Structural evolution of an enzyme specificity. The structure of rat carboxypeptidase A2 at 1.9-A resolution

High-resolution structure of the M14-type cytosolic carboxypeptidase from Burkholderia cenocepacia refined exploiting PDB_REDO strategies

A potential cytosolic metallocarboxypeptidase from Burkholderia cenocepacia has been crystallized and a synchrotron-radiation microfocus beamline allowed the acquisition of diffraction data to 1.9 Å resolution. The asymmetric unit comprises a tetramer containing over 1500 amino acids, and the high-throughput automated protocols embedded in PDB_REDO were coupled with model-map inspections in refinement. This approach has highlighted the value of such protocols for efficient analyses. The subunit is constructed from two domains. The N-terminal domain has previously only been observed in cytosolic carboxypeptidase (CCP) proteins. The C-terminal domain, which carries the Zn2+-containing active site, serves to classify this protein as a member of the M14D subfamily of carboxypeptidases. Although eukaryotic CCPs possess deglutamylase activity and are implicated in processing modified tubulin, the function and substrates of the bacterial family members remain unknown. The B. cenocepacia protein did not display deglutamylase activity towards a furylacryloyl glutamate derivative, a potential substrate. Residues previously shown to coordinate the divalent cation and that contribute to peptide-bond cleavage in related enzymes such as bovine carboxypeptidase are conserved. The location of a conserved basic patch in the active site adjacent to the catalytic Zn2+, where an acetate ion is identified, suggests recognition of the carboxy-terminus in a similar fashion to other carboxypeptidases. However, there are significant differences that indicate the recognition of substrates with different properties. Of note is the presence of a lysine in the S1' recognition subsite that suggests specificity towards an acidic substrate.

Proteome-derived peptide libraries to study the substrate specificity profiles of carboxypeptidases

Through processing peptide and protein C termini, carboxypeptidases participate in the regulation of various biological processes. Few tools are however available to study the substrate specificity profiles of these enzymes. We developed a proteome-derived peptide library approach to study the substrate preferences of carboxypeptidases. Our COFRADIC-based approach takes advantage of the distinct chromatographic behavior of intact peptides and the proteolytic products generated by the action of carboxypeptidases, to enrich the latter and facilitate its MS-based identification. Two different peptide libraries, generated either by chymotrypsin or by metalloendopeptidase Lys-N, were used to determine the substrate preferences of human metallocarboxypeptidases A1 (hCPA1), A2 (hCPA2), and A4 (hCPA4). In addition, our approach allowed us to delineate the substrate specificity profile of mouse mast cell carboxypeptidase (MC-CPA or mCPA3), a carboxypeptidase suggested to function in innate immune responses regulation and mast cell granule homeostasis, but which thus far lacked a detailed analysis of its substrate preferences. mCPA3 was here shown to preferentially remove bulky aromatic amino acids, similar to hCPA2. This was also shown by a hierarchical cluster analysis, grouping hCPA1 close to hCPA4 in terms of its P1 primed substrate specificity, whereas hCPA2 and mCPA3 cluster separately. The specificity profile of mCPA3 may further aid to elucidate the function of this mast cell carboxypeptidase and its biological substrate repertoire. Finally, we used this approach to evaluate the substrate preferences of prolylcarboxypeptidase, a serine carboxypeptidase shown to cleave C-terminal amino acids linked to proline and alanine.

A structural and functional analysis of Nna1 in Purkinje cell degeneration (pcd) mice

The axotomy-inducible enzyme Nna1 defines a subfamily of M14 metallocarboxypeptidases, and its mutation underlies the Purkinje cell degeneration (pcd) mouse. However, the relationship among its catalytic activity, substrate specificities, and the critical processes of neurodegeneration/axon regeneration is incompletely understood. Here we used a transgenic rescue strategy targeting expression of modified forms of Nna1 to Purkinje cells in pcd mice to determine structure-activity relationships for neuronal survival and in parallel characterized the enzymatic properties of purified recombinant Nna1. The Nna1 subfamily uniquely shares conserved substrate-determining residues with aspartoacylase that, when mutated, cause Canavan disease. Homologous mutations (D1007E and R1078E) inactivate Nna1 in vivo, as does mutation of its catalytic glutamate (E1094A), which implies that metabolism of acidic substrates is essential for neuronal survival. Consistent with reports that Nna1 is a tubulin glutamylase, recombinant Nna1-but not the catalytic mutants-removes glutamate from tubulin. Recombinant Nna1 metabolizes synthetic substrates with 2 or more C-terminal glutamate (but not aspartate) residues (V(max) for 3 glutamates is ∼7-fold higher than 2 glutamates although K(M) is similar). Catalysis is not ATP/GTP dependent, and mutating the ATP/GTP binding site of Nna1 has no effect in vivo. Nna1 is a monomeric enzyme essential for neuronal survival through hydrolysis of polyglutamate-containing substrates.

Characterization of the substrate specificity of human carboxypeptidase A4 and implications for a role in extracellular peptide processing

CPA4 (carboxypeptidase A4) is a member of the metallocarboxypeptidase family. CPA4 was originally found in a screen of mRNAs up-regulated by sodium butyrate-induced differentiation of cancer cells. Further studies suggested a relation between CPA4 and prostate cancer aggressiveness. In the present study, we determined that CPA4 is secreted from cells as a soluble proenzyme (pro-CPA4) that can be activated by endoproteases, such as trypsin. Three complementary approaches were used to study the substrate specificity of CPA4; kinetic analysis was performed using a new series of chromogenic substrates and some biologically relevant peptides, the cleavage of synthetic peptides was tested individually, and the cleavage of a mixture of >100 mouse brain peptides was examined using a quantitative peptidomics mass spectrometry-based approach. CPA4 was able to cleave hydrophobic C-terminal residues with a preference for Phe, Leu, Ile, Met, Tyr, and Val. However, not all peptides with C-terminal hydrophobic residues were cleaved, indicating the importance of additional residues within the peptide. Aliphatic, aromatic, and basic residues in the P1 position have a positive influence on the cleavage specificity. In contrast, acidic residues, Pro, and Gly have a negative influence in the P1 position. Some of the peptides identified as CPA4 substrates (such as neurotensin, granins, and opioid peptides) have been previously shown to function in cell proliferation and differentiation, potentially explaining the link between CPA4 and cancer aggressiveness. Taken together, these studies suggest that CPA4 functions in neuropeptide processing and regulation in the extracellular environment.

Dihydroorotase from Escherichia coli: Loop Movement and Cooperativity between Subunits

Escherichia coli dihydroorotase has been crystallized in the presence of the product, L-dihydroorotate (L-DHO), and the structure refined at 1.9A resolution. The structure confirms that previously reported (PDB entry 1J79), crystallized in the presence of the substrate N-carbamyl-D,L-aspartate (D, L-CA-asp), which had a dimer in the asymmetric unit, with one subunit having the substrate, L-CA-asp bound at the active site and the other having L-DHO. Importantly, no explanation for the unusual structure was given. Our results now show that a loop comprised of residues 105-115 has different conformations in the two subunits. In the case of the L-CA-asp-bound subunit, this loop reaches in toward the active site and makes hydrogen-bonding contact with the bound substrate molecule. For the L-DHO-bound subunit, the loop faces in the opposite direction and forms part of the surface of the protein. Analysis of the kinetics for conversion of L-DHO to L-CA-asp at low concentrations of L-DHO shows positive cooperativity with a Hill coefficient n=1.57(+/-0.13). Communication between subunits in the dimer may occur via cooperative conformational changes of the side-chains of a tripeptide from each subunit: Arg256-His257-Arg258, near the subunit interface.

Molecular cloning and cDNA sequence analysis of carboxypeptidases A1, A2 and B from the Japanese flounder Paralichthys olivaceus

Although pancreatic serine proteases have been cloned in teleosts, no sequence data are currently available on members of the carboxypeptidase (CP) family. Here, we cloned cDNAs coding for two preproCPAs, corresponding to mammalian preproCPA1 and preproCPA2, and one preproCPB from a pancreatic cDNA library of the Japanese flounder, Paralichthys olivaceus. The activation peptides of flounder proCPs completely retained the sequences for inhibition of enzymatic activity of proCPs just like mammalian proCPs. Of 306-309 amino acids in total, 95 amino acids are completely conserved between bovine CPA1 and CPB and flounder CPs. Notably, amino acid residues for Zn(2+) ligands, catalysis and substrate anchoring are completely conserved between flounder and bovine CPs. Three species of flounder preproCPs are all expressed in the pancreas of first feeding larvae.

Expression of a soluble and activatable form of bovine procarboxypeptidase A in Escherichia coli

Bovine pancreatic procarboxypeptidase A has been overexpressed in a soluble and activatable form in Escherichia coli. When the protein was expressed under the control of bacteriophage T7 promoter in E. coli ADA494 (a thioredoxin reductase deficient bacteria), a thioredoxin fusion protein was produced at relatively high level in the cytoplasm (4 mg/L culture medium). Although the recombinant protein essentially accumulated as inclusion bodies, as much as 30% of the fusion protein was recovered in a soluble form at low growth temperature and could therefore be purified to homogeneity in a single-step procedure by metal-affinity chromatography. The recombinant precursor form of bovine carboxypeptidase A was recognized by a monoclonal antibody directed against purified bovine pancreatic carboxypeptidase A. Moreover, upon tryptic activation it gave rise to an enzyme, the N-terminal sequence, molecular size,and specific activity of which were comparable to those of the enzyme derived from the native precursor purified from bovine pancreas.

Changes in zinc ligation promote remodeling of the active site in the zinc hydrolase superfamily

The zinc hydrolase superfamily is a group of divergently related proteins that are predominantly enzymes with a zinc-based catalytic mechanism. The common structural scaffold of the superfamily consists of an eight-stranded beta-sheet flanked by six alpha-helices. Previous analyses, while acknowledging the likely divergent origins of leucine aminopeptidase, carboxypeptidase A and the co-catalytic enzymes of the metallopeptidase H clan based on their structural scaffolds, have failed to find any homology between the active sites in leucine aminopeptidase and the metallopeptidase H clan enzymes. Here we show that these two groups of co-catalytic enzymes have overlapping dizinc centers where one of the two zinc atoms is conserved in each group. Carboxypeptidase A and leucine aminopeptidase, on the other hand, no longer share any homologous zinc-binding sites. At least three catalytic zinc-binding sites have existed in the structural scaffold over the period of history defined by available structures. Comparison of enzyme-inhibitor complexes show that major remodeling of the substrate-binding site has occurred in association with each change in zinc ligation in the binding site. These changes involve re-registration and re-orientation of the substrate. Some residues important to the catalytic mechanism are not conserved amongst members. We discuss how molecules acting in trans may have facilitated the mutation of catalytically important residues in the active site in this group.

Metallocarboxypeptidases and their protein inhibitors. Structure, function and biomedical properties

Among the different aspects of recent progress in the field of metallocarboxypeptidases has been the elucidation of the three dimensional structures of the pro-segments (in monomeric or oligomeric species) and their role in the expression, folding and inhibition/activation of the pancreatic and pancreatic-like forms. Also of great significance has been the cloning and characterization of several new regulatory carboxypeptidases, enzymes that are related with important functions in protein and peptide processing and that show significant structural differences among them and also with the digestive ones. Many regulatory carboxypeptidases lack a pro-region, unlike the digestive forms or others in between from the evolutionary point of view. Finally, important advances have been made on the finding and characterization of new protein inhibitors of metallocarboxypeptidases, some of them with interesting potential applications in the biotechnological/biomedical fields. These advances are analyzed here and compared with the earlier observations in this field, which was first explored by Hans Neurath and collaborators.

Glu300 of rat carboxypeptidase E is essential for enzymatic activity but not substrate binding or routing to the regulated secretory pathway

Several recently discovered members of the carboxypeptidase E (CPE) gene family lack critical active site residues that are conserved in other family members. For example, three CPE-like proteins contain a Tyr in place of Glu300 (equivalent to Glu270 of carboxypeptidase A and B). To investigate the importance of this position, Glu300 of rat CPE was converted into Gln, Lys, or Tyr, and the proteins expressed in Sf9 cells using the baculovirus system. All three mutants were secreted from the cells, but the media showed no enzyme activity above background levels. Wild-type CPE and the Gln300 point mutant bound to a p-aminobenzoyl-Arg-Sepharose affinity resin, and this binding was competed by an active site-directed inhibitor, guanidinoethylmercaptosuccinic acid. The affinity purified mutant CPE protein showed no detectable enzyme activity (<0.004% of wild-type CPE) toward dansyl-Phe-Ala-Arg. Expression of the Gln300 and Lys300 mutant CPE proteins in the NIT3 mouse pancreatic beta-cell line showed that these mutants are routed into secretory vesicles and secreted via the regulated pathway. Taken together, these results indicate that Glu300 of CPE is essential for enzyme activity, but not required for substrate binding or for routing into the regulated secretory pathway.

Cloning, expression, and substrate specificity of MeCPA, a zinc carboxypeptidase that is secreted into infected tissues by the fungal entomopathogen Metarhizium anisopliae

To date zinc carboxypeptidases have only been found in animals and actinomycete bacteria. A cDNA clone (MeCPA) for a novel fungal (Metarhizium anisopliae) carboxypeptidase (MeCPA) was obtained by using reverse transcription differential display polymerase chain reaction to identify pathogenicity genes. MeCPA resembles pancreatic carboxypeptidases in being synthesized as a precursor species (418 amino acids) containing a large amino-terminal fragment (99 amino acids). The mature (secreted) form of MeCPA shows closest amino acid identity to human carboxypeptidases A1 (35%) and A2 (37%). MeCPA was expressed in an insect cell line yielding an enzyme with dual A1 + A2 specificity for branched aliphatic and aromatic COOH-terminal amino acids. However, in contrast to the very broad spectrum A + B-type bacterial enzymes, MeCPA lacks B-type activity against charged amino acids. This is predictable as key catalytic residues determining the specificity of MeCPA are conserved with those of mammalian A-type carboxypeptidases. Thus, in evolutionary terms the fungal enzyme is an intermediate between the divergence of A and B forms and the differentiation of the A form into A1 and A2 isoforms. Ultrastructural immunocytochemistry of infected host (Manduca sexta) cuticle demonstrated that MeCPA participates with the concurrently produced endoproteases in procuring nutrients; an equivalent function to digestive pancreatic enzymes.

Cloning, sequencing and functional expression of a cDNA encoding porcine pancreatic preprocarboxypeptidase A1

A full-length cDNA clone coding for porcine pancreatic preprocarboxypeptidase A1 (prePCPA1) was isolated from a cDNA library. The open reading frame (ORF) of the nucleotide sequence was 1260 nt in length and encoded a protein of 419 amino acids (aa). The cDNA included a short signal peptide of 16 aa and a 94 aa-long activation segment. The calculated molecular mass of the mature proenzyme was 45561 Da, in accordance with that of the purified porcine pancreatic PCPA1. The deduced aa sequence of the corresponding enzyme differed from that predicted by the three-dimensional structure by 40 aa, and showed 85% identity and 55% identity to that of procarboxypeptidases A1 and A2, respectively. Moreover the sequence was identical to that of several independent cDNA clones, suggesting that it is the major transcribed gene. No evidence for a second variant was observed in the cDNA library and PCPA2 is apparently absent from the porcine pancreas. The cDNA was expressed in Saccharomyces cerevisiae under the control of the yeast triose phosphate isomerase promoter. The signal peptide of the PCPA protein efficiently directed its secretion into the culture medium (1.5 mg.L-1) as a protein of the predicted size. The recombinant proenzyme was analyzed by immunological and enzymological methods. Its activation behavior was comparable with that of the native form and led to a 35-kDa active enzyme.

Overexpression of human procarboxypeptidase A2 in Pichia pastoris and detailed characterization of its activation pathway

The cDNA of human procarboxypeptidase A2 has been overexpressed in the methylotrophic yeast Pichia pastoris and secreted into the culture medium by means of the alpha-mating factor signal sequence, yielding a major protein of identical size and N-terminal sequence as the wild-type form. Two other forms containing the proenzyme have also been overexpressed: one of them resulted from an incomplete processing of the signal peptide, whereas the other was a glycosylated derivative. Recombinant procarboxypeptidase A2 was purified to homogeneity, and it was shown that its mature active form displays functional properties similar to those of the enzyme directly isolated from human pancreas. The overall yield was approximately 250 mg of proenzyme or 180 mg of mature enzyme/liter of cell culture. The proteolysis-promoted activation process of the recombinant proenzyme has been studied in detail. During maturation by trypsin, the increase in activity of the enzyme is a rapid and monotonic event, which reflects the rate of the proteolytic release of the inhibitory pro-segment and the weaker nature of its interactions with the enzyme moiety compared with procarboxypeptidases of the A1 type. Three main forms of the pro-segment (96, 94, and 92 amino acids), with no inhibitory capability in the severed state, and a single mature carboxypeptidase A2 are produced during this process. No further proteolysis of these pro-segments by the generated carboxypeptidase A2 occurs, in contrast with observations made in other procarboxypeptidases (A1 and B). This differential behavior is a result of the extreme specificity of carboxypeptidase A2.

Characterisation and preliminary X-ray diffraction analysis of human pancreatic procarboxypeptidase A2

Human procarboxypeptidase A2 has been expressed in a Pichia pastoris heterologous system and purified by hydrophobic interaction and anion exchange chromatographies. The hydrolytic action of carboxypeptidase A2 on peptide substrates with different lengths and residues at the C-terminus was analysed, and a preference towards long substrates with aromatic amino acids in their C-terminal end, particularly tryptophan, was found; with such substrates its activity is similar or higher than that of bovine carboxypeptidase A1. Procarboxypeptidase A2 has been crystallised using a vapour diffusion approach; the crystals obtained belong to the monoclinic system, spacegroup P2(1), and present one procarboxypeptidase A2 molecule per asymmetric unit. The crystals diffract beyond 1.8 A resolution and are suitable for detailed X-ray analysis.

Rapid induction by a blood meal of a carboxypeptidase gene in the gut of the mosquito Anopheles gambiae

A search for genes induced rapidly (< 3 h) after a blood meal in the gut of the human malaria vector Anopheles gambiae led to the identification of a carboxypeptidase gene (AgCP). We report the sequence of the 1302 nt AgCP transcribed sequence, 710 nt of upstream and 585 nt of downstream DNA. The AgCP open reading frame is 60.4% identical at the nucleotide level to a blackfly, Simulium vittatum, carboxypeptidase gene. The transcriptional start site of AgCP was determined by primer extension. Expression of AgCP mRNA is detectable in the guts of pupae and sugar-fed adult female mosquitoes and is induced (approximately 10-fold) within 3 h of a blood meal. By 24 h after a blood meal, mRNA abundance returns to a level close to that present before a blood meal. Whole-mount in situ hybridization shows that AgCP mRNA expression is restricted to most or all cells of the posterior midgut. Expression of the AgCP and trypsin genes were compared and shown to differ in two fundamental ways: (1) the peak of AgCP expression after a blood meal occurs approximately 20 h before that of trypsin; and (2) induction of the AgCP gene is independent of the composition of the ingested meal whereas trypsin induction requires the presence of protein. The potential use of the AgCP promoter for driving the expression of genes that hinder the development of parasites in the mosquito gut is discussed.

Crystal structure of an oligomer of proteolytic zymogens: detailed conformational analysis of the bovine ternary complex and implications for their activation

The pancreas of ruminants secretes a 100 kDa non-covalent ternary complex of the zymogen of a metalloexopeptidase, carboxypeptidase A, and the proforms of two serine endopeptidases, chymotrypsin C and proteinase E. The crystal structure of the bovine complex has been solved and refined to an R-factor of 0.192 using synchrotron radiation X-ray data to 2.35 A resolution. In this heterotrimeric complex, the 403 residue procarboxypeptidase A takes a central position, with chymotrypsinogen C and proproteinase E attached to different surface sites of it. The procarboxypeptidase A subunit is composed of the active enzyme part and the 94 residue prodomain, similar to the monomeric porcine homologous form. The 251 residue subunit chymotrypsinogen structure, the first solved of an anionic (acidic pI) chymotrypsinogen, exhibits characteristics of both chymotrypsinogen A and elastases, with a potential specificity pocket of intermediate size (to accommodate apolar medium-sized residues) although not properly folded, as in bovine chymotrypsinogen A; this pocket displays a "zymogen triad" characteristic for zymogens of the chymotrypsinogen family, consisting of three non-catalytic residues (one serine, one histidine, and one aspartate) arranged in a fashion similar to the catalytic residues in the active enzymes. Following the traits of this family, the N terminus is clamped to the main molecular body by a disulphide bond, but the close six residue activation segment is completely disordered. The third zymogen, the 253 residue proproteinase E, bears close conformational resemblance to active porcine pancreatic elastase; its specificity pocket is buried, displaying the second "zymogen triad". Its five N-terminal residues are disordered, although the close activation site is fixed to the molecular surface. The structure of this native zymogen displays large conformational differences when compared with the recently solved crystal structure of bovine subunit III, an N-terminally truncated, non-activatable, proproteinase E variant lacking the first 13 residues of the native proenzyme. Most of the prosegment of procarboxypeptidase A and its activation sites are buried in the centre of the oligomer, whilst the activation sites of chymotrypsinogen C and proproteinase E are surface-located and not involved in intra or inter-trimer contacts. This organization confers a functional role to the oligomeric structure, establishing a sequential proteolytic activation for the different zymogens of the complex. The large surface and number of residues involved in the contacts among subunits, as well as the variety of non-bonded interactions, account for the high stability of the native ternary complex.

The sequence and conformation of human pancreatic procarboxypeptidase A2. cDNA cloning, sequence analysis, and three-dimensional model

A full-length cDNA clone coding for human pancreatic preprocarboxypeptidase A2 has been isolated from a lambda gt 11 human pancreatic library. Expression clones were identified by specific interaction with antisera raised against the native protein. The open reading frame of the polynucleotide sequence is 1254 base pairs in length and encodes a protein of 417 amino acids. This cDNA includes a short leader signal peptide of 16 amino acids and a 94-amino acid-long activation segment. The amino acid sequence shows 89% identity to that of rat procarboxypeptidase A2, the only A2 form sequenced so far, and 64% identity to that of human procarboxypeptidase A1. The newly determined sequence was modeled to the three-dimensional crystal structures of both bovine carboxypeptidase A and porcine procarboxypeptidase A1 by a novel distance geometry approach. Biases in the modeling were avoided by relying exclusively on automatic procedures and by using random structures as starting points. Information taken from the known homologous structures refers only to the backbone since no explicit data describing the conformation of side chains were transferred. Ten structures of human carboxypeptidase A2 were determined on the basis of each of the two known crystal structures. The root-mean-square distance for the backbone atoms between the 10 structures and their mean for 237 selected residues is 0.7 A when starting from the bovine protein and 0.8 A for 251 selected residues when starting from the porcine protein. The 94 residue-long activation segment was also determined in the modeling based on the porcine zymogen; its structure is well defined but not its orientation with respect to the enzyme moiety. The model obtained for human procarboxypeptidase A2 is discussed with respect to the specificity and activation of the enzyme.

Procarboxypeptidase in rat pancreas. Overall characterization and comparison of the activation processes

Three monomeric procarboxypeptidases and a binary complex consisting of a procarboxypeptidase and a chymotrypsinogen have been isolated from rat pancreas by HPLC. N-terminal sequence determination, substrate-specificity analysis and physico-chemical characterization showed that the carboxypeptidase precursors were the A1, A2 and B forms. No isomorphism could be detected for any of these proenzymes and no clear evidence was obtained for the presence of procarboxypeptidase-containing quaternary complexes of the types previously described for other species. Instead, we observed the presence of a binary complex between procarboxypeptidase A2 and chymotrypsinogen B. Among the major pancreatic endoproteinases, only trypsin was found to be a general activator of rat procarboxypeptidases in vitro. Time-course analysis of the products generated after trypsin addition confirmed that full activation of procarboxypeptidase A1 requires several cleavages in the C-terminal region (residues 87-94) of the activation segment, while procarboxypeptidases A2 and B require a single cleavage each. The carboxypeptidases released participate in the trimming of the activation segment in A1 and B, but not in A2, probably because of the high specificity of the latter in the active form.

Advances in metallo-procarboxypeptidases. Emerging details on the inhibition mechanism and on the activation process

Our knowledge on the structure and functionality of pancreatic carboxypeptidases is rapidly expanding to include that of their zymogen forms. The recent application of fast and mild isolation procedures, together with modern molecular genetic and biochemical-biophysical characterization approaches, has provided a clearer view of the basic structures and functional states in which these zymogens occur, and their evolutionary relationships. The same holds for related metallo-carboxypeptidases, either in the pro or active forms, that have been isolated and characterized in non-digestive fluids and tissues, where they probably play an important role in protein and peptide processing. The determination of the three-dimensional structure of the A and B pancreatic zymogens has revealed the molecular determinants of their inactivity and proteolytic activation. The folding of their 95-residue activation segment in a globular N-terminal domain (74-81 residues) and in a connecting region (20-14 residues), and the specific contacts of these pieces with the substrate binding sites of the enzyme, are important factors in zymogen inhibition. On the other hand, the different length of the alpha-helical connecting region and the stability of its contacts with the enzyme account for the different activation properties of A and B zymogens.