A model for the three-dimensional structure of glucagon

Mapping out the multistage fibrillation of glucagon

The 29-residue peptide hormone glucagon forms many different morphological types of amyloid-like fibrils, depending on solvent conditions. Here, we combine time-series far-UV CD with singular value decomposition analysis to reveal six different conformational states populated during fibrillation at 25 °C and pH 2.5. The existence of these states is supported by complementary fluorescence and electron microscopy data. This highlights a multitude of structural transitions of glucagon from unordered structure to β sheets, β turns and further tertiary-level changes. We attribute the observed unusual far-UV CD spectra to tertiary-level structural changes during the formation and maturation of fibrils. The fibrillation model for the whole process involves the formation of three oligomeric species and two different morphologies of fibrils in the same solution. The visualization of annular pore-like species in the early stages of glucagon fibrillation and the prevalence of such species in the amyloidogenesis of several proteins indicates that they may be a common feature of the fibrillation process. This study gives significant insights into the stepwise conversion of soluble glucagon to its fibrillar state and identifies the importance of fibril twisting for its thermodynamic stabilization.

Optimization of the native glucagon sequence for medicinal purposes

BACKGROUND

Glucagon is a life-saving medication used in the treatment of hypoglycemia. It possesses poor solubility in aqueous buffers at or near physiological pH values. At low and high pH, at which the peptide can be formulated to concentrations of a milligram or more per milliliter, the chemical integrity of the hormone is limited, as evidenced by the formation of multiple degradation-related peptides. Consequently, the commercial preparation is provided as a lyophilized solid with an acidic diluent and directions for rendering it soluble at the time of use. Any unused material is recommended for disposal immediately after initial use.

METHODS

A set of glucagon analogs was prepared by solid-phase peptide synthesis to explore the identification of a glucagon analog with enhanced solubility and chemical stability at physiological pH. The physical properties of the peptide analogs were studied by solubility determination, high-performance chromatography, and mass spectral analysis. The biochemical properties were determined in engineered human embryonic kidney cell line 293 (HEK293) cells that overexpressed either the human glucagon or glucagon-like peptide-1 (GLP-1) receptors linked to a luciferase reporter gene.

RESULTS

We observed the previously characterized formation of glucagon degradation products upon incubation of the peptide in dilute acid for extended periods or elevated temperature. Lowering the isoelectric point of the hormone through the substitution of asparagine-28 with aspartic acid significantly increased the solubility at physiological pH. Similarly, the C-terminal extension (Cex) of the hormone with an exendin-based, 10-residue, C-terminal sequence yielded a peptide of dramatically enhanced solubility. These two glucagon analogs, D28 and Cex, maintained high potency and selectivity for the glucagon receptor relative to GLP-1 receptor.

CONCLUSIONS

Glucagon presents unique structural challenges to the identification of an analog of high biological activity and selectivity that also possesses sufficient aqueous solubility and stability such that it might be developed as a ready-to-use medicine. The glucagon analogs D28 and Cex demonstrated all of the chemical, physical, and biochemical properties supportive of further study as potential clinical candidates for treatment of hypoglycemia.

The changing face of glucagon fibrillation: structural polymorphism and conformational imprinting

We have established a time-resolved fluorescence assay to study fibrillation of the 29 residue peptide hormone glucagon under a variety of different conditions in a high-throughput format. Fibrils formed at pH 2.5 differ in fibrillation kinetics, morphology, thioflavin T staining and FTIR/CD spectra depending on salts, glucagon concentration and fibrillation temperature. Apparent fibrillar stability correlates with spectral and kinetic properties; generally, fibrils formed under conditions favourable for rapid fibrillation (ambient temperatures, high glucagon concentration or high salt concentration) appear less thermostable than those formed under more challenging conditions (high temperatures, low glucagon or low salt concentrations). Properties of preformed fibrils used for seeding are inherited in a prion-like manner. Thus, we conclude that the structure of fibrils formed by glucagon is not the result of the global energy minimization, but rather kinetically controlled by solvent conditions and seed-imprinting. Fibrillar polymorphism, which is being reported for an increasing number of proteins, probably reflects that fibrils have not been under evolutionary constraints to retain a single active conformation. Our results highlight the complexity of the fibrillation mechanism of glucagon, since even subtle changes in fibrillation conditions can alter the type of fibrils formed, or result in formation of mixtures of several types of fibrils.

The relative rates of glutamine and asparagine deamidation in glucagon fragment 22-29 under acidic conditions

Our objective was to compare the relative rates of asparaginyl and glutaminyl deamidation in fragment 22-29 of the polypeptide hormone glucagon in acidic aqueous solutions. Reaction mixtures containing 22-29 (FVQWLMNT) or its degradation products were degraded at 60 degrees C in dilute hydrochloric acid or phosphate buffer in the pH range 1-3. Degradation products were separated by high-performance liquid chromatography and identified by amino acid sequencing, amino acid analysis, liquid chromatography-mass spectrometry (LC-MS), and matrix-assisted laser desorption and ionization (MALDI). Nine major degradation products were identified, including asparaginyl and glutaminyl deamidated forms, aspartyl peptide cleavage of the asparaginyl deamidated products, and a cyclic imide intermediate. The pH dependences of rate constants for individual pathways were consistent with acid catalysis. Previous investigators have reported a greater susceptibility of asparagine residues to deamidation in neutral and alkaline solutions due to the formation of a more stable five-membered succinimide intermediate. It has been suggested that asparagine may be more labile under acidic conditions also. We have observed a more facile deamidation for the glutamine residue under the acidic condition studied. It is proposed that the lower reactivity of the asparagine residue may be due to a decreased electrophilicity of its side chain carbonyl carbon imparted by a parallel cleavage pathway at this residue.

Development of potent glucagon antagonists: structure-activity relationship study of glycine at position 4

We examined the functional role of glycine at position 4 in the potent glucagon antagonist [desHis(1), Glu(9)]glucagon amide, by substituting the L- and D-enantiomers of alanine and leucine for Gly(4) in this antagonist. The methyl and isobutyl side-chain substituents were introduced to evaluate the preference shown by the glucagon receptor, if any, for the orientation of the N-terminal residues. The L-amino acids demonstrated only slightly better receptor recognition than the D-enantiomers. These results suggest that the Gly(4) residue in glucagon antagonists may be exposed to the outside of the receptor. The enhanced binding affinities of analogs 1 and 3 compared with the parent antagonist, [desHis(1), Glu(9)]glucagon amide, may have resulted from the strengthened hydrophobic patch in the N-terminal region and/or the increased propensity for a helical conformation due to the replacement of alanine and leucine for glycine. Thus, as a result of the increased receptor binding affinities, antagonist activities of analogs 1-4 were increased 10-fold compared with the parent antagonist, [desHis(1), Glu(9)]glucagon amide. These potent glucagon antagonists have among the highest pA(2) values of any glucagon analogs reported to date.

Comparison between the isocratic and gradient retention behaviour of polypeptides in reversed-phase liquid chromatographic environments

The isocratic and gradient elution behaviour of beta-endorphin and glucagon, two polypeptides known to exist in amphipathic alpha-helical conformations in lipophilic environments, have been examined under reversed-phase high-performance liquid chromatographic (RP-HPLC) conditions with low pH, aquo-acetonitrile mobile phases. The effects of changes in the volume fraction, psi, of the organic solvent modifier and temperature, T, on the magnitudes of the S and log k(o) values of these two polypeptides, obtained from the plots of logarithmic capacity factor (log k') vs. psi using isocratic elution conditions have been determined. These data have then been compared to the corresponding S and log k(o) values, obtained from the plots of logarithmic median capacity factor (log k) versus the median volume fraction of the organic solvent modifier (psi) derived from the linear gradient elution data, using the same n-butyl silica sorbent and related aquo-acetonitrile mobile phase conditions. As apparent from these studies, substantial differences occur in the temperature-dependent trends and magnitudes of the corresponding S and S values, or the log k(o) and log k(o) values, when these parameters are derived from experimental data acquired by these two different elution methods. Moreover, when gradient elution data for beta-endorphin and glucagon are utilised, the extrapolated values of the intercept and slope of the plots of log k vs. 1/T (corresponding to an apparent change in the median enthalpy of association, deltaH(o)assoc, or an apparent change in the median entropy of association, deltaS(o)assoc) substantially deviated from the values obtained for the thermodynamic parameters, deltaH(o)assoc and deltaS(o)assoc, derived from the log k' vs. 1/T plots using the corresponding isocratic data. These findings thus have important implications for biophysical and thermodynamic investigations when gradient elution data are employed to assess the molecular basis of the interaction of polypeptides with non-polar ligates.

Pure glucagon antagonists: biological activities and cAMP accumulation using phosphodiesterase inhibitors

Five new glucagon analogues have been designed, synthesized, characterized and their biological activities tested. The investigation was centered on modifications in the N-terminal region in particular, residues at Thr5, Phe6 and Tyr10 positions, with the goal of obtaining pure glucagon antagonists in our newly developed high sensitivity cAMP accumulation assay. The structures of the designed compounds are: [des-His1, des-Phe6, Glu9] glucagon-NH2 (1); [des-His1, des-Phe6, Glu9, Phe10]glucagon-NH2 (2); [des-His1, Tyr5, des-Phe6, Glu9]glucagon-NH2 (3); [des-His1, Phe5, des-Phe6, Glu9]glucagon-NH2 (4) and [des-His1, des-Phe6, Glu9, D-Arg18]glucagon-NH2 (5). The binding potencies IC50 values in (nM) were 48.0, 27.4, 26.0, 20.0 and 416.0, respectively. All of these analogues when tested in the classical adenylate cyclase assay demonstrate antagonist properties, and in competition experiments, all caused a rightward-shift of the glucagon stimulated adenylate cyclase dose-response curve. The pA2 values for these analogues were 8.20 (1); 6.25 (2); 6.10 (3); 6.25 (4); and 6.08 (5), respectively. A newly revised assay has been developed to determine the intracellular cAMP accumulation levels in hepatocytes at the highest possible sensitivity. Four of the five glucagon analogues in this report (analogues 1, 2, 4 and 5), did not activate the adenylate cyclase in the presence of Rolipram up to a maximal physiological concentration of 1 microM, and thus are pure antagonists.

Macromolecular bioactivity: is it resonant interaction between macromolecules?--Theory and applications

Biological processes in any living organism are based on selective interactions between particular biomolecules. In most cases, these interactions involve and are driven by proteins which are the main conductors of any living process within the organism. The physical nature of these interactions is still not well known. This paper represents a whole new view to biomolecular interactions, in particular protein-protein and protein-DNA interactions, based on the assumption that these interactions are electromagnetic in their nature. This new approach is incorporated in the Resonant Recognition Model (RRM), which was developed over the last 10 years. It has been shown initially that certain periodicities within the distribution of energies of delocalized electrons along a protein molecule are critical for protein biological function, i.e., interaction with its target. If protein conductivity was introduced, then a charge moving through protein backbone can produce electromagnetic irradiation or absorption with spectral characteristics corresponding to energy distribution along the protein. The RRM enables these spectral characteristics, which were found to be in the range of infrared and visible light, to be calculated. These theoretically calculated spectra were proved using experimentally obtained frequency characteristics of some light-induced biological processes. Furthermore, completely new peptides with desired spectral characteristics, and consequently corresponding biological activities, were designed.

Roles of aspartic acid 15 and 21 in glucagon action: receptor anchor and surrogates for aspartic acid 9

The discovery of aspartic acid at position 9 in glucagon to be a critical residue for transduction has spurred renewed efforts to identify other strategic residues in the peptide sequence that dictate either receptor binding or biological activity. It also became apparent from further studies that Asp9 operates in conjunction with His1 in the activation mechanism that follows binding to the glucagon receptor. Indeed, it was later demonstrated that the protonatable histidine imidazole is important for transduction. It is likely that the interaction of a positively charged histidine 1 with a negatively charged aspartic acid 9 might be part of the triggering step at the molecular level. Two other aspartic acid residues in glucagon are capable of assuming a similar role, namely that of contributing to an electrostatic attraction with histidine via a negative carboxylate. These studies were conducted to investigate the role of aspartic acid 15 and 21 in glucagon action. Evidence reported here, gathered from 31 replacement analogs, supports the idea that in the absence of the requisite carboxyl group at position 9, histidine utilizes Asp21 or Asp15 as a compensatory site. Asp15 was also found to be indispensable for binding and may serve to tether the hormone to the receptor protein at the binding site. It is also demonstrated that these new findings promote the design of better glucagon antagonists.

New glucagon analogues with conformational restrictions and altered amphiphilicity: effects on binding, adenylate cyclase and glycogenolytic activities

In an effort to obtain highly potent glucagon antagonists, we have investigated glucagon (1) structure-function relationships utilizing the following design principles: (1) structural changes known to lead to partial agonist activities; (2) conformational restrictions; (3) changes in the conformational probabilities of the primary sequence; and (4) increased amphiphilicity. In this report we present the total synthesis, purification, receptor binding, adenylate cyclase activity, in vivo glycogenolytic activity and CD spectrum of the following four glucagon analogues: [Ahx17,18]glucagon (2), [D-Phe4,Tyr5, 3,5-diiodo-Tyr10,Arg12,Lys17,18,Glu21]glucagon (3), [Asp9,Lys12,Lys17,18,Glu21]glucagon 4, and [Glu15,Lys17,18]glucagon 5. Compound 2 binds exclusively to the high affinity receptor and compound 3 was a highly potent antagonist with respect to adenylate cyclase activity. Analog 4 showed distinct biphasic binding (IC50 5.6 nM and 630 nM), with only the low affinity binding leading to adenylate cyclase activity. Furthermore in analogue 5 receptor binding and adenylate cyclase activity were dissociated by a factor of 5. The results are consistent with a multistep binding mechanism in which glucagon interacts first nonspecifically with the anisotropic interphase of the cell membrane, followed by a conformational transition which occurs in the sequences 10-14 and 15-18 when the membrane bound peptide binds to its receptor.

Identification of the guanine binding domain peptide of the GTP-binding site of glucagon

Glucagon, a peptide hormone synthesized and secreted by alpha islet cells, regulates glucose homeostasis by several mechanisms. Using [gamma 32P]8N3GTP, a proven photoaffinity probe for GTP, a specific nucleotide binding site on human glucagon was detected that showed preference for GTP. Half-maximal saturation of photoinsertion into the polypeptide hormone was at 8-12 microM with either [alpha 32P]8N3GTP or [gamma 32P]8N3GTP. GTP protected photolabeling with an apparent kd of 15 microM, whereas ATP was less effective as a protector, exhibiting an apparent kd of about 30 microM. Maximal protection by GTP and ATP was over 90%. UTP, CTP, GDP, ADP, GMP, AMP, guanosine, adenosine, guanine, and adenine were much less effective protectors, indicating that binding is specific for purine nucleoside triphosphates, particularly GTP. Mg2+ at 150 microM enhanced photoinsertion (twofold), whereas at 2-10 mM, it inhibited photoinsertion. Both Ca2+ and Zn2+ at 0.2 mM decreased photoinsertion about 45%. Purification of chymotryptic and tryptic digests of photolabeled glucagon by reverse-phase high performance liquid chromatography (HPLC) revealed that the N-terminal peptide, HSQGTF, was the only peptide region covalently photomodified by [32P]8N3GTP. GTP, if present during photolysis, greatly reduced both photoinsertion into glucagon and the amount of radiolabeled peptide recovered on HPLC. The specificity of binding to the N-terminal region is suggestive of a physiological role for a glucagon-GTP complex in the mechanism of action of this hormone.

High-performance liquid chromatography of amino acids, peptides and proteins. CXV. Thermodynamic behaviour of peptides in reversed-phase chromatography

The thermodynamic behaviour of three peptides, bombesin, beta-endorphin and glucagon, was studied under reversed-phase high-performance liquid chromatographic conditions. Experimental data related to the interactive surface contact area (S values) and solute affinity (log k0) were derived over a range of temperatures between 5 and 85 degrees C. These experimental conditions allowed changes in the secondary structure of the solute to be monitored. The influence of the nature of the stationary phase ligand on the relative conformational stability of the three peptides was analysed by acquiring data with n-octadecyl silica (C18) and n-butyl silica (C4) sorbents. Values for the relative changes in entropy and enthalpy associated with the interactive process were also determined. The results provide further insight into the factors involved with the stabilization of secondary structure and the mechanism of the interaction of peptides with hydrophobic surfaces.

Investigation of the membrane-active peptides melittin and glucagon by photochemically induced dynamic-nuclear-polarization (photo-CIDNP) NMR

The photochemically induced dynamic-nuclear-polarization (photo-CIDNP) NMR technique was used to investigate the membrane-active peptides melittin and glucagon. The experiments were performed both in the absence and presence of phospholipid vesicles in order to study the topography of the membrane-bound state. From the results it can be concluded that the melittin peptide chain is oriented in such a way that the single tryptophan residue (Trp19) reaches into the membrane. In the case of glucagon, a binding interaction with vesicle membranes is indicated within the pH range 2-10, whereby the single tryptophan residue (Trp25) is buried in the lipid bilayer and the tyrosine and histidine residues are exposed to the aqueous solvent.

Chemical properties of the histidine residue of secretin: evidence for a specific intramolecular interaction

Secretin has a single histidine residue located at the amino terminus which plays a crucial role in its biological activity. The chemical properties, viz. pK and reactivity, of the alpha-amino and imidazole groups of this residue were determined at a secretin concentration of 10(-6) M in 0.1 M KCl at 37 degrees C. Competitive labelling using tritiated 1-fluoro-2,4-dinitrobenzene (DNP-F) as the labelling reagent was the experimental approach employed. The alpha-amino group was found to have a pK value of 8.83 and a reactivity 5-times that of the alpha-amino group in the model compound, histidylglycine. For the imidazole function a pK value of 8.24 and a reactivity 26-times that of the imidazole function in histidylglycine was found. Both these groups in secretin had pK values which were shifted one pK unit higher than in histidylglycine, but like the model compound the reactivity of the imidazole function was still linked to the state of ionization of the alpha-amino group. These observations are interpreted as evidence for the existence of a major conformational state in dilute aqueous solution in which the amino-terminal histidine of secretion is interacting with a negatively charged carboxyl group.

Importance of the 10-13 region of glucagon for its receptor interactions and activation of adenylate cyclase

The role of the Tyr10-Ser11-Lys12-Tyr13 region of glucagon in the binding interaction and activation of the glucagon receptor was investigated by means of the synthetic glucagon analogues [Phe13]glucagonamide, [Phe10]glucagonamide, [Phe10]glucagon, [Phe10,13]glucagon, [Pro11]glucagon, [Pro11,Gly12]glucagonamide, [Ala11]glucagon, and [Oac11-13]glucagonamide. These analogues were synthesized by solid-phase peptide synthesis on p-methylbenzhydrylamine or Merrifield resins with protected N alpha-tert-butyloxycarbonyl amino acids. Purification by dialysis, cation-exchange chromatography, gel filtration, and preparative reverse-phase high-performance liquid chromatography (HPLC) gave products that proved homogeneous by thin-layer chromatography and HPLC and on analysis by amino acid analysis, by sequencing, and by alpha-chymotryptic peptide mapping with HPLC. Biological activities were examined by measurement of the stimulation of liver plasma membrane adenylate cyclase and by specific displacement of [125I]glucagon from glucagon receptors. The results of these studies indicate that while the biological "message" region of glucagon is located elsewhere, the 10-13 region has multiple roles in the glucagon-glucagon receptor interaction: this region provides functional groups for direct binding interaction with the receptor, and this region interacts with the receptor in such a way as to allow the "transduction message" portion of glucagon to interact and activate the receptor.

The receptors of the VIP family peptides (VIP, secretin, GRF, PHI, PHM, GIP, glucagon and oxyntomodulin). Specificities and identity

A model is proposed for the receptors of the VIP family peptides including a ligand and a cellular domain. Specificities of the receptors are due to different ligand binding sites. Three subgroups of the family can be distinguished accordingly: glucagon and oxyntomodulin; GIP; VIP, secretin r and hGRF, PHI and PHM. In the same species, the expression of these different sites is cell-specific resulting in a stoichiometry of the ligand-receptor interaction which is compatible with physiological regulation of cell function. Specificities of the interaction as studied by native and synthetic analogs is supported both by restricted sequences of amino acids (such as that including the N-terminal histidine residue), and membrane-induced configuration of the ligand. Identity of the receptors is related to their interactions with subunits of the adenylate cyclase system. Arguments are put forward indicating that the alpha subunit of the guanyl regulatory protein is a reasonable candidate for directly transducing to the adenylyl cyclase the information contained in the activated ligand-binding site subunits. Evidence of functional and molecular heterogeneity of the recognizing site and of the alpha subunits leads to the supposition that some types of specific complementarity is retained at this level of interaction, further enhancing the possibility of species and cell differences. On the other hand, the identities found in other sequences of the alpha and ras oncogene products extend to the receptor of the VIP family peptides a pattern of organization which is similar to that recently described for the insulin family of receptors. The role of ligand specific receptor mediated regulation in homologous or heterologous desensitization is reviewed in brief for the peptides of the VIP family as well as the appearance of the specific receptor during the ontogenesis or the cell differentiation. The co-distribution of plasma membrane receptors from other families further adds to the cell specificity resulting for each differentiated cell in unique patterns of recognizing site. Some examples of receptor-receptor interaction are given, indicating that the integration of the different signals by cells might occur at an early step through the transmembranair domain of the receptor.

Structure-function relationships of S-carboxymethyl methionine27 glucagon

Carboxymethylation of glucagon and subsequent purification of the hormone has provided a derivative modified by the addition of bulk to the methionine at position 27 without a net charge alteration in the side chain. Unreacted glucagon was removed after methylation of the methionine which provides a positively charged chromatographic handle. The derivative has a half-maximum concentration for binding of 5.3 nM and is a full agonist. These findings along with those provided by methylation of the methionine indicate that a positive charge rather than bulk on the methionine side chain disrupts the binding of hormone to its receptor. The S-carboxymethyl derivative lacks the concentration-dependent aggregation characteristic of glucagon at pH 10.2 as does the S-methyl derivative but increases its helical content in 30% 2-chloroethanol to the same extent as native and S-methyl hormone. Full activity of the S-carboxymethyl methionine27 glucagon does not favor the existence of the globular structure proposed by Korn and Ottensmeyer [(1983) J. Theor. Biol. 105, 403] as the binding species whereas multiple considerations do favor a flexible hormone with nucleation followed by conformational changes for complete binding and activation. Isotopic enrichment using labeled iodoacetate is feasible and can provide more definitive structural information.

Unusual chemical properties of N-terminal histidine residues of glucagon and vasoactive intestinal peptide

An N-terminal histidine residue of a protein or peptide has two functional groups, viz., an alpha-amino group and an imidazole group. A new procedure, based on the competitive labeling approach described by Duggleby and Kaplan [Duggleby, R. G., & Kaplan, H. (1975) Biochemistry 14, 5168-5175], has been developed by which the chemical reactivity of each functional group in such a residue can be determined as a function of pH. Only very small amounts of material are required, which makes it possible to determine the chemical properties in dilute solution or in proteins and polypeptides that can be obtained in only minute quantities. With this approach, the reactivity of the alpha-amino group of histidylglycine toward 1-fluoro-2,4-dinitrobenzene gave an apparent pKa value of 7.64 +/- 0.07 at 37 degrees C, in good agreement with a value of 7.69 +/- 0.02 obtained by acid-base titration. However, the reactivity of the imidazole function gave an apparent pKa value of 7.16 +/- 0.07 as compared to the pKa value of 5.85 +/- 0.01 obtained by acid-base titration. Similarly, in glucagon and vasoactive intestinal peptide (VIP), apparent pKa values of 7.60 +/- 0.04 and 7.88 +/- 0.18, respectively, were obtained for the alpha-amino of their N-terminal histidine, and pKa values of 7.43 +/- 0.09 and 7.59 +/- 0.18 were obtained for the imidazole function.(ABSTRACT TRUNCATED AT 250 WORDS)

Nucleosome reconstruction via phosphorus mapping

Electron spectroscopic imaging was combined with reconstruction algorithms to derive the three-dimensional structure of the nucleosome core particle to a resolution of 1.5 nanometers. Images of phosphorus distributions within individual nucleosomes were interpreted as projections of a supercoil of DNA. These were used to orient the corresponding individual nucleosome images, making it possible to reconstruct the entire nucleosome in three dimensions. The structure is consistent with known biochemical and biophysical data and explains site-specific nuclease sensitivity, although differing in part with other nucleosome models.