organized). The same order pertains to (a) overall α-helix content as probed by circular dichroism, (b) synthetic yields of corresponding three-disulfide insulin analogs, and (c) ER stress induced in cell culture by corresponding mutant proinsulins. These findings suggest that this and related peptide models will provide a general platform for classification of MIDY mutations based on molecular mechanisms by which nascent disulfide pairing is impaired. We propose that the syndrome's variable phenotypic spectrum-onsets ranging from the neonatal period to later in childhood or adolescence-reflects structural features of respective folding intermediates.

Structural principles of insulin formulation and analog design: A century of innovation

BACKGROUND

The discovery of insulin in 1921 and its near-immediate clinical use initiated a century of innovation. Advances extended across a broad front, from the stabilization of animal insulin formulations to the frontiers of synthetic peptide chemistry, and in turn, from the advent of recombinant DNA manufacturing to structure-based protein analog design. In each case, a creative interplay was observed between pharmaceutical applications and then-emerging principles of protein science; indeed, translational objectives contributed to a growing molecular understanding of protein structure, aggregation and misfolding.

SCOPE OF REVIEW

Pioneering crystallographic analyses-beginning with Hodgkin's solving of the 2-Zn insulin hexamer-elucidated general features of protein self-assembly, including zinc coordination and the allosteric transmission of conformational change. Crystallization of insulin was exploited both as a step in manufacturing and as a means of obtaining protracted action. Forty years ago, the confluence of recombinant human insulin with techniques for site-directed mutagenesis initiated the present era of insulin analogs. Variant or modified insulins were developed that exhibit improved prandial or basal pharmacokinetic (PK) properties. Encouraged by clinical trials demonstrating the long-term importance of glycemic control, regimens based on such analogs sought to resemble daily patterns of endogenous β-cell secretion more closely, ideally with reduced risk of hypoglycemia.

MAJOR CONCLUSIONS

Next-generation insulin analog design seeks to explore new frontiers, including glucose-responsive insulins, organ-selective analogs and biased agonists tailored to address yet-unmet clinical needs. In the coming decade, we envision ever more powerful scientific synergies at the interface of structural biology, molecular physiology and therapeutics.

Structural Lessons From the Mutant Proinsulin Syndrome

Insight into folding mechanisms of proinsulin has been provided by analysis of dominant diabetes-associated mutations in the human insulin gene (INS). Such mutations cause pancreatic β-cell dysfunction due to toxic misfolding of a mutant proinsulin and impairment in trans of wild-type insulin secretion. Anticipated by the "Akita" mouse (a classical model of monogenic diabetes mellitus; DM), this syndrome illustrates the paradigm endoreticulum (ER) stress leading to intracellular proteotoxicity. Diverse clinical mutations directly or indirectly perturb native disulfide pairing leading to protein misfolding and aberrant aggregation. Although most introduce or remove a cysteine (Cys; leading in either case to an unpaired thiol group), non-Cys-related mutations identify key determinants of folding efficiency. Studies of such mutations suggest that the hormone's evolution has been constrained not only by structure-function relationships, but also by the susceptibility of its single-chain precursor to impaired foldability. An intriguing hypothesis posits that INS overexpression in response to peripheral insulin resistance likewise leads to chronic ER stress and β-cell dysfunction in the natural history of non-syndromic Type 2 DM. Cryptic contributions of conserved residues to folding efficiency, as uncovered by rare genetic variants, define molecular links between biophysical principles and the emerging paradigm of Darwinian medicine: Biosynthesis of proinsulin at the edge of non-foldability provides a key determinant of "diabesity" as a pandemic disease of civilization.

'Smart' insulin-delivery technologies and intrinsic glucose-responsive insulin analogues

Insulin replacement therapy for diabetes mellitus seeks to minimise excursions in blood glucose concentration above or below the therapeutic range (hyper- or hypoglycaemia). To mitigate acute and chronic risks of such excursions, glucose-responsive insulin-delivery technologies have long been sought for clinical application in type 1 and long-standing type 2 diabetes mellitus. Such 'smart' systems or insulin analogues seek to provide hormonal activity proportional to blood glucose levels without external monitoring. This review highlights three broad strategies to co-optimise mean glycaemic control and time in range: (1) coupling of continuous glucose monitoring (CGM) to delivery devices (algorithm-based 'closed-loop' systems); (2) glucose-responsive polymer encapsulation of insulin; and (3) mechanism-based hormone modifications. Innovations span control algorithms for CGM-based insulin-delivery systems, glucose-responsive polymer matrices, bio-inspired design based on insulin's conformational switch mechanism upon insulin receptor engagement, and glucose-responsive modifications of new insulin analogues. In each case, innovations in insulin chemistry and formulation may enhance clinical outcomes. Prospects are discussed for intrinsic glucose-responsive insulin analogues containing a reversible switch (regulating bioavailability or conformation) that can be activated by glucose at high concentrations.

Diabetes mellitus due to toxic misfolding of proinsulin variants

BACKGROUND

Dominant mutations in the human insulin gene (INS) lead to pancreatic β-cell dysfunction and diabetes mellitus (DM) due to toxic misfolding of a mutant proinsulin. Analogous to a classical mouse model of monogenic DM ("Akita"), this syndrome highlights the susceptibility of β-cells to endoreticulum (ER) stress due to protein misfolding and aberrant aggregation.

SCOPE OF REVIEW

Diverse clinical mutations directly or indirectly perturb native disulfide pairing. Whereas most introduce or remove a cysteine (Cys; leading in either case to an unpaired thiol group), non-Cys-related mutations identify key determinants of folding efficiency. Studies of such mutations suggest that the hormone's evolution has been constrained not only by structure-function relationships but also by the susceptibility of its single-chain precursor to impaired foldability. An intriguing hypothesis posits that INS overexpression in response to peripheral insulin resistance likewise leads to chronic ER stress and β-cell dysfunction in the natural history of nonsyndromic Type 2 DM.

MAJOR CONCLUSIONS

Cryptic contributions of conserved residues to folding efficiency, as uncovered by rare genetic variants, define molecular links between biophysical principles and the emerging paradigm of Darwinian medicine: Biosynthesis of proinsulin at the edge of nonfoldability provides a key determinant of "diabesity" as a pandemic disease of civilization.

Evolution of insulin at the edge of foldability and its medical implications

Proteins have evolved to be foldable, and yet determinants of foldability may be inapparent once the native state is reached. Insight has emerged from studies of diseases of protein misfolding, exemplified by monogenic diabetes mellitus due to mutations in proinsulin leading to endoplasmic reticulum stress and β-cell death. Cellular foldability of human proinsulin requires an invariant Phe within a conserved crevice at the receptor-binding surface (position B24). Any substitution, even related aromatic residue TyrB24, impairs insulin biosynthesis and secretion. As a seeming paradox, a monomeric TyrB24 insulin analog exhibits a native-like structure in solution with only a modest decrement in stability. Packing of TyrB24 is similar to that of PheB24, adjoining core cystine B19-A20 to seal the core; the analog also exhibits native self-assembly. Although affinity for the insulin receptor is decreased ∼20-fold, biological activities in cells and rats were within the range of natural variation. Together, our findings suggest that the invariance of PheB24 among vertebrate insulins and insulin-like growth factors reflects an essential role in enabling efficient protein folding, trafficking, and secretion, a function that is inapparent in native structures. In particular, we envision that the para-hydroxyl group of TyrB24 hinders pairing of cystine B19-A20 in an obligatory on-pathway folding intermediate. The absence of genetic variation at B24 and other conserved sites near this disulfide bridge-excluded due to β-cell dysfunction-suggests that insulin has evolved to the edge of foldability. Nonrobustness of a protein's fitness landscape underlies both a rare monogenic syndrome and "diabesity" as a pandemic disease of civilization.

Secretagogin Regulates Insulin Signaling by Direct Insulin Binding

Secretagogin (SCGN) is a β-cell enriched, secretory/cytosolic Ca²⁺-binding protein with unknown secretory regulation and functions. Recent findings suggest that SCGN deficiency correlates with compromised insulin response and diabetes. However, the (patho)physiological SCGN-insulin nexus remains unexplored. We here report that SCGN is an insulin-interacting protein. The protein-protein interaction between SCGN and insulin regulates insulin stability and increases insulin potency in vitro and in vivo. Mutagenesis studies suggest an indispensable role for N-terminal domain of SCGN in modulating insulin stability and function. SCGN supplementation in diabetogenic-diet-fed mice preserves physiological insulin responsiveness while relieving obesity and cardiovascular risk. SCGN-insulin interaction mediated alleviation of hyperinsulinemia by increased insulin internalization, which translates to reduced body fat and hepatic lipid accumulation, emerges as a plausible mechanism for the preservation of insulin responsiveness. These findings establish SCGN as a functional insulin-binding protein (InsBP) with therapeutic potential against diabetes.

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SCGN is an insulin-interacting calcium sensor protein

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SCGN binding protects insulin from aggregation

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SCGN potentiates insulin action in vivo

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SCGN administration into HFD-fed animals impedes insulin resistance

Biosynthesis, structure, and folding of the insulin precursor protein

Insulin synthesis in pancreatic β-cells is initiated as preproinsulin. Prevailing glucose concentrations, which oscillate pre- and postprandially, exert major dynamic variation in preproinsulin biosynthesis. Accompanying upregulated translation of the insulin precursor includes elements of the endoplasmic reticulum (ER) translocation apparatus linked to successful orientation of the signal peptide, translocation and signal peptide cleavage of preproinsulin-all of which are necessary to initiate the pathway of proper proinsulin folding. Evolutionary pressures on the primary structure of proinsulin itself have preserved the efficiency of folding ("foldability"), and remarkably, these evolutionary pressures are distinct from those protecting the ultimate biological activity of insulin. Proinsulin foldability is manifest in the ER, in which the local environment is designed to assist in the overall load of proinsulin folding and to favour its disulphide bond formation (while limiting misfolding), all of which is closely tuned to ER stress response pathways that have complex (beneficial, as well as potentially damaging) effects on pancreatic β-cells. Proinsulin misfolding may occur as a consequence of exuberant proinsulin biosynthetic load in the ER, proinsulin coding sequence mutations, or genetic predispositions that lead to an altered ER folding environment. Proinsulin misfolding is a phenotype that is very much linked to deficient insulin production and diabetes, as is seen in a variety of contexts: rodent models bearing proinsulin-misfolding mutants, human patients with Mutant INS-gene-induced Diabetes of Youth (MIDY), animal models and human patients bearing mutations in critical ER resident proteins, and, quite possibly, in more common variety type 2 diabetes.

A thing of beauty: Structure and function of insulin's "aromatic triplet"

The classical crystal structure of insulin was determined in 1969 by D.C. Hodgkin et al. following a 35-year program of research. This structure depicted a hexamer remarkable for its self-assembly as a zinc-coordinated trimer of dimer. Prominent at the dimer interface was an "aromatic triplet" of conserved residues at consecutive positions in the B chain: PheB24 , PheB25 and TyrB26 . The elegance of this interface inspired the Oxford team to poetry: "A thing of beauty is a joy forever" (John Keats as quoted by Blundell, T.L., et al. Advances in Protein Chemistry 26:279-286 [1972]). Here, we revisit this aromatic triplet in light of recent advances in the structural biology of insulin bound as a monomer to fragments of the insulin receptor. Such co-crystal structures have defined how these side chains pack at the primary hormone-binding surface of the receptor ectodomain. On receptor binding, the B-chain β-strand (residues B24-B28) containing the aromatic triplet detaches from the α-helical core of the hormone. Whereas TyrB26 lies at the periphery of the receptor interface and may functionally be replaced by a diverse set of substitutions, PheB24 and PheB25 engage invariant elements of receptor domains L1 and αCT. These critical contacts were anticipated by the discovery of diabetes-associated mutations at these positions by Donald Steiner et al. at the University of Chicago. Conservation of PheB24 , PheB25 and TyrB26 among vertebrate insulins reflects the striking confluence of structure-based evolutionary constraints: foldability, protective self-assembly and hormonal activity.

Solution structure of an ultra-stable single-chain insulin analog connects protein dynamics to a novel mechanism of receptor binding

Domain-minimized insulin receptors (IRs) have enabled crystallographic analysis of insulin-bound "micro-receptors." In such structures, the C-terminal segment of the insulin B chain inserts between conserved IR domains, unmasking an invariant receptor-binding surface that spans both insulin A and B chains. This "open" conformation not only rationalizes the inactivity of single-chain insulin (SCI) analogs (in which the A and B chains are directly linked), but also suggests that connecting (C) domains of sufficient length will bind the IR. Here, we report the high-resolution solution structure and dynamics of such an active SCI. The hormone's closed-to-open transition is foreshadowed by segmental flexibility in the native state as probed by heteronuclear NMR spectroscopy and multiple conformer simulations of crystallographic protomers as described in the companion article. We propose a model of the SCI's IR-bound state based on molecular-dynamics simulations of a micro-receptor complex. In this model, a loop defined by the SCI's B and C domains encircles the C-terminal segment of the IR α-subunit. This binding mode predicts a conformational transition between an ultra-stable closed state (in the free hormone) and an active open state (on receptor binding). Optimization of this switch within an ultra-stable SCI promises to circumvent insulin's complex global cold chain. The analog's biphasic activity, which serendipitously resembles current premixed formulations of soluble insulin and microcrystalline suspension, may be of particular utility in the developing world.

Insulin in motion: The A6-A11 disulfide bond allosterically modulates structural transitions required for insulin activity

The structural transitions required for insulin to activate its receptor and initiate regulation of glucose homeostasis are only partly understood. Here, using ring-closing metathesis, we substitute the A6-A11 disulfide bond of insulin with a rigid, non-reducible dicarba linkage, yielding two distinct stereo-isomers (cis and trans). Remarkably, only the cis isomer displays full insulin potency, rapidly lowering blood glucose in mice (even under insulin-resistant conditions). It also posseses reduced mitogenic activity in vitro. Further biophysical, crystallographic and molecular-dynamics analyses reveal that the A6-A11 bond configuration directly affects the conformational flexibility of insulin A-chain N-terminal helix, dictating insulin’s ability to engage its receptor. We reveal that in native insulin, contraction of the C_α-C_α distance of the flexible A6-A11 cystine allows the A-chain N-terminal helix to unwind to a conformation that allows receptor engagement. This motion is also permitted in the cis isomer, with its shorter C_α-C_α distance, but prevented in the extended trans analogue. These findings thus illuminate for the first time the allosteric role of the A6-A11 bond in mediating the transition of the hormone to an active conformation, significantly advancing our understanding of insulin action and opening up new avenues for the design of improved therapeutic analogues.

An ultra-stable single-chain insulin analog resists thermal inactivation and exhibits biological signaling duration equivalent to the native protein

Thermal degradation of insulin complicates its delivery and use. Previous efforts to engineer ultra-stable analogs were confounded by prolonged cellular signaling in vivo, of unclear safety and complicating mealtime therapy. We therefore sought an ultra-stable analog whose potency and duration of action on intravenous bolus injection in diabetic rats are indistinguishable from wild-type (WT) insulin. Here, we describe the structure, function, and stability of such an analog, a 57-residue single-chain insulin (SCI) with multiple acidic substitutions. Cell-based studies revealed native-like signaling properties with negligible mitogenic activity. Its crystal structure, determined as a novel zinc-free hexamer at 2.8 Å, revealed a native insulin fold with incomplete or absent electron density in the C domain; complementary NMR studies are described in the accompanying article. The stability of the analog (ΔG_U 5.0(±0.1) kcal/mol at 25 °C) was greater than that of WT insulin (3.3(±0.1) kcal/mol). On gentle agitation, the SCI retained full activity for >140 days at 45 °C and >48 h at 75 °C. These findings indicate that marked resistance to thermal inactivation in vitro is compatible with native duration of activity in vivo Further, whereas WT insulin forms large and heterogeneous aggregates above the standard 0.6 mm pharmaceutical strength, perturbing the pharmacokinetic properties of concentrated formulations, dynamic light scattering, and size-exclusion chromatography revealed only limited SCI self-assembly and aggregation in the concentration range 1-7 mm Such a combination of favorable biophysical and biological properties suggests that SCIs could provide a global therapeutic platform without a cold chain.

Extending Halogen-based Medicinal Chemistry to Proteins: IODO-INSULIN AS A CASE STUDY

Insulin, a protein critical for metabolic homeostasis, provides a classical model for protein design with application to human health. Recent efforts to improve its pharmaceutical formulation demonstrated that iodination of a conserved tyrosine (TyrB26) enhances key properties of a rapid-acting clinical analog. Moreover, the broad utility of halogens in medicinal chemistry has motivated the use of hybrid quantum- and molecular-mechanical methods to study proteins. Here, we (i) undertook quantitative atomistic simulations of 3-[iodo-TyrB26]insulin to predict its structural features, and (ii) tested these predictions by X-ray crystallography. Using an electrostatic model of the modified aromatic ring based on quantum chemistry, the calculations suggested that the analog, as a dimer and hexamer, exhibits subtle differences in aromatic-aromatic interactions at the dimer interface. Aromatic rings (TyrB16, PheB24, PheB25, 3-I-TyrB26, and their symmetry-related mates) at this interface adjust to enable packing of the hydrophobic iodine atoms within the core of each monomer. Strikingly, these features were observed in the crystal structure of a 3-[iodo-TyrB26]insulin analog (determined as an R6 zinc hexamer). Given that residues B24-B30 detach from the core on receptor binding, the environment of 3-I-TyrB26 in a receptor complex must differ from that in the free hormone. Based on the recent structure of a "micro-receptor" complex, we predict that 3-I-TyrB26 engages the receptor via directional halogen bonding and halogen-directed hydrogen bonding as follows: favorable electrostatic interactions exploiting, respectively, the halogen's electron-deficient σ-hole and electronegative equatorial band. Inspired by quantum chemistry and molecular dynamics, such "halogen engineering" promises to extend principles of medicinal chemistry to proteins.

Contribution of TyrB26 to the Function and Stability of Insulin: STRUCTURE-ACTIVITY RELATIONSHIPS AT A CONSERVED HORMONE-RECEPTOR INTERFACE

Crystallographic studies of insulin bound to receptor domains have defined the primary hormone-receptor interface. We investigated the role of Tyr(B26), a conserved aromatic residue at this interface. To probe the evolutionary basis for such conservation, we constructed 18 variants at B26. Surprisingly, non-aromatic polar or charged side chains (such as Glu, Ser, or ornithine (Orn)) conferred high activity, whereas the weakest-binding analogs contained Val, Ile, and Leu substitutions. Modeling of variant complexes suggested that the B26 side chains pack within a shallow depression at the solvent-exposed periphery of the interface. This interface would disfavor large aliphatic side chains. The analogs with highest activity exhibited reduced thermodynamic stability and heightened susceptibility to fibrillation. Perturbed self-assembly was also demonstrated in studies of the charged variants (Orn and Glu); indeed, the Glu(B26) analog exhibited aberrant aggregation in either the presence or absence of zinc ions. Thus, although Tyr(B26) is part of insulin's receptor-binding surface, our results suggest that its conservation has been enjoined by the aromatic ring's contributions to native stability and self-assembly. We envisage that such classical structural relationships reflect the implicit threat of toxic misfolding (rather than hormonal function at the receptor level) as a general evolutionary determinant of extant protein sequences.

Proinsulin misfolding and endoplasmic reticulum stress during the development and progression of diabetes

To maintain copious insulin granule stores in the face of ongoing metabolic demand, pancreatic beta cells must produce large quantities of proinsulin, the insulin precursor. Proinsulin biosynthesis can account for up to 30-50% of total cellular protein synthesis of beta cells. This puts pressure on the beta cell secretory pathway, especially the endoplasmic reticulum (ER), where proinsulin undergoes its initial folding, including the formation of three evolutionarily conserved disulfide bonds. In normal beta cells, up to 20% of newly synthesized proinsulin may fail to reach its native conformation, suggesting that proinsulin is a misfolding-prone protein. Misfolded proinsulin molecules can either be refolded to their native structure or degraded through ER associated degradation (ERAD) and autophagy. These degraded molecules decrease proinsulin yield but do not otherwise compromise beta cell function. However, under certain pathological conditions, proinsulin misfolding increases, exceeding the genetically determined threshold of beta cells to handle the misfolded protein load. This results in accumulation of misfolded proinsulin in the ER - a causal factor leading to beta cell failure and diabetes. In patients with Mutant INS-gene induced diabetes of Youth (MIDY), increased proinsulin misfolding due to insulin gene mutations is the primary defect operating as a "first hit" to beta cells. Additionally, increased proinsulin misfolding can be secondary to an unfavorable ER folding environment due to genetic and/or environmental factors. Under these conditions, increased wild-type proinsulin misfolding becomes a "second hit" to the ER and beta cells, aggravating beta cell failure and diabetes. In this article, we describe our current understanding of the normal proinsulin folding pathway in the ER, and then review existing links between proinsulin misfolding, ER dysfunction, and beta cell failure in the development and progression of type 2, type 1, and some monogenic forms of diabetes.

Aromatic anchor at an invariant hormone-receptor interface: function of insulin residue B24 with application to protein design

Crystallographic studies of insulin bound to fragments of the insulin receptor have recently defined the topography of the primary hormone-receptor interface. Here, we have investigated the role of Phe(B24), an invariant aromatic anchor at this interface and site of a human mutation causing diabetes mellitus. An extensive set of B24 substitutions has been constructed and tested for effects on receptor binding. Although aromaticity has long been considered a key requirement at this position, Met(B24) was found to confer essentially native affinity and bioactivity. Molecular modeling suggests that this linear side chain can serve as an alternative hydrophobic anchor at the hormone-receptor interface. These findings motivated further substitution of Phe(B24) by cyclohexanylalanine (Cha), which contains a nonplanar aliphatic ring. Contrary to expectations, [Cha(B24)]insulin likewise exhibited high activity. Furthermore, its resistance to fibrillation and the rapid rate of hexamer disassembly, properties of potential therapeutic advantage, were enhanced. The crystal structure of the Cha(B24) analog, determined as an R6 zinc-stabilized hexamer at a resolution of 1.5 Å, closely resembles that of wild-type insulin. The nonplanar aliphatic ring exhibits two chair conformations with partial occupancies, each recapitulating the role of Phe(B24) at the dimer interface. Together, these studies have defined structural requirements of an anchor residue within the B24-binding pocket of the insulin receptor; similar molecular principles are likely to pertain to insulin-related growth factors. Our results highlight in particular the utility of nonaromatic side chains as probes of the B24 pocket and suggest that the nonstandard Cha side chain may have therapeutic utility.

Deciphering a molecular mechanism of neonatal diabetes mellitus by the chemical synthesis of a protein diastereomer, [D-AlaB8]human proinsulin

Misfolding of proinsulin variants in the pancreatic β-cell, a monogenic cause of permanent neonatal-onset diabetes mellitus, provides a model for a disease of protein toxicity. A hot spot for such clinical mutations is found at position B8, conserved as glycine within the vertebrate insulin superfamily. We set out to investigate the molecular basis of the aberrant properties of a proinsulin clinical mutant in which residue Gly(B8) is replaced by Ser(B8). Modular total chemical synthesis was used to prepare the wild-type [Gly(B8)]proinsulin molecule and three analogs: [D-Ala(B8)]proinsulin, [L-Ala(B8)]proinsulin, and the clinical mutant [L-Ser(B8)]proinsulin. The protein diastereomer [D-Ala(B8)]proinsulin produced higher folding yields at all pH values compared with the wild-type proinsulin and the other two analogs, but showed only very weak binding to the insulin receptor. The clinical mutant [L-Ser(B8)]proinsulin impaired folding at pH 7.5 even in the presence of protein-disulfide isomerase. Surprisingly, although [L-Ser(B8)]proinsulin did not fold well under the physiological conditions investigated, once folded the [L-Ser(B8)]proinsulin protein molecule bound to the insulin receptor more effectively than wild-type proinsulin. Such paradoxical gain of function (not pertinent in vivo due to impaired secretion of the mutant insulin) presumably reflects induced fit in the native mechanism of hormone-receptor engagement. This work provides insight into the molecular mechanism of a clinical mutation in the insulin gene associated with diabetes mellitus. These results dramatically illustrate the power of total protein synthesis, as enabled by modern chemical ligation methods, for the investigation of protein folding and misfolding.

Biophysical optimization of a therapeutic protein by nonstandard mutagenesis: studies of an iodo-insulin derivative

Insulin provides a model for the therapeutic application of protein engineering. A paradigm in molecular pharmacology was defined by design of rapid-acting insulin analogs for the prandial control of glycemia. Such analogs, a cornerstone of current diabetes regimens, exhibit accelerated subcutaneous absorption due to more rapid disassembly of oligomeric species relative to wild-type insulin. This strategy is limited by a molecular trade-off between accelerated disassembly and enhanced susceptibility to degradation. Here, we demonstrate that this trade-off may be circumvented by nonstandard mutagenesis. Our studies employed Lys(B28), Pro(B29)-insulin ("lispro") as a model prandial analog that is less thermodynamically stable and more susceptible to fibrillation than is wild-type insulin. We have discovered that substitution of an invariant tyrosine adjoining the engineered sites in lispro (Tyr(B26)) by 3-iodo-Tyr (i) augments its thermodynamic stability (ΔΔGu 0.5 ± 0.2 kcal/mol), (ii) delays onset of fibrillation (lag time on gentle agitation at 37 °C was prolonged by 4-fold), (iii) enhances affinity for the insulin receptor (1.5 ± 0.1-fold), and (iv) preserves biological activity in a rat model of diabetes mellitus. (1)H NMR studies suggest that the bulky iodo-substituent packs within a nonpolar interchain crevice. Remarkably, the 3-iodo-Tyr(B26) modification stabilizes an oligomeric form of insulin pertinent to pharmaceutical formulation (the R6 zinc hexamer) but preserves rapid disassembly of the oligomeric form pertinent to subcutaneous absorption (T6 hexamer). By exploiting this allosteric switch, 3-iodo-Tyr(B26)-lispro thus illustrates how a nonstandard amino acid substitution can mitigate the unfavorable biophysical properties of an engineered protein while retaining its advantages.

Diabetes mellitus due to the toxic misfolding of proinsulin variants

Dominant mutations in the human insulin gene can lead to pancreatic β-cell dysfunction and diabetes mellitus due to toxic folding of a mutant proinsulin. Analogous to a classical mouse model (the Akita mouse), this monogenic syndrome highlights the susceptibility of human β-cells to endoreticular stress due to protein misfolding and aberrant aggregation. The clinical mutations directly or indirectly perturb native disulfide pairing. Whereas the majority of mutations introduce or remove a cysteine (leading in either case to an unpaired residue), non-cysteine-related mutations identify key determinants of folding efficiency. Studies of such mutations suggest that the evolution of insulin has been constrained not only by its structure and function, but also by the susceptibility of its single-chain precursor to impaired foldability.

Structural integrity of the B24 site in human insulin is important for hormone functionality

Despite the recent first structural insight into the insulin-insulin receptor complex, the role of the C terminus of the B-chain of insulin in this assembly remains unresolved. Previous studies have suggested that this part of insulin must rearrange to reveal amino acids crucial for interaction with the receptor. The role of the invariant Phe(B24), one of the key residues of the hormone, in this process remains unclear. For example, the B24 site functionally tolerates substitutions to D-amino acids but not to L-amino acids. Here, we prepared and characterized a series of B24-modified insulin analogues, also determining the structures of [D-HisB24]-insulin and [HisB24]-insulin. The inactive [HisB24]-insulin molecule is remarkably rigid due to a tight accommodation of the L-His side chain in the B24 binding pocket that results in the stronger tethering of B25-B28 residues to the protein core. In contrast, the highly active [D-HisB24]-insulin is more flexible, and the reverse chirality of the B24C(α) atom swayed the D-His(B24) side chain into the solvent. Furthermore, the pocket vacated by Phe(B24) is filled by Phe(B25), which mimics the Phe(B24) side and main chains. The B25→B24 downshift results in a subsequent downshift of Tyr(B26) into the B25 site and the departure of B26-B30 residues away from the insulin core. Our data indicate the importance of the aromatic L-amino acid at the B24 site and the structural invariance/integrity of this position for an effective binding of insulin to its receptor. Moreover, they also suggest limited, B25-B30 only, unfolding of the C terminus of the B-chain upon insulin activation.

All-atom structural models of insulin binding to the insulin receptor in the presence of a tandem hormone-binding element

Insulin regulates blood glucose levels in higher organisms by binding to and activating insulin receptor (IR), a constitutively homodimeric glycoprotein of the receptor tyrosine kinase (RTK) superfamily. Therapeutic efforts in treating diabetes have been significantly impeded by the absence of structural information on the activated form of the insulin/IR complex. Mutagenesis and photo-crosslinking experiments and structural information on insulin and apo-IR strongly suggest that the dual-chain insulin molecule, unlike the related single-chain insulin-like growth factors, binds to IR in a very different conformation than what is displayed in storage forms of the hormone. In particular, hydrophobic residues buried in the core of the folded insulin molecule engage the receptor. There is also the possibility of plasticity in the receptor structure based on these data, which may in part be due to rearrangement of the so-called CT-peptide, a tandem hormone-binding element of IR. These possibilities provide opportunity for large-scale molecular modeling to contribute to our understanding of this system. Using various atomistic simulation approaches, we have constructed all-atom structural models of hormone/receptor complexes in the presence of CT in its crystallographic position and a thermodynamically favorable displaced position. In the "displaced-CT" complex, many more insulin-receptor contacts suggested by experiments are satisfied, and our simulations also suggest that R-insulin potentially represents the receptor-bound form of hormone. The results presented in this work have further implications for the design of receptor-specific agonists/antagonists.

Dissecting the role of disulfide bonds on the amyloid formation of insulin

Disulfide bonds play a critical role in the stability and folding of proteins. Here, we used insulin as a model system, to investigate the role of its individual disulfide bond during the amyloid formation of insulin. Tris(2-carboxyethyl)phosphine (TCEP) was applied to reduce two of the three disulfide bonds in porcine insulin and the reduced disulfide bonds were then alkylated by iodoacetamide. Three disulfide bond-modified insulin analogs, INS-2 (lack of A6-A11), INS-3 (lack of A7-B7) and INS-6 (lack of both A6-A11 and A7-B7), were obtained. Far-UV circular dichroism (CD) spectroscopy results indicated that the secondary structure of INS-2 was the closest to insulin under neutral conditions, followed by INS-3 and INS-6, whereas in an acidic solution all analogs were essentially unfolded. To test how these modifications affect the amyloidogenicity of insulin, thioflavin-T (ThT) fluorescence and transmission electronic microscopy (TEM) were performed. Our results showed that all analogs were more prone to aggregation than insulin, with the order of aggregation rates being INS-6>INS-3>INS-2. Cross-linking of unmodified proteins (PICUP) assay results showed that analogs without A6-A11 (INS-2 and INS-6) have a higher potential for oligomerization than insulin and INS-3, which is accompanied with a higher cytotoxicity as the hemolytic assays of human erythrocytes suggested. The results indicated that breakage of A7-B7 induced more unfolding of the insulin structure and a higher amyloidogenicity than breakage of A6-A11, but breakage of A6-A11 caused a significant cytotoxicity increase and a higher potency to form high order toxic oligomers.

Insulin fibrillation and protein design: topological resistance of single-chain analogs to thermal degradation with application to a pump reservoir

Insulin is susceptible to thermal fibrillation, a misfolding process that leads to nonnative cross-β assembly analogous to pathological amyloid deposition. Pharmaceutical formulations are ordinarily protected from such degradation by sequestration of the susceptible monomer within native protein assemblies. With respect to the safety and efficacy of insulin pumps, however, this strategy imposes an intrinsic trade-off between pharmacokinetic goals (rapid absorption and clearance) and the requisite physical properties of a formulation (prolonged shelf life and stability within the reservoir). Available rapid-acting formulations are suboptimal in both respects; susceptibility to fibrillation is exacerbated even as absorption is delayed relative to the ideal specifications of a closed-loop system. To circumvent this molecular trade-off, we exploited structural models of insulin fibrils and amyloidogenic intermediates to define an alternative protective mechanism. Single-chain insulin (SCI) analogs were shown to be refractory to thermal fibrillation with maintenance of biological activity for more than 3 months under conditions that promote the rapid fibrillation and inactivation of insulin. The essential idea exploits an intrinsic incompatibility between SCI topology and the geometry of cross-β assembly. A peptide tether was thus interposed between the A- and B-chains whose length was (a) sufficiently long to provide the "play" needed for induced fit of the hormone on receptor binding and yet (b) sufficiently short to impose a topological barrier to fibrillation. Our findings suggest that ultrastable monomeric SCI analogs may be formulated without protective self-assembly and so permit simultaneous optimization of pharmacokinetics and reservoir life.

Insulin analogs for the treatment of diabetes mellitus: therapeutic applications of protein engineering

The engineering of insulin analogs represents a triumph of structure-based protein design. A framework has been provided by structures of insulin hexamers. Containing a zinc-coordinated trimer of dimers, such structures represent a storage form of the active insulin monomer. Initial studies focused on destabilization of subunit interfaces. Because disassembly facilitates capillary absorption, such targeted destabilization enabled development of rapid-acting insulin analogs. Converse efforts were undertaken to stabilize the insulin hexamer and promote higher-order self-assembly within the subcutaneous depot toward the goal of enhanced basal glycemic control with reduced risk of hypoglycemia. Current products either operate through isoelectric precipitation (insulin glargine, the active component of Lantus(®); Sanofi-Aventis) or employ an albumin-binding acyl tether (insulin detemir, the active component of Levemir(®); Novo-Nordisk). To further improve pharmacokinetic properties, modified approaches are presently under investigation. Novel strategies have recently been proposed based on subcutaneous supramolecular assembly coupled to (a) large-scale allosteric reorganization of the insulin hexamer (the TR transition), (b) pH-dependent binding of zinc ions to engineered His-X(3)-His sites at hexamer surfaces, or (c) the long-range vision of glucose-responsive polymers for regulated hormone release. Such designs share with wild-type insulin and current insulin products a susceptibility to degradation above room temperature, and so their delivery, storage, and use require the infrastructure of an affluent society. Given the global dimensions of the therapeutic supply chain, we envisage that concurrent engineering of ultra-stable protein analog formulations would benefit underprivileged patients in the developing world.

Insulin: a small protein with a long journey

Insulin is a hormone that is essential for regulating energy storage and glucose metabolism in the body. Insulin in liver, muscle, and fat tissues stimulates the cell to take up glucose from blood and store it as glycogen in liver and muscle. Failure of insulin control causes diabetes mellitus (DM). Insulin is the unique medicine to treat some forms of DM. The population of diabetics has dramatically increased over the past two decades, due to high absorption of carbohydrates (or fats and proteins), lack of physical exercise, and development of new diagnostic techniques. At present, the two largest developing countries (India and China) and the largest developed country (United States) represent the top three countries in terms of diabetic population. Insulin is a small protein, but contains almost all structural features typical of proteins: α-helix, β-sheet, β-turn, high order assembly, allosteric T®R-transition, and conformational changes in amyloidal fibrillation. More than ten years' efforts on studying insulin disulfide intermediates by NMR have enabled us to decipher the whole picture of insulin folding coupled to disulfide pairing, especially at the initial stage that forms the nascent peptide. Two structural switches are also known to regulate insulin binding to receptors and progress has been made to identify the residues involved in binding. However, resolving the complex structure of insulin and its receptor remains a challenge in insulin research. Nevertheless, the accumulated knowledge of insulin structure has allowed us to specifically design a new ultra-stable and active single-chain insulin analog (SCI-57), and provides a novel way to design super-stable, fast-acting and cheaper insulin formulations for DM patients. Continuing this long journey of insulin study will benefit basic research in proteins and in pharmaceutical therapy.

Conformational dynamics of insulin

We have exploited a prandial insulin analog to elucidate the underlying structure and dynamics of insulin as a monomer in solution. A model was provided by insulin lispro (the active component of Humalog(®); Eli Lilly and Co.). Whereas NMR-based modeling recapitulated structural relationships of insulin crystals (T-state protomers), dynamic anomalies were revealed by amide-proton exchange kinetics in D(2)O. Surprisingly, the majority of hydrogen bonds observed in crystal structures are only transiently maintained in solution, including key T-state-specific inter-chain contacts. Long-lived hydrogen bonds (as defined by global exchange kinetics) exist only at a subset of four α-helical sites (two per chain) flanking an internal disulfide bridge (cystine A20-B19); these sites map within the proposed folding nucleus of proinsulin. The anomalous flexibility of insulin otherwise spans its active surface and may facilitate receptor binding. Because conformational fluctuations promote the degradation of pharmaceutical formulations, we envisage that "dynamic re-engineering" of insulin may enable design of ultra-stable formulations for humanitarian use in the developing world.

Mutant INS-gene induced diabetes of youth: proinsulin cysteine residues impose dominant-negative inhibition on wild-type proinsulin transport

Recently, a syndrome of Mutant INS-gene-induced Diabetes of Youth (MIDY, derived from one of 26 distinct mutations) has been identified as a cause of insulin-deficient diabetes, resulting from expression of a misfolded mutant proinsulin protein in the endoplasmic reticulum (ER) of insulin-producing pancreatic beta cells. Genetic deletion of one, two, or even three alleles encoding insulin in mice does not necessarily lead to diabetes. Yet MIDY patients are INS-gene heterozygotes; inheritance of even one MIDY allele, causes diabetes. Although a favored explanation for the onset of diabetes is that insurmountable ER stress and ER stress response from the mutant proinsulin causes a net loss of beta cells, in this report we present three surprising and interlinked discoveries. First, in the presence of MIDY mutants, an increased fraction of wild-type proinsulin becomes recruited into nonnative disulfide-linked protein complexes. Second, regardless of whether MIDY mutations result in the loss, or creation, of an extra unpaired cysteine within proinsulin, Cys residues in the mutant protein are nevertheless essential in causing intracellular entrapment of co-expressed wild-type proinsulin, blocking insulin production. Third, while each of the MIDY mutants induces ER stress and ER stress response; ER stress and ER stress response alone appear insufficient to account for blockade of wild-type proinsulin. While there is general agreement that ultimately, as diabetes progresses, a significant loss of beta cell mass occurs, the early events described herein precede cell death and loss of beta cell mass. We conclude that the molecular pathogenesis of MIDY is initiated by perturbation of the disulfide-coupled folding pathway of wild-type proinsulin.

Deciphering the hidden informational content of protein sequences: foldability of proinsulin hinges on a flexible arm that is dispensable in the mature hormone

Protein sequences encode both structure and foldability. Whereas the interrelationship of sequence and structure has been extensively investigated, the origins of folding efficiency are enigmatic. We demonstrate that the folding of proinsulin requires a flexible N-terminal hydrophobic residue that is dispensable for the structure, activity, and stability of the mature hormone. This residue (Phe(B1) in placental mammals) is variably positioned within crystal structures and exhibits (1)H NMR motional narrowing in solution. Despite such flexibility, its deletion impaired insulin chain combination and led in cell culture to formation of non-native disulfide isomers with impaired secretion of the variant proinsulin. Cellular folding and secretion were maintained by hydrophobic substitutions at B1 but markedly perturbed by polar or charged side chains. We propose that, during folding, a hydrophobic side chain at B1 anchors transient long-range interactions by a flexible N-terminal arm (residues B1-B8) to mediate kinetic or thermodynamic partitioning among disulfide intermediates. Evidence for the overall contribution of the arm to folding was obtained by alanine scanning mutagenesis. Together, our findings demonstrate that efficient folding of proinsulin requires N-terminal sequences that are dispensable in the native state. Such arm-dependent folding can be abrogated by mutations associated with β-cell dysfunction and neonatal diabetes mellitus.

An Achilles' heel in an amyloidogenic protein and its repair: insulin fibrillation and therapeutic design

Insulin fibrillation provides a model for a broad class of amyloidogenic diseases. Conformational distortion of the native monomer leads to aggregation-coupled misfolding. Whereas beta-cells are protected from proteotoxicity by hexamer assembly, fibrillation limits the storage and use of insulin at elevated temperatures. Here, we have investigated conformational distortions of an engineered insulin monomer in relation to the structure of an insulin fibril. Anomalous (13)C NMR chemical shifts and rapid (15)N-detected (1)H-(2)H amide-proton exchange were observed in one of the three classical alpha-helices (residues A1-A8) of the hormone, suggesting a conformational equilibrium between locally folded and unfolded A-chain segments. Whereas hexamer assembly resolves these anomalies in accordance with its protective role, solid-state (13)C NMR studies suggest that the A-chain segment participates in a fibril-specific beta-sheet. Accordingly, we investigated whether helicogenic substitutions in the A1-A8 segment might delay fibrillation. Simultaneous substitution of three beta-branched residues (Ile(A2) --> Leu, Val(A3) --> Leu, and Thr(A8) --> His) yielded an analog with reduced thermodynamic stability but marked resistance to fibrillation. Whereas amide-proton exchange in the A1-A8 segment remained rapid, (13)Calpha chemical shifts exhibited a more helical pattern. This analog is essentially without activity, however, as Ile(A2) and Val(A3) define conserved receptor contacts. To obtain active analogs, substitutions were restricted to A8. These analogs exhibit high receptor-binding affinity; representative potency in a rodent model of diabetes mellitus was similar to wild-type insulin. Although (13)Calpha chemical shifts remain anomalous, significant protection from fibrillation is retained. Together, our studies define an "Achilles' heel" in a globular protein whose repair may enhance the stability of pharmaceutical formulations and broaden their therapeutic deployment in the developing world.

Solution structure of proinsulin: connecting domain flexibility and prohormone processing

The folding of proinsulin, the single-chain precursor of insulin, ensures native disulfide pairing in pancreatic beta-cells. Mutations that impair folding cause neonatal diabetes mellitus. Although the classical structure of insulin is well established, proinsulin is refractory to crystallization. Here, we employ heteronuclear NMR spectroscopy to characterize a monomeric analogue. Proinsulin contains a native-like insulin moiety (A- and B-domains); the tethered connecting (C) domain (as probed by {(1)H}-(15)N nuclear Overhauser enhancements) is progressively less ordered. Although the BC junction is flexible, residues near the CA junction exhibit alpha-helical-like features. Relative to canonical alpha-helices, however, segmental (13)C(alpha/beta) chemical shifts are attenuated, suggesting that this junction and contiguous A-chain residues are molten. We propose that flexibility at each C-domain junction facilitates prohormone processing. Studies of protease SPC3 (PC1/3) suggest that C-domain sequences contribute to cleavage site selection. The structure of proinsulin provides a foundation for studies of insulin biosynthesis and its impairment in monogenic forms of diabetes mellitus.

Insulin dimer dissociation and unfolding revealed by amide I two-dimensional infrared spectroscopy

A structurally sensitive probe of the monomer/dimer equilibrium of insulin was developed using 2DIR spectroscopy and interpreted using calculated spectra.

Contribution of residue B5 to the folding and function of insulin and IGF-I: constraints and fine-tuning in the evolution of a protein family

Proinsulin exhibits a single structure, whereas insulin-like growth factors refold as two disulfide isomers in equilibrium. Native insulin-related growth factor (IGF)-I has canonical cystines (A6-A11, A7-B7, and A20-B19) maintained by IGF-binding proteins; IGF-swap has alternative pairing (A7-A11, A6-B7, and A20-B19) and impaired activity. Studies of mini-domain models suggest that residue B5 (His in insulin and Thr in IGFs) governs the ambiguity or uniqueness of disulfide pairing. Residue B5, a site of mutation in proinsulin causing neonatal diabetes, is thus of broad biophysical interest. Here, we characterize reciprocal B5 substitutions in the two proteins. In insulin, His(B5) --> Thr markedly destabilizes the hormone (DeltaDeltaG(u) 2.0 +/- 0.2 kcal/mol), impairs chain combination, and blocks cellular secretion of proinsulin. The reciprocal IGF-I substitution Thr(B5) --> His (residue 4) specifies a unique structure with native (1)H NMR signature. Chemical shifts and nuclear Overhauser effects are similar to those of native IGF-I. Whereas wild-type IGF-I undergoes thiol-catalyzed disulfide exchange to yield IGF-swap, His(B5)-IGF-I retains canonical pairing. Chemical denaturation studies indicate that His(B5) does not significantly enhance thermodynamic stability (DeltaDeltaG(u) 0.2 +/- 0.2 kcal/mol), implying that the substitution favors canonical pairing by destabilizing competing folds. Whereas the activity of Thr(B5)-insulin is decreased 5-fold, His(B5)-IGF-I exhibits 2-fold increased affinity for the IGF receptor and augmented post-receptor signaling. We propose that conservation of Thr(B5) in IGF-I, rescued from structural ambiguity by IGF-binding proteins, reflects fine-tuning of signal transduction. In contrast, the conservation of His(B5) in insulin highlights its critical role in insulin biosynthesis.

Crystal structure of a "nonfoldable" insulin: impaired folding efficiency despite native activity

Protein evolution is constrained by folding efficiency ("foldability") and the implicit threat of toxic misfolding. A model is provided by proinsulin, whose misfolding is associated with beta-cell dysfunction and diabetes mellitus. An insulin analogue containing a subtle core substitution (Leu(A16) --> Val) is biologically active, and its crystal structure recapitulates that of the wild-type protein. As a seeming paradox, however, Val(A16) blocks both insulin chain combination and the in vitro refolding of proinsulin. Disulfide pairing in mammalian cell culture is likewise inefficient, leading to misfolding, endoplasmic reticular stress, and proteosome-mediated degradation. Val(A16) destabilizes the native state and so presumably perturbs a partial fold that directs initial disulfide pairing. Substitutions elsewhere in the core similarly destabilize the native state but, unlike Val(A16), preserve folding efficiency. We propose that Leu(A16) stabilizes nonlocal interactions between nascent alpha-helices in the A- and B-domains to facilitate initial pairing of Cys(A20) and Cys(B19), thus surmounting their wide separation in sequence. Although Val(A16) is likely to destabilize this proto-core, its structural effects are mitigated once folding is achieved. Classical studies of insulin chain combination in vitro have illuminated the impact of off-pathway reactions on the efficiency of native disulfide pairing. The capability of a polypeptide sequence to fold within the endoplasmic reticulum may likewise be influenced by kinetic or thermodynamic partitioning among on- and off-pathway disulfide intermediates. The properties of [Val(A16)]insulin and [Val(A16)]proinsulin demonstrate that essential contributions of conserved residues to folding may be inapparent once the native state is achieved.

Human insulin A-chain peptide analog(s) with in vitro biological activity

In a previous study, we showed that a synthetic human insulin 1-chain analog, named analog (3) was capable of mimicking in vitro effects of native insulin, including stimulation of cell proliferation, glucose uptake and glycogen synthesis. Here, we have synthesized three new analogs (6, 9, 12) of the human A-chain, bearing or not their N- or C-terminal residue, to determine the structural features which are responsible for their biological properties. In vitro experiments clearly demonstrated that the N-terminal part of the peptides is required for the biological activity of the molecules, suggesting its crucial role in the mechanism underlying the cellular effect. Our findings may help to better understand the mechanism of interaction between insulin and its receptor. In addition, the present data demonstrate that some mini-insulin derived from the A-chain can exert similar effects as native insulin. These small peptides may offer specific advantages over insulin in the definition of new strategies for diabetes treatment.

Proinsulin and the genetics of diabetes mellitus

Insulin plays a central role in the regulation of vertebrate metabolism. The hormone, the post-translational product of a single-chain precursor, is a globular protein containing two chains, A (21 residues) and B (30 residues). Recent advances in human genetics have identified dominant mutations in the insulin gene causing permanent neonatal-onset DM(2) (1-4). The mutations are predicted to block folding of the precursor in the ER of pancreatic beta-cells. Although expression of the wild-type allele would in other circumstances be sufficient to maintain homeostasis, studies of a corresponding mouse model (5-7) suggest that the misfolded variant perturbs wild-type biosynthesis (8, 9). Impaired beta-cell secretion is associated with ER stress, distorted organelle architecture, and cell death (10). These findings have renewed interest in insulin biosynthesis (11-13) and the structural basis of disulfide pairing (14-19). Protein evolution is constrained not only by structure and function but also by susceptibility to toxic misfolding.

Decoding the cryptic active conformation of a protein by synthetic photoscanning: insulin inserts a detachable arm between receptor domains

Proteins evolve in a fitness landscape encompassing a complex network of biological constraints. Because of the interrelation of folding, function, and regulation, the ground-state structure of a protein may be inactive. A model is provided by insulin, a vertebrate hormone central to the control of metabolism. Whereas native assembly mediates storage within pancreatic beta-cells, the active conformation of insulin and its mode of receptor binding remain elusive. Here, functional surfaces of insulin were probed by photocross-linking of an extensive set of azido derivatives constructed by chemical synthesis. Contacts are circumferential, suggesting that insulin is encaged within its receptor. Mapping of photoproducts to the hormone-binding domains of the insulin receptor demonstrated alternating contacts by the B-chain beta-strand (residues B24-B28). Whereas even-numbered probes (at positions B24 and B26) contact the N-terminal L1 domain of the alpha-subunit, odd-numbered probes (at positions B25 and B27) contact its C-terminal insert domain. This alternation corresponds to the canonical structure of abeta-strand (wherein successive residues project in opposite directions) and so suggests that the B-chain inserts between receptor domains. Detachment of a receptor-binding arm enables photo engagement of surfaces otherwise hidden in the free hormone. The arm and associated surfaces contain sites also required for nascent folding and self-assembly of storage hexamers. The marked compression of structural information within a short polypeptide sequence rationalizes the diversity of diabetes-associated mutations in the insulin gene. Our studies demonstrate that photoscanning mutagenesis can decode the active conformation of a protein and so illuminate cryptic constraints underlying its evolution.

Enhancing the activity of a protein by stereospecific unfolding: conformational life cycle of insulin and its evolutionary origins

A central tenet of molecular biology holds that the function of a protein is mediated by its structure. An inactive ground-state conformation may nonetheless be enjoined by the interplay of competing biological constraints. A model is provided by insulin, well characterized at atomic resolution by x-ray crystallography. Here, we demonstrate that the activity of the hormone is enhanced by stereospecific unfolding of a conserved structural element. A bifunctional beta-strand mediates both self-assembly (within beta-cell storage vesicles) and receptor binding (in the bloodstream). This strand is anchored by an invariant side chain (Phe(B24)); its substitution by Ala leads to an unstable but native-like analog of low activity. Substitution by d-Ala is equally destabilizing, and yet the protein diastereomer exhibits enhanced activity with segmental unfolding of the beta-strand. Corresponding photoactivable derivatives (containing l- or d-para-azido-Phe) cross-link to the insulin receptor with higher d-specific efficiency. Aberrant exposure of hydrophobic surfaces in the analogs is associated with accelerated fibrillation, a form of aggregation-coupled misfolding associated with cellular toxicity. Conservation of Phe(B24), enforced by its dual role in native self-assembly and induced fit, thus highlights the implicit role of misfolding as an evolutionary constraint. Whereas classical crystal structures of insulin depict its storage form, signaling requires engagement of a detachable arm at an extended receptor interface. Because this active conformation resembles an amyloidogenic intermediate, we envisage that induced fit and self-assembly represent complementary molecular adaptations to potential proteotoxicity. The cryptic threat of misfolding poses a universal constraint in the evolution of polypeptide sequences.

The structure and function of insulin: decoding the TR transition

Crystal structures of insulin are remarkable for a long-range reorganization among three families of hexamers (designated T(6), T(3)R(3)(f), and R(6)). Although these structures are well characterized at atomic resolution, the biological implications of the TR transition remain the subject of speculation. Recent studies indicate that such allostery reflects a structural switch between distinct folding-competent and active conformations. Stereospecific modulation of this switch by corresponding d- and l-amino-acid substitutions yields reciprocal effects on protein stability and receptor-binding activity. Naturally occurring human mutations at the site of conformational change impair the folding of proinsulin and cause permanent neonatal-onset diabetes mellitus. The repertoire of classical structures thus foreshadows the conformational lifecycle of insulin in vivo. By highlighting the richness of information provided by protein crystallography-even in a biological realm far removed from conditions of crystallization-these findings validate the prescient insights of the late D. C. Hodgkin. Future studies of the receptor-bound structure of insulin may enable design of novel agonists for the treatment of diabetes mellitus.

The structure of a mutant insulin uncouples receptor binding from protein allostery. An electrostatic block to the TR transition

The zinc insulin hexamer undergoes allosteric reorganization among three conformational states, designated T(6), T(3)R(3)(f), and R(6). Although the free monomer in solution (the active species) resembles the classical T-state, an R-like conformational change is proposed to occur upon receptor binding. Here, we distinguish between the conformational requirements of receptor binding and the crystallographic TR transition by design of an active variant refractory to such reorganization. Our strategy exploits the contrasting environments of His(B5) in wild-type structures: on the T(6) surface but within an intersubunit crevice in R-containing hexamers. The TR transition is associated with a marked reduction in His(B5) pK(a), in turn predicting that a positive charge at this site would destabilize the R-specific crevice. Remarkably, substitution of His(B5) (conserved among eutherian mammals) by Arg (occasionally observed among other vertebrates) blocks the TR transition, as probed in solution by optical spectroscopy. Similarly, crystallization of Arg(B5)-insulin in the presence of phenol (ordinarily a potent inducer of the TR transition) yields T(6) hexamers rather than R(6) as obtained in control studies of wild-type insulin. The variant structure, determined at a resolution of 1.3A, closely resembles the wild-type T(6) hexamer. Whereas Arg(B5) is exposed on the protein surface, its side chain participates in a solvent-stabilized network of contacts similar to those involving His(B5) in wild-type T-states. The substantial receptor-binding activity of Arg(B5)-insulin (40% relative to wild type) demonstrates that the function of an insulin monomer can be uncoupled from its allosteric reorganization within zinc-stabilized hexamers.

Design of an active ultrastable single-chain insulin analog: synthesis, structure, and therapeutic implications

Single-chain insulin (SCI) analogs provide insight into the inter-relation of hormone structure, function, and dynamics. Although compatible with wild-type structure, short connecting segments (<3 residues) prevent induced fit upon receptor binding and so are essentially without biological activity. Substantial but incomplete activity can be regained with increasing linker length. Here, we describe the design, structure, and function of a single-chain insulin analog (SCI-57) containing a 6-residue linker (GGGPRR). Native receptor-binding affinity (130 +/- 8% relative to the wild type) is achieved as hindrance by the linker is offset by favorable substitutions in the insulin moiety. The thermodynamic stability of SCI-57 is markedly increased (DeltaDeltaG(u) = 0.7 +/- 0.1 kcal/mol relative to the corresponding two-chain analog and 1.9 +/- 0.1 kcal/mol relative to wild-type insulin). Analysis of inter-residue nuclear Overhauser effects demonstrates that a native-like fold is maintained in solution. Surprisingly, the glycine-rich connecting segment folds against the insulin moiety: its central Pro contacts Val(A3) at the edge of the hydrophobic core, whereas the final Arg extends the A1-A8 alpha-helix. Comparison between SCI-57 and its parent two-chain analog reveals striking enhancement of multiple native-like nuclear Overhauser effects within the tethered protein. These contacts are consistent with wild-type crystal structures but are ordinarily attenuated in NMR spectra of two-chain analogs, presumably due to conformational fluctuations. Linker-specific damping of fluctuations provides evidence for the intrinsic flexibility of an insulin monomer. In addition to their biophysical interest, ultrastable SCIs may enhance the safety and efficacy of insulin replacement therapy in the developing world.

Insulin structure and function

Throughout much of the last century insulin served a central role in the advancement of peptide chemistry, pharmacology, cell signaling and structural biology. These discoveries have provided a steadily improved quantity and quality of life for those afflicted with diabetes. The collective work serves as a foundation for the development of insulin analogs and mimetics capable of providing more tailored therapy. Advancements in patient care have been paced by breakthroughs in core technologies, such as semisynthesis, high performance chromatography, rDNA-biosynthesis and formulation sciences. How the structural and conformational dynamics of this endocrine hormone elicit its biological response remains a vigorous area of study. Numerous insulin analogs have served to coordinate structural biology and biochemical signaling to provide a first level understanding of insulin action. The introduction of broad chemical diversity to the study of insulin has been limited by the inefficiency in total chemical synthesis, and the inherent limitations in rDNA-biosynthesis and semisynthetic approaches. The goals of continued investigation remain the delivery of insulin therapy where glycemic control is more precise and hypoglycemic liability is minimized. Additional objectives for medicinal chemists are the identification of superagonists and insulins more suitable for non-injectable delivery. The historical advancements in the synthesis of insulin analogs by multiple methods is reviewed with the specific structural elements of critical importance being highlighted. The functional refinement of this hormone as directed to improved patient care with insulin analogs of more precise pharmacology is reported.