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.

Progression of atherosclerosis is associated with variation in the alpha1-antitrypsin gene

OBJECTIVE

Alpha1-Antitrypsin (AAT) protects elastic tissue and may play a role in atherogenesis. The association of atherosclerosis progression with common AAT variants was considered in 2 clinical trials.

METHODS AND RESULTS

We examined the association of AAT V213A, S and Z deficiency alleles, and the functional 3' UTR 11478G>A with change in minimal luminal diameter, a measure of focal disease, in the Lopid Coronary Angiography Trial gemfibrozil study of post-bypass men. S or Z carriers (n=14) showed strong progression of disease on placebo (11.5%) but responded well to treatment (3% regression). 11478A carriers treated with placebo or gemfibrozil showed significantly more disease progression (n=8, -14.5% and n=16, -4.0%, respectively) than 11478GG men (n=107, -7.0% and n=108, -1.4%, respectively; overall, P=0.003). VV213 men treated with gemfibrozil (n=68) showed -4.8% progression, whereas A213 carriers (n=55) showed +1.4% regression of disease (P=0.001). No V213A effect was seen on placebo (P=0.11). In the Diabetes Atherosclerosis Intervention Study fenofibrate trial of angiographic progression in type 2 diabetes, the association of 11478A with increased disease progression was confirmed in the treatment group, but not the gemfibrozil-treated A213 association with regression, suggesting a pharmacogenetic difference.

CONCLUSIONS

Disease progression is associated with variation in AAT, and low AAT levels promote atherogenesis.

A review and comparison of the murine alpha1-antitrypsin and alpha1-antichymotrypsin multigene clusters with the human clade A serpins

The major human plasma protease inhibitors, alpha(1)-antitrypsin and alpha(1)-antichymotrypsin, are each encoded by a single gene, whereas in the mouse they are represented by clusters of 5 and 14 genes, respectively. Although there is a high degree of overall sequence similarity within these groupings, the reactive-center loop (RCL) domain, which determines target protease specificity, is markedly divergent. The literature dealing with members of these mouse serine protease inhibitor (serpin) clusters has been complicated by inconsistent nomenclature. Furthermore, some investigators, unaware of the complexity of the family, have failed to distinguish between closely related genes when measuring expression levels or functional activity. We have reviewed the literature dealing with the mouse equivalents of human alpha(1)-antitrypsin and alpha(1)-antichymotrypsin and made use of the recently completed mouse genome sequence to propose a systematic nomenclature. We have also examined the extended mouse clade "a" serpin cluster at chromosome 12F1 and compared it with the syntenic region at human chromosome 14q32. In summarizing the literature and suggesting a standardized nomenclature, we aim to provide a logical structure on which future research may be based.

EDEM as an acceptor of terminally misfolded glycoproteins released from calnexin

Terminally misfolded proteins in the endoplasmic reticulum (ER) are retrotranslocated to the cytoplasm and degraded by proteasomes through a mechanism known as ER-associated degradation (ERAD). EDEM, a postulated Man8B-binding protein, accelerates the degradation of misfolded proteins in the ER. Here, EDEM was shown to interact with calnexin, but not with calreticulin, through its transmembrane region. Both binding of substrates to calnexin and their release from calnexin were required for ERAD to occur. Overexpression of EDEM accelerated ERAD by promoting the release of terminally misfolded proteins from calnexin. Thus, EDEM appeared to function in the ERAD pathway by accepting substrates from calnexin.

The spontaneous polymerization of plasminogen activator inhibitor type-2 and Z-antitrypsin are due to different molecular aberrations

The wild-type form of plasminogen activator inhibitor type-2 (PAI-2) and the pathogenic Z-mutant of alpha(1)-antitrypsin (alpha(1)AT) are serpins that spontaneously polymerize by the loop-sheet mechanism. Compared to the consensus serpin sequence, both PAI-2 and Z-alpha(1)AT have deviations in the so-called breach region located at the top of the A beta-sheet. In the case of Z-alpha(1)AT, conformational perturbations caused by a single amino acid substitution result in polymerization in vivo and predisposes to disease. To test whether the polymerization of PAI-2 is due to aberrations in the breach region, we constructed substitution mutants of PAI-2 with conserved residues in this region. Analysis of the mutants revealed that deviations in the breach region modulate but are not the major cause of PAI-2 polymerization. Rather, PAI-2 exists in a highly polymerogenic conformation and does not require conformational rearrangements before polymerization can take place.

The underglycosylation of plasma alpha 1-antitrypsin in congenital disorders of glycosylation type I is not random

Conditions under which the glycosylation capacity of cells is limited provide an opportunity for studying the efficiency of site-specific glycosylation and the role of glycosylation in the maturation of glycoproteins. Congenital disorders of glycosylation type 1 (CDG-I) provide such a system. CDG-I is characterized by underglycosylation of glycoproteins due to defects in the assembly or transfer of the common dolichol-pyrophosphate-linked oligosaccharide precursor of asparagine-linked glycans. Human plasma alpha1-antitrypsin is normally fully glycosylated at three asparagine residues (46, 83, and 247), but un-, mono-, di-, and fully glycosylated forms of alpha1-antitrypsin were detected by 2D PAGE in the plasma from patients with CDG-I. The state of glycosylation of the three asparagine residues was analyzed in all the underglycosylated forms of alpha1-antitrypsin by peptide mass fingerprinting using matrix-assisted laser desorption ionization time-of-flight mass spectrometry. It was found that asparagine 46 was always glycosylated and that asparagine 83 was never glycosylated in the underglycosylated glycoforms of alpha1-antitrypsin. This showed that the asparagine residues are preferentially glycosylated in the order 46>247>83 in the mature underglycosylated forms of alpha1-antitrypsin found in plasma. It is concluded that the nonoccupancy of glycosylation sites is not random under conditions of decreased glycosylation capacity and that the efficiency of glycosylation site occupancy depends on structural features at each site. The implications of this observation for the intracellular transport and sorting of glycoproteins are discussed.

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.

Polyethylene glycol conjugation at Cys232 prolongs the half-life of alpha1 proteinase inhibitor

alpha1 Proteinase inhibitor (alpha1PI), a natural inhibitor of the serine proteinase leukocyte elastase, is also an intravenous therapeutic agent used to treat hereditary emphysema and may be useful in other respiratory disorders. However, to achieve sustained suppression of leukocyte elastase, alpha1PI must be given frequently and in large amounts, thus limiting its clinical use. We hypothesized that conjugating alpha1PI with polyethylene glycol (PEG) at Cys(232) could extend the in vivo half-life of alpha1PI in blood and lung. We present evidence that site-specific conjugation with either 20 or 40 kD PEG at Cys(232) of nonglycosylated recombinant human alpha1PI (rhalpha1PI) results in an active inhibitor with prolonged in vivo stability. In addition, 72 h after airway instillation PEG-rhalpha1PI was found to be significantly better than glycosylated alpha1PI in protecting the lung against leukocyte elastase-mediated lung hemorrhage. We conclude that thiol-specific PEGylation markedly improves the in vivo pharmacokinetic profile of rhalpha1PI and represents a simple, novel strategy to address the therapeutic goal of human leukocyte elastase inhibition.

Suppression of cholesterol 7alpha-hydroxylase transcription and bile acid synthesis by an alpha1-antitrypsin peptide via interaction with alpha1-fetoprotein transcription factor

alpha1-Antitrypsin (alpha1-AT) is a serum protease inhibitor that is synthesized mainly in the liver, and its rate of synthesis markedly increases in response to inflammation. This increase in alpha1-AT synthesis results in an increase in peptides, like its carboxyl-terminal C-36 peptide (C-36), resulting from alpha1-AT cleavage by proteases. Atherosclerosis is a form of chronic inflammation, and one of the risk factors is elevated plasma cholesterol levels. Because of the correlation between atherosclerosis, plasma cholesterol content, inflammation, and alpha1-AT rate of synthesis, we investigated the effect of the C-36 serpin peptide on hepatic bile acid biosynthesis. We discovered that C-36 is a powerful and specific transcriptional down-regulator of bile acid synthesis in primary rat hepatocytes, through inhibition of the cholesterol 7alpha-hydroxylase/CYP7A1 (7alpha-hydroxylase) promoter. Mice injected with the C-36 peptide also showed a decrease in 7alpha-hydroxylase mRNA. A mutated but very similar peptide did not have any effect on 7alpha-hydroxylase mRNA or its promoter. The sterol 12alpha-hydroxylase/CYP8B1 (12alpha-hydroxylase) promoter is also down-regulated by the C-36 peptide in HepG2 cells but not by the mutated peptide. The DNA element involved in the C-36-mediated regulation of 7alpha- and 12alpha-hydroxylase promoters mapped to the alpha1-fetoprotein transcription factor (FTF) site in both promoters. The C-36 peptide prevented binding of FTF to its target DNA recognition site by direct interaction with FTF. We hypothesize that the C-36 peptide specifically interacts with FTF and induces a conformational change that results in loss of its DNA binding ability, which results in suppression of 7alpha- and 12alpha-hydroxylase transcription. These results suggest that peptides derived from specific serum proteins may alter hepatic gene expression in a highly specific manner.

The murine alpha(1)-proteinase inhibitor gene family: polymorphism, chromosomal location, and structure

alpha(1)-Proteinase inhibitor (alpha(1)-PI) is a member of the serpin superfamily of serine proteinase inhibitors, which function in maintaining homeostasis through regulation of numerous proteolytic processes. In laboratory mice (Mus musculus domesticus), alpha(1)-PI occurs in multiple isoforms encoded by a family of three to five genes that are polymorphic among inbred strains and that are located at the Serpina1 locus on chromosome 12. In the present study, we have characterized the alpha(1)-PI gene family of inbred mice in more detail. We show that mice express seven isoforms, all of which are encoded by genes that map to the Serpina1 locus. In addition, polymorphism at the locus is defined by three haplotypes (Serpina1(b), Serpina1(c), and Serpina1(l)) that differ with regard to both the number and identity of alpha(1)-PI genes. Finally, we present the complete sequence of an 84-kb region of Serpina1 containing a tandem repeat of two alpha(1)-PI genes.

Interactions Causing the Kinetic Trap in Serpin Protein Folding

Conformational transition is fundamental to the mechanism of functional regulation in proteins, and serpins (serine protease inhibitors) can provide insight into this process. Serpins are metastable in their native forms, and they ordinarily undergo conformational transition to a stable state only when they form a tight complex with target proteases. The metastable native form is thus considered to be a kinetically trapped folding intermediate. We sought to understand the nature of the serpin kinetic trap as a step toward discovering how conformational transition is regulated. We found that mutations of the B/C beta-barrel of native alpha(1)-antitrypsin, a prototypical serpin, allowed conversion of the molecule into a more stable state. A 2.2 A resolution crystal structure of the stable form (PDB code, ) showed that the reactive site loop is inserted into an A beta-sheet, as in the latent plasminogen activator inhibitor-1. Mutational analyses suggest strongly that interactions not found in the final stable form cause the kinetic trap in serpin protein folding.

Glycoforms of six serum glycoproteins in a patient with congenital disorder of glycosylation type I

In this paper the occurrence and relative content of defectively glycosylated serum glycoforms in transferrin (Tf), alpha1-acid glycoprotein (AGP), haptoglobin (Hp), alpha1-antitrypsin (alpha1-AT), alpha2-macroglobulin (alpha2-MG) and ceruloplasmin (Cpl) in the serum of a patient with congenital disorder of glycosylation type I are reported. Blood samples were taken when the patient was 14 years old and then after a one-year interval. The patterns of glycoforms in both samples were compared. In 4 out of 6 examined glycoproteins, glycoforms lacking one and two oligosaccharide chains occurred. "Underglycosylated" glycoforms of alpha2-MG and Cpl were not clearly detectable. Tf was shown to be affected with this defect to a higher extent than other glycoproteins, containing only 30% properly glycosylated molecules and also as much as 30% of the molecules lacking two glycan units. In Hp and alpha1-AT the proportions of properly and defectively glycosylated forms were similar. This properly glycosylated form comprised 47% of the Hp and 51-55% of the alpha1-AT molecules. As in AGP and Tf, about 30% the of molecules lacked one glycan unit. Twenty-one percent of the Hp molecules were devoid of two glycans, and this amount slightly increased in the course of the year. In alpha1-AT, 19 and 17% of the molecules lacked two glycans in both samples, respectively. Only in AGP we did find a substantial difference between the two blood samples. In the course of the year, the amount of the form lacking 2-chains decreased from 12 to 3%, resulting in a simultaneous increase in the forms lacking one chain and the properly glycosylated. Our work also indicates, that applying a simple method of biochemical analysis such as SDS-PAGE/Western-blotting could be helpful in preliminary diagnosis and could improve the identification of congenital disorders of glycosylation.

A comparison between SDS-PAGE and size exclusion chromatography as analytical methods for determining product composition in protein conjugation reactions

Horseradish peroxidase (HRP) was conjugated with bovine serum albumin (BSA) or human alpha(1)-proteinase inhibitor (alpha(1)-PI). The enzyme was maleimidylated using N-succinimidyl 4-(N-maleimidomethyl)cyclohexane-1-carboxylate (SMCC) and then allowed to react with thiolated BSA or reduced alpha(1)-PI. The conjugation products were analysed both by SDS-PAGE and size exclusion chromatography (SEC) on Sephadex G200. The two methods of evaluating conjugative processes were compared with respect to information provided in relation to the behaviour of the products in solution. The results showed that neither SDS-PAGE nor SEC alone provides sufficient information about conjugate structure. The basic conjugate units observed in electrophoresis tend to form dimeric or higher-order aggregates under gel chromatographic conditions.

The reactivity and oxidation pathway of cysteine 232 in recombinant human alpha 1-antitrypsin

Oxidative damage to the sulfur-containing amino acids, methionine and cysteine, is a major concern in biotechnology and medicine. alpha1-Antitrypsin, which is a metastable and conformationally flexible protein that belongs to the serpin family of protease inhibitors, contains nine methionines and a single cysteine in its primary sequence. Although it is known that methionine oxidation in the protein active site results in a loss of biological activity, there is little specific knowledge regarding the reactivity of its unpaired thiol, Cys-232. In this study, the thiol-modifying reagent NBD-Cl (7-chloro-4-nitrobenz-2-oxa-1,3-diazole) was used to label peroxide-modified alpha1-antitrypsin and demonstrate that the Cys-232 in vitro oxidation pathway begins with a stable sulfenic acid intermediate and is followed by the formation of sulfinic and cysteic acid in successive steps. pH-dependent reactivity with hydrogen peroxide showed that Cys-232 has a pK(a) of 6.86 +/- 0.05, a value that is more than 1.5 pH units lower than that of a typical protein thiol. pH-induced conformational changes in the region surrounding Cys-232 were also examined and indicate that mildly acidic conditions induce a conformation that enhances Cys-232 reactivity. In summary, this work provides new insights into alpha1-antitrypsin reactivity in oxidizing environments and shows that a unique structural environment renders its unpaired thiol, Cys-232, its most reactive amino acid.

Familial conformational diseases and dementias

Familial conformational diseases occur when a mutation alters the conformation of a protein resulting in abnormal intermolecular interactions, protein aggregation, and consequent tissue damage. The molecular mechanisms of conformational disease are best understood for the serine protease inhibitor (serpin) superfamily of proteins. The serpinopathies include alpha(1)-antitrypsin (SERPINA1) deficiency and the newly characterized familial encephalopathy with neuroserpin inclusion bodies (FENIB) resulting from mutations in the neuroserpin (SERPINI1) gene. This review discusses how insights gained from the study of the serpins may be used to guide our research into other common diseases such as Alzheimer disease, Huntington disease, and Parkinson disease.

Uromodulin promoter directs high-level expression of biologically active human alpha1-antitrypsin into mouse urine

We have recently shown that the regulatory sequence of the uromodulin gene, containing the 3.7 kb promoter, exon 1 and a part of exon 2, provided for kidney-specific expression of the reporter lacZ gene in transgenic mice [Zbikowska, Soukhareva, Behnam, Chang, Drews, Lubon, Hammond and Soukharev (2002) Transgenic Res., in the press]. In the present study, we generated transgenic mice harbouring the regulatory sequence of the uromodulin gene to direct the expression of human alpha1-antitrypsin (alpha1AT) into urine. Of the 13 founder mice that tested positive by PCR, seven showed the presence of the human protein in their urine. The concentration of the recombinant human (rh) alpha1AT in the urine, estimated by using ELISA, ranged from 0.5 to 14 microg/ml in the F(0)-generation mice, and reached up to 65 microg/ml in the F1 generation. The transgenically produced rh alpha1AT was found to be N-glycosylated and biologically active. The N-terminal sequence analysis confirmed the identity of the human protein and revealed that the recombinant alpha1AT was correctly processed with the signal peptide cleaved off. Our results demonstrate for the first time that the uromodulin regulatory sequence provides a very attractive option for the potential large-scale production of functional therapeutic proteins in livestock.

Relationship between protein structure and methionine oxidation in recombinant human alpha 1-antitrypsin

alpha 1-Antitrypsin is a metastable and conformationally flexible protein that belongs to the serpin family of protease inhibitors. Although it is known that methionine oxidation in the protein's active site results in a loss of biological activity, there is little specific knowledge regarding the reactivity of each of the protein's methionine residues. In this study, we have used peptide mapping to study the oxidation kinetics of each of alpha 1-antitrypsin's methionines in alpha 1-AT((C232S)) as well as M351L and M358V mutants. These kinetic studies establish that Met1, Met226, Met242, Met351, and Met358 are reactive with hydrogen peroxide at neutral pH and that each reactive methionine is oxidized in a bimolecular, rather than coupled, mechanism. Analysis of Met226, Met351, and Met358 oxidation provides insights regarding the structure of alpha 1-antitrypsin's active site that allow us to relate conformation to experimentally observed reactivity. The relationship between solution pH and methionine oxidation was also examined to evaluate methionine reactivity under conditions that perturb the native structure. Methionine oxidation data show that at pH 5, global conformational changes occur that alter the oxidation susceptibility of each of alpha 1-antitrypsin's 10 methionine residues. Between pH 6 and 9, however, more localized conformational changes occur that affect primarily the reactivity of Met242. In sum, this work provides a detailed analysis of methionine oxidation in alpha 1-antitrypsin and offers new insights into the protein's solution structure.

Alterations in the mouse and human proteome caused by Huntington's disease

Huntington's disease is an autosomal dominantly inherited disease that usually starts in midlife and inevitably leads to death. In our effort to identify proteins involved in processes upstream or downstream of the disease-causing huntingtin, we studied the proteome of a well established mouse model by large gel two-dimensional electrophoresis. We could demonstrate for the first time at the protein level that alpha1-antitrypsin and alphaB-crystalline both decrease in expression over the course of disease. Importantly, the alpha1-antitrypsin decrease in the brain precedes that in liver and testes in mice. Reduced expression of the serine protease inhibitors alpha1-antitrypsin and contraspin was found in liver, heart, and testes close to terminal disease. Decreased expression of the chaperone alphaB-crystallin was found exclusively in the brain. In three brain regions obtained post-mortem from Huntington's disease patients, alpha1-antitrypsin expression was also altered. Reduced expression of the major urinary proteins not found in the brain was seen in the liver of affected mice, demonstrating that the disease exerts its influence outside the brain of transgenic mice at the protein level. Maintaining alpha1-antitrypsin and alphaB-crystallin availability during the course of Huntington's disease might prevent neuronal cell death and therefore could be useful in delaying the disease progression.

Functional diversification during evolution of the murine alpha(1)-proteinase inhibitor family: role of the hypervariable reactive center loop

Alpha(1)-proteinase inhibitor (alpha(1)-PI) is a member of the serpin superfamily of serine proteinase inhibitors that are involved in the regulation of a number of proteolytic processes. Alpha(1)-PI, like most serpins, functions by covalent binding to, and inhibition of, target proteinases. The interaction between alpha(1)-PI and its target is directed by the so-called reactive center loop (RCL), an approximately 20 residue domain that extends out from the body of the alpha(1)-PI polypeptide and determines the inhibitor's specificity. Mice express at least seven closely related alpha(1)-PI isoforms, encoded by a family of genes clustered at the Spi1 locus on chromosome 12. The amino acid sequence of the RCL region is hypervariable among alpha(1)-PIs, a phenomenon that has been attributed to high rates of evolution driven by positive Darwinian selection. This suggests that the various isoforms are functionally diverse. To test this notion, we have compared the proteinase specificities of individual alpha(1)-PIs from each of the two mouse species. As predicted from the positive Darwinian selection hypothesis, the various alpha(1)-PIs differ in their ability to form covalent complexes with serine proteinases, such as elastase, trypsin, chymotrypsin, and cathepsin G. In addition, they differ in their binding ability to proteinases in crude snake venoms. Importantly, the RCL region of the alpha(1)-PI polypeptide is the primary determinant of isoform-specific differences in proteinase recognition, indicating that hypervariability within this region drives the functional diversification of alpha(1)-PIs during evolution. The possible physiological benefits of alpha(1)-PI diversity are discussed.

Concerted regulation of inhibitory activity of alpha 1-antitrypsin by the native strain distributed throughout the molecule

The native forms of common globular proteins are in their most stable state but the native forms of plasma serpins (serine protease inhibitors) show high energy state interactions. The high energy state strain of alpha(1)-antitrypsin, a prototype serpin, is distributed throughout the whole molecule, but the strain that regulates the function directly appears to be localized in the region where the reactive site loop is inserted during complex formation with a target protease. To examine the functional role of the strain at other regions of alpha(1)-antitrypsin, we increased the stability of the molecule greatly via combining various stabilizing single amino acid substitutions that did not affect the activity individually. The results showed that a substantial increase of stability, over 13 kcal mol(-1), affected the inhibitory activity with a correlation of 11% activity loss per kcal mol(-1). Addition of an activity affecting single residue substitution in the loop insertion region to these very stable substitutions caused a further activity decrease. The results suggest that the native strain of alpha(1)-antitrypsin distributed throughout the molecule regulates the inhibitory function in a concerted manner.

Evidence that serpin architecture intrinsically supports papain-like cysteine protease inhibition: engineering alpha(1)-antitrypsin to inhibit cathepsin proteases

The closely related serpins squamous cell carcinoma antigen-1 and -2 (SCCA-1 and -2, respectively) are capable of inhibiting cysteine proteases of the papain superfamily. To ascertain whether the ability to inhibit cysteine proteases is an intrinsic property of serpins in general, the reactive center loop (RCL) of the archetypal serine protease inhibitor alpha(1)-antitrypsin was replaced with that of SCCA-1. It was found that this simple substitution could convert alpha(1)-antitrypsin into a cysteine protease inhibitor, albeit an inefficient one. The RCL of SCCA-1 is three residues longer than that of alpha(1)-antitrypsin, and therefore, the effect of loop length on the cysteine protease inhibitory activity was investigated. Mutants in which the RCL was shortened by one, two, or three residues were effective inhibitors with second-order rate constants of 10(5)-10(7) M(-)(1) s(-)(1). In addition to loop length, the identity of the cysteine protease was of considerable importance, since the chimeric molecules inhibited cathepsins L, V, and K efficiently, but not papain or cathepsin B. By testing complexes between an RCL-mimicking peptide and the mutants, it was found that the formation of a stable serpin-cysteine protease complex and the inhibition of a cysteine protease were both critically dependent on RCL insertion. The results strongly indicate that the serpin body is intrinsically capable of supporting cysteine protease inhibition, and that the complex with a papain-like cysteine protease would be expected to be analogous to that seen with serine proteases.