Intrinsic fluorescence changes and rapid kinetics of proteinase deformation during serpin inhibition

Replacing the eleven native tryptophans by directed evolution produces an active P-glycoprotein with site-specific, non-conservative substitutions

P-glycoprotein (Pgp) pumps an array of hydrophobic compounds out of cells, and has major roles in drug pharmacokinetics and cancer multidrug resistance. Yet, polyspecific drug binding and ATP hydrolysis-driven drug export in Pgp are poorly understood. Fluorescence spectroscopy using tryptophans (Trp) inserted at strategic positions is an important tool to study ligand binding. In Pgp, this method will require removal of 11 endogenous Trps, including highly conserved Trps that may be important for function, protein-lipid interactions, and/or protein stability. Here, we developed a directed evolutionary approach to first replace all eight transmembrane Trps and select for transport-active mutants in Saccharomyces cerevisiae. Surprisingly, many Trp positions contained non-conservative substitutions that supported in vivo activity, and were preferred over aromatic amino acids. The most active construct, W(3Cyto), served for directed evolution of the three cytoplasmic Trps, where two positions revealed strong functional bias towards tyrosine. W(3Cyto) and Trp-less Pgp retained wild-type-like protein expression, localization and transport function, and purified proteins retained drug stimulation of ATP hydrolysis and drug binding affinities. The data indicate preferred Trp substitutions specific to the local context, often dictated by protein structural requirements and/or membrane lipid interactions, and these new insights will offer guidance for membrane protein engineering.

Inhibitory serpins. New insights into their folding, polymerization, regulation and clearance

Serpins are a widely distributed family of high molecular mass protein proteinase inhibitors that can inhibit both serine and cysteine proteinases by a remarkable mechanism-based kinetic trapping of an acyl or thioacyl enzyme intermediate that involves massive conformational transformation. The trapping is based on distortion of the proteinase in the complex, with energy derived from the unique metastability of the active serpin. Serpins are the favoured inhibitors for regulation of proteinases in complex proteolytic cascades, such as are involved in blood coagulation, fibrinolysis and complement activation, by virtue of the ability to modulate their specificity and reactivity. Given their prominence as inhibitors, much work has been carried out to understand not only the mechanism of inhibition, but how it is fine-tuned, both spatially and temporally. The metastability of the active state raises the question of how serpins fold, whereas the misfolding of some serpin variants that leads to polymerization and pathologies of liver disease, emphysema and dementia makes it clinically important to understand how such polymerization might occur. Finally, since binding of serpins and their proteinase complexes, particularly plasminogen activator inhibitor-1 (PAI-1), to the clearance and signalling receptor LRP1 (low density lipoprotein receptor-related protein 1), may affect pathways linked to cell migration, angiogenesis, and tumour progression, it is important to understand the nature and specificity of binding. The current state of understanding of these areas is addressed here.

Revisiting antithrombotic therapeutics; sculptin, a novel specific, competitive, reversible, scissile and tight binding inhibitor of thrombin

Thrombin is a multifunctional enzyme with a key role in the coagulation cascade. Its functional modulation can culminate into normal blood coagulation or thrombosis. Thus, the identification of novel potent inhibitors of thrombin are of immense importance. Sculptin is the first specific thrombin inhibitor identified in the transcriptomics analysis of tick’s salivary glands. It consists of 168 residues having four similar repeats and evolutionary diverged from hirudin. Sculptin is a competitive, specific and reversible inhibitor of thrombin with a K_i of 18.3 ± 1.9 pM (k_on 4.04 ± 0.03 × 10⁷ M⁻¹ s⁻¹ and k_off 0.65 ± 0.04 × 10⁻³ s⁻¹). It is slowly consumed by thrombin eventually losing its activity. Contrary, sculptin is hydrolyzed by factor Xa and each polypeptide fragment is able to inhibit thrombin independently. A single domain of sculptin alone retains ~45% of inhibitory activity, which could bind thrombin in a bivalent fashion. The formation of a small turn/helical-like structure by active site binding residues of sculptin might have made it a more potent thrombin inhibitor. In addition, sculptin prolongs global coagulation parameters. In conclusion, sculptin and its independent domain(s) have strong potential to become novel antithrombotic therapeutics.

Kinetic intermediates en route to the final serpin-protease complex: studies of complexes of α1-protease inhibitor with trypsin

Serpin protein protease inhibitors inactivate their target proteases through a unique mechanism in which a major serpin conformational change, resulting in a 70-Å translocation of the protease from its initial reactive center loop docking site to the opposite pole of the serpin, kinetically traps the acyl-intermediate complex. Although the initial Michaelis and final trapped acyl-intermediate complexes have been well characterized structurally, the intermediate stages involved in this remarkable transformation are not well understood. To better characterize such intermediate steps, we undertook rapid kinetic studies of the FRET and fluorescence perturbation changes of site-specific fluorophore-labeled derivatives of the serpin, α1-protease inhibitor (α1PI), which report the serpin and protease conformational changes involved in transforming the Michaelis complex to the trapped acyl-intermediate complex in reactions with trypsin. Two kinetically resolvable conformational changes were observed in the reactions, ascribable to (i) serpin reactive center loop insertion into sheet A with full protease translocation but incomplete protease distortion followed by, (ii) full conformational distortion and movement of the protease and coupled serpin conformational changes involving the F helix-sheet A interface. Kinetic studies of calcium effects on the labeled α1PI-trypsin reactions demonstrated both inactive and low activity states of the distorted protease in the final complex that were distinct from the intermediate distorted state. These studies provide new insights into the nature of the serpin and protease conformational changes involved in trapping the acyl-intermediate complex in serpin-protease reactions and support a previously proposed role for helix F in the trapping mechanism.

Structural change in β-sheet A of Z α(1)-antitrypsin is responsible for accelerated polymerization and disease

The presence of the Z mutation (Glu342Lys) is responsible for more than 95% of α(1)-antitrypsin (α(1)AT) deficiency cases. It leads to increased polymerization of the serpin α(1)AT during its synthesis and in circulation. It has been proposed that the Z mutation results in a conformational change within the folded state of antitrypsin that enhances its polymerization. In order to localize the conformational change, we have created two single tryptophan mutants of Z α(1)AT and analyzed their fluorescence properties. α(1)AT contains two tryptophan residues that are located in distinct regions of the molecule: Trp194 at the top of β-sheet A and Trp238 on β-sheet B. We have replaced each tryptophan residue individually with a phenylalanine in order to study the local environment of the remaining tryptophan residue in both M and Z α(1)AT. A detailed fluorescence spectroscopic analysis of each mutant was carried out, and we detected differences in the emission spectrum, the Stern-Volmer constant for potassium iodide quenching and the anisotropy of only Trp194 in Z α(1)AT compared to M α(1)AT. Our data reveal that the Z mutation results in a conformational change at the top of β-sheet A but does not affect the structural integrity of β-sheet B.

Broadening the cofactor specificity of a thermostable alcohol dehydrogenase using rational protein design introduces novel kinetic transient behavior

Cofactor specificity in the aldo-keto reductase (AKR) superfamily has been well studied, and several groups have reported the rational alteration of cofactor specificity in these enzymes. Although most efforts have focused on mesostable AKRs, several putative AKRs have recently been identified from hyperthermophiles. The few that have been characterized exhibit a strong preference for NAD(H) as a cofactor, in contrast to the NADP(H) preference of the mesophilic AKRs. Using the design rules elucidated from mesostable AKRs, we introduced two site-directed mutations in the cofactor binding pocket to investigate cofactor specificity in a thermostable AKR, AdhD, which is an alcohol dehydrogenase from Pyrococcus furiosus. The resulting double mutant exhibited significantly improved activity and broadened cofactor specificity as compared to the wild-type. Results of previous pre-steady-state kinetic experiments suggest that the high affinity of the mesostable AKRs for NADP(H) stems from a conformational change upon cofactor binding which is mediated by interactions between a canonical arginine and the 2'-phosphate of the cofactor. Pre-steady-state kinetics with AdhD and the new mutants show a rich conformational behavior that is independent of the canonical arginine or the 2'-phosphate. Additionally, experiments with the highly active double mutant using NADPH as a cofactor demonstrate an unprecedented transient behavior where the binding mechanism appears to be dependent on cofactor concentration. These results suggest that the structural features involved in cofactor specificity in the AKRs are conserved within the superfamily, but the dynamic interactions of the enzyme with cofactors are unexpectedly complex.

Computational methods for studying serpin conformational change and structural plasticity

Currently, over a hundred high-resolution structures of serpins are available, exhibiting a wide range of conformations. However, our understanding of serpin dynamics and conformational change is still limited, mainly due to challenges of monitoring structural changes and characterizing transient conformations using experimental methods. Insight can be provided, however, by employing theoretical and computational approaches. In this chapter, we present an overview of such methods, focusing on molecular dynamics and simulation. As serpin conformational dynamics span a wide range of timescales, we discuss the relative merits of each method and suggest which method is suited to specific conformational phenomena.

Determining serpin conformational distributions with single molecule fluorescence

Conformational plasticity is key to inhibitory serpin function, and this plasticity gives serpins relatively easy access to alternative, dysfunctional conformations. Thus, a given serpin population may contain both functional and dysfunctional proteins. Single molecule fluorescence (SMF), with its ability to interrogate one fluorescently labeled protein at a time, is a powerful method for elucidating conformational distributions and monitoring how these distributions change over time. SMF and related methods have been particularly valuable for characterizing serpin polymerization. Fluorescence correlation spectroscopy experiments have revealed a second lag phase during in vitro α(1)-antitrypsin polymerization associated with the formation of smaller oligomers that then condense to form longer polymers [Purkayastha, P., Klemke, J. W., Lavender, S., Oyola, R., Cooperman, B. S., and Gai, F. (2005). Alpha 1-antitrypsin polymerization: A fluorescence correlation spectroscopic study. Biochemistry44, 2642-2649.]. SMF studies of in vitro neuroserpin polymerization have confirmed that a monomeric intermediate is required for polymer formation while providing a test of proposed polymerization mechanisms [Chiou, A., Hägglöf, P., Orte, A., Chen, A. Y., Dunne, P. D., Belorgey, D., Karlsson-Li, S., Lomas, D., and Klenerman, D. (2009). Probing neuroserpin polymerization and interaction with amyloid-beta peptides using single molecule fluorescence. Biophys. J.97, 2306-2315.]. SMF has also been used to monitor protease-serpin interactions. Single pair Förster resonance energy transfer studies of covalent protease-serpin complexes suggest that the extent of protease structural disruption in the complex is protease dependent [Liu, L., Mushero, N., Hedstrom, L., and Gershenson, A. (2006). Conformational distributions of protease-serpin complexes: A partially translocated complex. Biochemistry45, 10865-10872.]. SMF techniques are still evolving and the combination of SMF with encapsulation methods has the potential to provide more detailed information on the conformational changes associated with serpin polymerization, protease-serpin complex formation, and serpin folding.

Serpin polymerization and its role in disease--the molecular basis of alpha1-antitrypsin deficiency

Protein aggregation is the cause of several human diseases. Understanding the molecular mechanisms involved in protein aggregation requires knowledge of the kinetics and structures populated during the reaction. Arguably, the best structurally characterized misfolding reaction is that of alpha(1)-antitrypsin. Alpha(1)-antitrypsin misfolding leads to both liver disease and emphysema and affect approximately 1 in 2000 of the population. This review will focus on the mechanism of alpha(1)-antitrypsin misfolding and the development of potential therapeutic strategies.

Short-lived protease serpin complexes: partial disruption of the rat trypsin active site

Serpins inhibit serine proteases by mechanically disrupting the protease active site. The protease first reacts with the serpin's reactive center loop (RCL) to form an acylenzyme. Then the RCL inserts into a beta-sheet in the body of the serpin, translocating the attached protease approximately 70 A and deforming the protease active site, thereby trapping the acylenzyme. Loop insertion (approximately 1 s(-1)) is an order of magnitude slower than hydrolysis of a typical substrate acylenzyme (approximately 50 s(-1)), indicating that the protease is inhibited during translocation. We have previously trapped a partially translocated covalent complex of rat trypsin and alpha1-proteinase inhibitor (EpartI*) resulting from attractive interactions between cationic dyes and anionic rat trypsin. Here, using single pair Förster resonance energy transfer, we demonstrate that EpartI* is a metastable complex that can dissociate to free protease and cleaved serpin (I*) as well as convert to the canonical fully translocated complex EfullI*. The partitioning between these two pathways is pH dependent, with conversion favored at low pH and dissociation favored at high pH. The short lifetime of EpartI* (approximately 3 h at pH 7.4) and the pH dependence of EpartI* dissociation suggest that, unlike in EfullI*, the catalytic triad is intact in EpartI*. These results also demonstrate that interactions between target proteases and the body of the serpin can hinder protease translocation leading to short-lived covalent complexes.

The loss of tryptophan 194 in antichymotrypsin lowers the kinetic barrier to misfolding

Antichymotrypsin, a member of the serpin superfamily, has been shown to form inactive polymers in vivo, leading to chronic obstructive pulmonary disease. At present, however, the molecular determinants underlying the polymerization transition are unclear. Within a serpin, the breach position is implicated in conformational change, as it is the first point of contact for the reactive center loop and the body of the molecule. W194, situated within the breach, represents one of the most highly conserved residues within the serpin architecture. Using a range of equilibrium and kinetic experiments, the contribution of W194 to proteinase inhibition, stability and polymerization was studied for antichymotrypsin. Replacement of W194 with phenylalanine resulted in a fully active inhibitor that was destabilized relative to the wild-type protein. The aggregation kinetics were significantly altered; wild-type antichymotrypsin exhibits a lag phase followed by chain elongation. The loss of W194 almost entirely removed the lag phase and accelerated the elongation phase. On the basis of our data, we propose that one of the main roles of W194 in antichymotrypsin is in preventing polymerization.

An overview of the serpin superfamily

Serpins are a broadly distributed family of protease inhibitors that use a conformational change to inhibit target enzymes. They are central in controlling many important proteolytic cascades, including the mammalian coagulation pathways. Serpins are conformationally labile and many of the disease-linked mutations of serpins result in misfolding or in pathogenic, inactive polymers.

The human serpin proteinase inhibitor-9 self-associates at physiological temperatures

The metastable serpin architecture is perturbed by extremes of temperature, pH, or changes in primary sequence resulting in the formation of inactive, polymeric conformations. Polymerization of a number of human serpins in vivo leads to diseases such as emphysema, thrombosis, and dementia, and in these cases mutations are present within the gene encoding the aggregating protein. Here we show that aggregation of the human serpin, proteinase inhibitor-9 (PI-9), occurs under physiological conditions, and forms aggregates that are morphologically distinct from previously characterized serpin polymers. Incubation of monomeric PI-9 at 37 degrees C leads to the rapid formation of aggregated PI-9. Using a variety of spectroscopic methods we analyzed the nature of the structures formed after incubation at 37 degrees C. Electron microscopy showed that PI-9 forms ordered circular and elongated-type aggregates, which also bind the fluorescent dye Thioflavin T. Our data show that in vitro wild-type PI-9 forms aggregates at physiological temperatures. The biological implications of PI-9 aggregates at physiological temperatures are discussed.

Identification of residual structure within denatured antichymotrypsin: implications for serpin folding and misfolding

The native serpin fold is metastable and possesses the inherent ability to convert into more stable, but inactive, conformations. In order to understand why serpins attain the native fold instead of other more thermodynamically favourable folds we have investigated the presence of residual structure within denatured antichymotrypsin (ACT). Through mutagenesis we created a single tryptophan variant of ACT in which a Trp residue (276) is situated on the H-helix, located within a region known as the B/C barrel. The presence of residual structure around Trp 276 in 5 M guanidine hydrochloride (GdnHCl) was shown by fluorescence and circular dichroism spectroscopy and fluorescence lifetime experiments. The residual structure was disrupted in the presence of 5 M guanidine thiocyanate (GdnSCN). Protein refolding studies showed that significant refolding could be achieved from the GdnHCl denatured state but not the GdnSCN denatured form. The implications of these data on the folding and misfolding of the serpin superfamily are discussed.

How do proteins avoid becoming too stable? Biophysical studies into metastable proteins

The vast majority of theoretical and experimental folding studies have shown that as a protein folds, it attempts to adopt a conformation that occurs at its lowest free energy minimum. However, studies on a small number of proteins have now shown that this is a generality. In this review we discuss recent data on how two proteins, alpha-lytic protease and alpha1-antitrypsin, successfully fold to their metastable native states, whilst avoiding more stable but inactive conformations.

Distortion of the catalytic domain of tissue-type plasminogen activator by plasminogen activator inhibitor-1 coincides with the formation of stable serpin-proteinase complexes

Plasminogen activator inhibitor-1 (PAI-1) is a typical member of the serpin family that kinetically traps its target proteinase as a covalent complex by distortion of the proteinase domain. Incorporation of the fluorescently silent 4-fluorotryptophan analog into PAI-1 permitted us to observe changes in the intrinsic tryptophan fluorescence of two-chain tissue-type plasminogen activator (tPA) and the proteinase domain of tPA during the inhibition reaction. We demonstrated three distinct conformational changes of the proteinase that occur during complex formation and distortion. A conformational change occurred during the initial formation of the non-covalent Michaelis complex followed by a large conformational change associated with the distortion of the proteinase catalytic domain that occurs concurrently with the formation of stable proteinase-inhibitor complexes. Following distortion, a very slow structural change occurs that may be involved in the stabilization or regulation of the trapped complex. Furthermore, by comparing the inhibition rates of two-chain tPA and the proteinase domain of tPA by PAI-1, we demonstrate that the accessory domains of tPA play a prominent role in the initial formation of the non-covalent Michaelis complex.

The 1.5 A crystal structure of a prokaryote serpin: controlling conformational change in a heated environment

Serpins utilize conformational change to inhibit target proteinases; the price paid for this conformational flexibility is that many undergo temperature-induced polymerization. Despite this thermolability, serpins are present in the genomes of thermophilic prokaryotes, and here we characterize the first such serpin, thermopin. Thermopin is a proteinase inhibitor and, in comparison with human alpha(1)-antitrypsin, possesses enhanced stability at 60 degrees C. The 1.5 A crystal structure reveals novel structural features in regions implicated in serpin folding and stability. Thermopin possesses a C-terminal "tail" that interacts with the top of the A beta sheet and plays an important role in the folding/unfolding of the molecule. These data provide evidence as to how this unusual serpin has adapted to fold and function in a heated environment.

Mapping of the epitope of a monoclonal antibody protecting plasminogen activator inhibitor-1 against inactivating agents

Plasminogen activator inhibitor-1 (PAI-1) belongs to the serpin family of serine proteinase inhibitors. Serpins inhibit their target proteinases by an ester bond being formed between the active site serine of the proteinase and the P1 residue of the reactive centre loop (RCL) of the serpin, followed by insertion of the RCL into beta-sheet A of the serpin. Concomitantly, there are conformational changes in the flexible joint region lateral to beta-sheet A. We have now, by site-directed mutagenesis, mapped the epitope for a monoclonal antibody, which protects the inhibitory activity of PAI-1 against inactivation by a variety of agents acting on beta-sheet A and the flexible joint region. Curiously, the epitope is localized in alpha-helix C and the loop connecting alpha-helix I and beta-strand 5A, on the side of PAI-1 opposite to beta-sheet A and distantly from the flexible joint region. By a combination of site-directed mutagenesis and antibody protection against an inactivating organochemical ligand, we were able to identify a residue involved in conferring the antibody-induced conformational change from the epitope to the rest of the molecule. We have thus provided evidence for communication between secondary structural elements not previously known to interact in serpins.

Conformational change in elastase following complexation with alpha1-proteinase inhibitor: a CD investigation

The CD spectrum of porcine pancreatic elastase in complex with alpha1-proteinase inhibitor (alpha1-PI) was calculated by subtracting the CD spectrum of the proteolytically cleaved inhibitor from that of the elastase-alpha1-PI complex. Elastase undergoes a moderate secondary structure change: its beta-structure is partially disordered while its alpha-helix content is poorly affected. In contrast, its tertiary structure undergoes a significant structural loosening upon complexation. These alterations have been compared with those following chemical and thermal unfolding of free elastase. Inhibitor-bound elastase and the denaturation intermediate of free elastase share secondary but not tertiary structural features. On the other hand, both free and complexed elastases undergo a single-step transition in tertiary structure upon thermal unfolding. These data are discussed in terms of the inhibition and structural modification of elastase induced by alpha1-PI observed by previous investigators.

In vitro and in silico design of alpha1-antitrypsin mutants with different conformational stabilities

Alpha(1)-antitrypsin, a protein belonging to the serine protease inhibitor (serpin) superfamily, is characterized by the ability to undergo dramatic conformational changes leading to inactive polymers. Serpin polymerization, which causes a range of diseases such as emphysema, thrombosis and dementia, occurs through a process in which the reactive center loop residues of one serpin molecule insert into the A beta-sheet of another. PoPMuSiC, a program that uses database-derived mean force potentials to predict changes in folding free energy resulting from single-site mutations, was used to modulate rationally the polymerization propensity of alpha(1)-antitrypsin. This was accomplished by generating mutants with a stabilized active form and destabilized polymerized form, or the converse. Of these mutants, five were expressed and characterized experimentally. In agreement with the predictions, three of them, K331F, K331I and K331V, were shown to stabilize the active form and decrease the polymerization rate, and one of them, S330R, to destabilize the active form and to increase polymerization. Only one mutant (K331T) did not display the expected behavior. Thus, strikingly, the adjacent positions 330 and 331, which are located at the beginning of the beta-strand next to the additionally inserted beta-strand in the polymerized form, have opposite effects on the conformational change. These residues therefore appear to play a key role in inducing or preventing such conformational change.

Acid Denaturation of alpha1-antitrypsin: characterization of a novel mechanism of serpin polymerization

The native serpin architecture is extremely sensitive to mutation and environmental factors. These factors induce the formation of a partially folded species that results in the production of inactive loop-sheet polymers. The deposition of these aggregates in tissue, results in diseases such as liver cirrhosis, thrombosis, angioedema and dementia. In this study, we characterize the kinetics and conformational changes of alpha(1)-antitrypsin polymerization at pH 4 using tryptophan fluorescence, circular dichroism, turbidity changes and thioflavin T binding. These biophysical techniques have demonstrated that polymerization begins with a reversible conformational change that results in partial loss of secondary structure and distortion at the top of beta-sheet A. This is followed by two bimolecular processes. First, protodimers are formed, which can be dissociated by changing the pH back to 8. Then, an irreversible conformational change occurs, resulting in the stabilization of the dimers with a concomitant increase in beta-sheet structure, allowing for subsequent polymer extension. Electron microscopy analysis of the polymers, coupled with the far-UV CD and thioflavin T properties of the pH 4 polymers suggest they do not form via the classical loop-beta-sheet A linkage. However, they more closely resemble those formed by the pathological variant M(malton). Taken together, these data describe a novel kinetic mechanism of serine proteinase inhibitor polymerization.

Comparative trajectories of active and S195A inactive trypsin upon binding to serpins

Serpins inhibit proteinases through a complicated multistep mechanism. The precise nature of these steps and the order by which they occur are still debated. We compared the fate of active and S195A inactive rat trypsin upon binding to alpha(1)-antitrypsin and P(1)-Arg-antichymotrypsin using stopped-flow kinetics with fluorescence resonance energy transfer detection and time-resolved fluorescence resonance energy transfer. We show that inhibition of active trypsin by these serpins leads to two irreversible complexes, one being compatible with the full insertion of the serpin-reactive site loop but not the other one. Binding of inactive trypsin to serpins triggers a large multistep reversible rearrangement leading to the migration of the proteinase to an intermediate position. Binding of inactive trypsin, unlike that of active trypsin, does not perturb the rhodamine fluorescence at position 150 on the helix F of the serpin. Thus, inactive proteinases do not migrate past helix F and do not trigger full serpin loop insertion.

The molecular basis for anti-proteolytic and non-proteolytic functions of plasminogen activator inhibitor type-1: roles of the reactive centre loop, the shutter region, the flexible joint region and the small serpin fragment

The serine proteinase inhibitor plasminogen activator inhibitor type-1 (PAI-1) is the primary physiological inhibitor of the tissue-type and the urokinase-type plasminogen activator (tPA and uPA, respectively) and as such an important regulator of proteolytic events taking place in the circulation and in the extracellular matrix. Moreover, a few non-proteolytic functions have been ascribed to PAI-1, mediated by its interaction with vitronectin or the interaction between the uPA-PAI-1 complex bound to the uPA receptor and members of the low density lipoprotein receptor family. PAI-1 belongs to the serpin family, characterised by an unusual conformational flexibility, which governs its molecular interactions. In this review we describe the anti-proteolytic and non-proteolytic functions of PAI-1 from both a biological and a biochemical point of view. We will relate the various biological roles of PAI-1 to its biochemistry in general and to the different conformations of PAI-1 in particular. We put emphasis on the intramolecular rearrangements of PAI-1 that are required for its antiproteolytic as well as its non-proteolytic functions.

The role of strand 1 of the C beta-sheet in the structure and function of alpha(1)-antitrypsin

Serpins inhibit cognate serine proteases involved in a number of important processes including blood coagulation and inflammation. Consequently, loss of serpin function or stability results in a number of disease states. Many of the naturally occurring mutations leading to disease are located within strand 1 of the C beta-sheet of the serpin. To ascertain the structural and functional importance of each residue in this strand, which constitutes the so-called distal hinge of the reactive center loop of the serpin, an alanine scanning study was carried out on recombinant alpha(1)-antitrypsin Pittsburgh mutant (P1 = Arg). Mutation of the P10' position had no effect on its inhibitory properties towards thrombin. Mutations to residues P7' and P9' caused these serpins to have an increased tendency to act as substrates rather than inhibitors, while mutations at P6' and P8' positions caused the serpin to behave almost entirely as a substrate. Mutations at the P6' and P8' residues of the C beta-sheet, which are buried in the hydrophobic core in the native structure, caused the serpin to become highly unstable and polymerize much more readily. Thus, P6' and P8' mutants of alpha(1)-antitrypsin had melting temperatures 14 degrees lower than wild-type alpha(1)-antitrypsin. These results indicate the importance of maintaining the anchoring of the distal hinge to both the inhibitory mechanism and stability of serpins, the inhibitory mechanism being particularly sensitive to any perturbations in this region. The results of this study allow more informed analysis of the effects of mutations found at these positions in disease-associated serpin variants.