Predictors for neoplastic progression in patients with Barrett's Esophagus: a prospective cohort study.

OBJECTIVES: Patients with Barrett's esophagus (BE) have an increased risk of developing esophageal adenocarcinoma (EAC). As the absolute risk remains low, there is a need for predictors of neoplastic progression to tailor more individualized surveillance programs. The aim of this study was to identify such predictors of progression to high-grade dysplasia (HGD) and EAC in patients with BE after 4 years of surveillance and to develop a prediction model based on these factors. METHODS: We included 713 patients with BE (≥ 2 cm) with no dysplasia (ND) or low-grade dysplasia (LGD) in a multicenter, prospective cohort study. Data on age, gender, body mass index (BMI), reflux symptoms, tobacco and alcohol use, medication use, upper gastrointestinal (GI) endoscopy findings, and histology were prospectively collected. As part of this study, patients with ND underwent surveillance every 2 years, whereas those with LGD were followed on a yearly basis. Log linear regression analysis was performed to identify risk factors associated with the development of HGD or EAC during surveillance. RESULTS: After 4 years of follow-up, 26/713 (3.4%) patients developed HGD or EAC, with the remaining 687 patients remaining stable with ND or LGD. Multivariable analysis showed that a known duration of BE of ≥ 10 years (risk ratio (RR) 3.2; 95% confidence interval (CI) 1.3-7.8), length of BE (RR 1.11 per cm increase in length; 95% CI 1.01-1.2), esophagitis (RR 3.5; 95% CI 1.3-9.5), and LGD (RR 9.7; 95% CI 4.4-21.5) were significant predictors of progression to HGD or EAC. In a prediction model, we found that the annual risk of developing HGD or EAC in BE varied between 0.3% and up to 40%. Patients with ND and no other risk factors had the lowest risk of developing HGD or EAC (<1%), whereas those with LGD and at least one other risk factor had the highest risk of neoplastic progression (18-40%). CONCLUSIONS: In patients with BE, the risk of developing HGD or EAC is predominantly determined by the presence of LGD, a known duration of BE of ≥10 years, longer length of BE, and presence of esophagitis. One or combinations of these risk factors are able to identify patients with a low or high risk of neoplastic progression and could therefore be used to individualize surveillance intervals in BE.

The structure of a complex of recombinant hirudin and human alpha-thrombin.

The crystallographic structure of a recombinant hirudin-thrombin complex has been solved at 2.3 angstrom (A) resolution. Hirudin consists of an NH2-terminal globular domain and a long (39 A) COOH-terminal extended domain. Residues Ile1 to Tyr3 of hirudin form a parallel beta-strand with Ser214 to Glu217 of thrombin with the nitrogen atom of Ile1 making a hydrogen bond with Ser195 O gamma atom of the catalytic site, but the specificity pocket of thrombin is not involved in the interaction. The COOH-terminal segment makes numerous electrostatic interactions with an anion-binding exosite of thrombin, whereas the last five residues are in a helical loop that forms many hydrophobic contacts. In all, 27 of the 65 residues of hirudin have contacts less than 4.0 A with thrombin (10 ion pairs and 23 hydrogen bonds). Such abundant interactions may account for the high affinity and specificity of hirudin.

The clot thickens: clues provided by thrombin structure.

Thrombin is the primary promoter of blood clotting; it also plays an important role in the regulation of the coagulation cascade and has been implicated in a number of other cellular processes. How can one molecule catalyse such a variety of events? The recent X-ray structure determination of human a-thrombin and related structures shows that the molecule can be divided into several functional regions that recognize different chemical moieties. By using different combinations of these elements, thrombin can interact with a variety of macromolecules with high specificity.

A pro-inflammatory genotype predisposes to Barrett's esophagus.

INTRODUCTION: Severity of mucosal inflammation is shown to be associated with Barrett's esophagus (BE) development in animals. It has therefore been postulated that a strong pro-inflammatory host response predisposes to BE. AIM: To determine the impact of cytokine gene polymorphisms on the development of BE. METHODS: The multiplex SNaPshot method was used to determine interleukin (IL)-12B (A+1188C), IL-10 (C-592A, C-819T, A-1082G), IL-8 (A-251T), IL-6 (G-174C) and IL-2 (G-330T) gene polymorphisms in 255 patients with BE and 247 patients with reflux esophagitis (RE). RESULTS: The presence of the IL-12B C-allele, which is associated with increased IL-12p70 expression, was more frequently observed in BE than in RE patients [odds ratio (OR) 1.8; 95% confidence interval (CI) 1.2-2.7; P = 0.007). The risk of BE was increased in patients in whom the IL-12B C-allele coincided with a hiatal hernia (OR 2.9; 95% CI 1.32-6.58; P = 0.008). The IL-10(-1082) GG genotype, which is associated with higher IL-10 levels, was also associated with a decreased risk of BE when it was associated with the IL-12B C-allele, indicating IL-10-dependent down-regulation of IL-12p70 expression. A combination of the IL-12B AA genotype and the IL-10 AA or AG genotypes was associated with RE (OR 1.4; 95% CI 1.05-1.85; P = 0.011). CONCLUSION: A genetic profile predisposing to a strong pro-inflammatory host response, mediated by IL-12p70 and partially dependent on IL-10, is associated with BE. This risk further increases when this genotype coincides with a hiatal hernia, suggesting that exposure to gastroesophageal reflux in the presence of a pro-inflammatory genetic background is a driving force in the development of BE.

Flexibility and variability of TIMP binding: X-ray structure of the complex between collagenase-3/MMP-13 and TIMP-2.

The excessive activity of matrix metalloproteinases (MMPs) contributes to pathological processes such as arthritis, tumor growth and metastasis if not balanced by the tissue inhibitors of metalloproteinases (TIMPs). In arthritis, the destruction of fibrillar (type II) collagen is one of the hallmarks, with MMP-1 (collagenase-1) and MMP-13 (collagenase-3) being identified as key players in arthritic cartilage. MMP-13, furthermore, has been found in highly metastatic tumors. We have solved the 2.0 A crystal structure of the complex between the catalytic domain of human MMP-13 (cdMMP-13) and bovine TIMP-2. The overall structure resembles our previously determined MT1-MMP/TIMP-2 complex, in that the wedge-shaped TIMP-2 inserts with its edge into the entire MMP-13 active site cleft. However, the inhibitor is, according to a relative rotation of approximately 20 degrees, oriented differently relative to the proteinase. Upon TIMP binding, the catalytic zinc, the zinc-ligating side chains, the enclosing MMP loop and the S1' wall-forming segment move significantly and in concert relative to the rest of the cognate MMP, and the active site cleft constricts slightly, probably allowing a more favourable interaction between the Cys1(TIMP) alpha-amino group of the inhibitor and the catalytic zinc ion of the enzyme. Thus, this structure supports the view that the central N-terminal TIMP segment essentially defines the relative positioning of the TIMP, while the flanking edge loops determine the relative orientation, depending on the individual target MMP.

Structural features of a superfamily of zinc-endopeptidases: the metzincins.

A large number of zinc endopeptidases contain an HEXXHXXGXXH consensus motif in their catalytic site (single letter code; X is any amino acid residue). These enzymes can be grouped into four distinct families, the astacins, the adamalysins, the serralysins and the matrix metalloproteinases (matrixins). Despite a low degree of sequence similarity, their catalytic modules are topologically similar. A topology derived sequence alignment suggests that the four families form a superfamily, called the metzincins because of a perfectly superimposable methionine residue close to the zinc-binding active site. Topological similarity to the thermolysin-like enzymes indicates that these enzymes may have had a common ancestor.

Tissue-type plasminogen activator: variants and crystal/solution structures demarcate structural determinants of function.

NMR and crystal structure of many components of tissue-type plasminogen activator (t-PA) are now available: the finger-EGF pair and the kringle-2 domain structures have been solved, as have the proteolytic domains of vampire bat PA and human t-PA in two- and single-chain forms. These structures confirm the trypsin-like arrangement of the proteolytic domain of t-PA and show how surface loops near the catalytic centre contribute to the narrow specificity of t-PA. Together with mutational experiments, they identify the Lys156 sidechain as a cause of the amidolytic activity of single-chain t-PA, as it can provide a substitute salt bridge partner for Asp194 in the absence of the Ile16 N terminus of the two-chain form. These new findings provide new ideas for the design of PA variants with improved therapeutic properties.

Coagulation factors and their inhibitors.

A comprehensive three-dimensional picture of the coagulation process is beginning to emerge. Crystallographic structure determinations of prothrombin, factor Xa, factor IXa, tissue factor and factor XIII represent important advances in our understanding of the coagulation cascade. Similarly, structures of antithrombin, tissue factor pathway inhibitor and thrombomodulin provide details of endogenous anticoagulatory mechanisms. NMR spectroscopy of multiple domains of coagulation proteins represents an important contribution to the analysis of flexibility and rigidity of modular proteins. Thrombin, as the prime candidate for antithrombotic drug design, continues to be an object of intense efforts in applied crystallography.

Crystal structure of the E. coli dipeptidyl carboxypeptidase Dcp: further indication of a ligand-dependent hinge movement mechanism.

Dcp from Escherichia coli is a 680 residue cytoplasmic peptidase, which shows a strict dipeptidyl carboxypeptidase activity. Although Dcp had been assigned to the angiotensin I-converting enzymes (ACE) due to blockage by typical ACE inhibitors, it is currently grouped into the M3 family of mono zinc peptidases, which also contains the endopeptidases neurolysin and thimet oligopeptidase (TOP). We have cloned, expressed, purified, and crystallized Dcp in the presence of an octapeptide "inhibitor", and have determined its 2.0A crystal structure using MAD methods. The analysis revealed that Dcp consists of two half shell-like subdomains, which enclose an almost closed two-chamber cavity. In this cavity, two dipeptide products presumably generated by Dcp cleavage of the octapeptide bind to the thermolysin-like active site fixed to side-chains, which are provided by both subdomains. In particular, an Arg side-chain backed by a Glu residue, together with two Tyr phenolic groups provide a charged anchor for fixing the C-terminal carboxylate group of the P2' residue of a bound substrate, explaining the strict dipeptidyl carboxypeptidase specificity of Dcp. Tetrapeptidic substrates are fixed only via their main-chain functions from P2 to P2', suggesting a broad residue specificity for Dcp. Both subdomains exhibit very similar chain folds as the equivalent but abducted subdomains of neurolysin and TOP. Therefore, this "product-bound" Dcp structure seems to represent the inhibitor/substrate-bound "closed" form of the M3 peptidases, generated from the free "open" substrate-accessible form by a hinge-bending mechanism. A similar mechanism has recently been demonstrated experimentally for ACE2.

A helping hand for collagenases: the haemopexin-like domain.

The structures of the C-terminal domains of rabbit haemopexin and full-length porcine fibroblast collagenase reveal a common beta-propeller fold with pseudo-fourfold symmetry. The interactions of these haemopexin-like domains with target proteins and ligands is, however, still a matter of debate.

Specific inhibition of insect alpha-amylases: yellow meal worm alpha-amylase in complex with the amaranth alpha-amylase inhibitor at 2.0 A resolution.

BACKGROUND: alpha-Amylases constitute a family of enzymes that catalyze the hydrolysis of alpha-D-(1,4)-glucan linkages in starch and related polysaccharides. The Amaranth alpha-amylase inhibitor (AAI) specifically inhibits alpha-amylases from insects, but not from mammalian sources. AAI is the smallest proteinaceous alpha-amylase inhibitor described so far and has no known homologs in the sequence databases. Its mode of inhibition of alpha-amylases was unknown until now. RESULTS: The crystal structure of yellow meal worm alpha-amylase (TMA) in complex with AAI was determined at 2.0 A resolution. The overall fold of AAI, its three-stranded twisted beta sheet and the topology of its disulfide bonds identify it as a knottin-like protein. The inhibitor binds into the active-site groove of TMA, blocking the central four sugar-binding subsites. Residues from two AAI segments target the active-site residues of TMA. A comparison of the TMA-AAI complex with a modeled complex between porcine pancreatic alpha-amylase (PPA) and AAI identified six hydrogen bonds that can be formed only in the TMA-AAI complex. CONCLUSIONS: The binding of AAI to TMA presents a new inhibition mode for alpha-amylases. Due to its unique specificity towards insect alpha-amylases, AAI might represent a valuable tool for protecting crop plants from predatory insects. The close structural homology between AAI and 'knottins' opens new perspectives for the engineering of various novel activities onto the small scaffold of this group of proteins.

Amino-acid sequence and three-dimensional structure of the Staphylococcus aureus metalloproteinase at 1.72 A resolution.

BACKGROUND: Aureolysin is an extracellular zinc-dependent metalloproteinase from the pathogenic bacterium Staphylococcus aureus. This enzyme exhibits in vitro activity against several molecules of biological significance for the host, indicating that it is involved in the pathology of staphylococcal diseases. RESULTS: Here we report the amino-acid sequence and inhibitor-free X-ray crystal structure of aureolysin, a member of the thermolysin family of zinc-dependent metalloproteinases. This enzyme, which binds one zinc and three calcium ions, comprises a single chain of 301 amino acids that consists of a beta-strand-rich upper domain and an alpha-helix-rich lower domain. CONCLUSIONS: The overall structure of aureolysin is very similar to that of the other three members of this family whose structures are known - thermolysin (TLN) from Bacillus thermoproteolyticus, neutral protease (NP) from Bacillus cereus and elastase (PAE) from Pseudomonas aeruginosa. But an important difference has been encountered: in contrast to what has been observed in the other three members of this family (TLN, NP and PAE), inhibitor-free aureolysin displays a 'closed' active site cleft conformation. This new structure therefore raises questions about the universality of the hinge-bending motion model for the neutral metalloproteinases.

The crystal structure of the novel snake venom plasminogen activator TSV-PA: a prototype structure for snake venom serine proteinases.

BACKGROUND: Trimeresurus stejnejeri venom plasminogen activator (TSV-PA) is a snake venom serine proteinase that specifically activates plasminogen. Snake venom serine proteinases form a subfamily of trypsin-like proteinases that are characterised by a high substrate specificity and resistance to inhibition. Many of these venom enzymes specifically interfere with haemostatic mechanisms and display a long circulating half-life. For these reasons several of them have commercial applications and are potentially attractive pharmacological tools. RESULTS: The crystal structure of TSV-PA has been determined to 2.5 A resolution and refined to an R factor of 17.8 (R free, 24.4). The enzyme, showing the overall polypeptide fold of trypsin-like serine proteinases, displays unique structural elements such as the presence of a phenylalanine at position 193, a C-terminal tail clamped via a disulphide bridge to the 99-loop, and a structurally conserved Asp97 residue. The presence of a cis proline at position 218 is in agreement with evolutionary relationships to glandular kallikrein. CONCLUSIONS: We postulate that Phe 193 accounts for the high substrate specificity of TSV-PA and renders it incapable of forming a stable complex with bovine pancreatic trypsin inhibitor and other extended substrates and inhibitors. Mutational studies previously showed that Asp97 is crucial for the plasminogenolytic activity of TSV-PA, here we identify the conservation of Asp97 in both types of mammalian plasminogen activator - tissue-type (tPA) and urokinase-type (uPA). It seems likely that Asp97 of tPA and uPA will have a similar role in plasminogen recognition. The C-terminal extension of TSV-PA is conserved among snake venom serine proteinases, although its function is unknown. The three-dimensional structure presented here is the first of a snake venom serine proteinase and provides an excellent template for modelling other homologous family members.

Enzyme flexibility, solvent and 'weak' interactions characterize thrombin-ligand interactions: implications for drug design.

BACKGROUND: The explosive growth in the rate of X-ray determination of protein structures is fuelled largely by the expectation that structural information will be useful for pharmacological and biotechnological applications. For example, there have been intensive efforts to develop orally administrable antithrombotic drugs using information about the crystal structures of blood coagulation factors, including thrombin. Most of the low molecular weight thrombin inhibitors studied so far are based on arginine and benzamidine. We sought to expand the database of information on thrombin-inhibitor binding by studying new classes of inhibitors. RESULTS: We report the structures of three new inhibitors complexed with thrombin, two based on 4-aminopyridine and one based on naphthamidine. We observe several geometry changes in the protein main chain and side chains which accompany inhibitor binding. The two inhibitors based on 4-aminopyridine bind in notably different ways: one forms a water-mediated hydrogen bond to the active site Ser195, the other induces a rotation of the Ser214-Trp215 peptide plane that is unprecedented in thrombin structures. These binding modes also differ in their 'weak' interactions, including CH-O hydrogen bonds and interactions between water molecules and aromatic pi-clouds. Induced-fit structural changes were also seen in the structure of the naphthamidine inhibitor complex. CONCLUSIONS: Protein flexibility and variable water structures are essential elements in protein-ligand interactions. Ligand design strategies that fail to take this into account may overlook or underestimate the potential of lead structures. Further, the significance of 'weak' interactions must be considered both in crystallographic refinement and in analysis of binding mechanisms.

Coagulation factor IXa: the relaxed conformation of Tyr99 blocks substrate binding.

BACKGROUND: Among the S1 family of serine proteinases, the blood coagulation factor IXa (fIXa) is uniquely inefficient against synthetic peptide substrates. Mutagenesis studies show that a loop of residues at the S2-S4 substrate-binding cleft (the 99-loop) contributes to the low efficiency. The crystal structure of porcine fIXa in complex with the inhibitor D-Phe-Pro-Arg-chloromethylketone (PPACK) was unable to directly clarify the role of the 99-loop, as the doubly covalent inhibitor induced an active conformation of fIXa. RESULTS: The crystal structure of a recombinant two-domain construct of human fIXa in complex with p-aminobenzamidine shows that the Tyr99 sidechain adopts an atypical conformation in the absence of substrate interactions. In this conformation, the hydroxyl group occupies the volume corresponding to the mainchain of a canonically bound substrate P2 residue. To accommodate substrate binding, Tyr99 must adopt a higher energy conformation that creates the S2 pocket and restricts the S4 pocket, as in fIXa-PPACK. The energy cost may contribute significantly to the poor K(M) values of fIXa for chromogenic substrates. In homologs, such as factor Xa and tissue plasminogen activator, the different conformation of the 99-loop leaves Tyr99 in low-energy conformations in both bound and unbound states. CONCLUSIONS: Molecular recognition of substrates by fIXa seems to be determined by the action of the 99-loop on Tyr99. This is in contrast to other coagulation enzymes where, in general, the chemical nature of residue 99 determines molecular recognition in S2 and S3-S4. This dominant role on substrate interaction suggests that the 99-loop may be rearranged in the physiological fX activation complex of fIXa, fVIIIa, and fX.

Structural basis of the endoproteinase-protein inhibitor interaction.

Proteolytic enzymes are potentially hazardous to their protein environment, so that their activity must be carefully controlled. Living organisms use protein inhibitors as a major tool to regulate the proteolytic activity of proteinases. Most of the inhibitors for which 3D structures are available are directed towards serine proteinases, interacting with the active sites in a 'canonical' i.e. substrate-like manner via an exposed reactive site loop of conserved conformation. More recently, some non-canonically binding serine proteinase inhibitors directed against coagulation factors, in particular thrombin, a few cysteine proteinase inhibitors inhibitory towards papain-like proteinases, and three zinc endopeptidase inhibitors directed against metzincins and thermolysin have been characterised in the free and complexed state, displaying novel mechanisms of inhibition with their target proteinases. These different interaction modes are presented and briefly discussed with respect to the different strategies applied by nature.

Involvement of tyrosine residues in the protomer-protomer interaction of Proteus mirabilis flagella as studied by spectroscopic methods, chemical modification and aggregation experiments.

Using spectrophotometrical titration, chemical modification, and ultraviolet difference spectral methods, the existence of at least two distinct tyrosine groups in the isolated flagellin of Proteus mirabilis flagella has been established. Three of the five flagellin tyrosines are buried in the protein matrix, whereas the other two seem to lie on the protein surface accessible to perturbants. Also about two tyrosine residues, presumably the latter ones exposed to the environment, can be nitrated with tetranitromethane in the monomeric flagellin with a concomitant loss of the polymerization ability after about one tyrosine per mol flagellin has been nitrated. Nitrated flagellin, homogeneous with respect to molecular weight, degree of nitration and isoelectric point, could be isolated and characterized. On the other hand, it could be shown that in the polymeric flagellum the phenolic groups of all five tyrosine residues are inaccessible to perturbing and modifying reagents. It seems, therefore, that the integrity of the phenolic groups is necessary for the proper folding and aggregation of the flagellin subunits to form the stable helical flagella.

The human mast cell tryptase tetramer: a fascinating riddle solved by structure.

Tryptases, the predominant proteins of human mast cells, have been implicated as pathogenetic mediators of allergic and inflammatory conditions, most notably asthma. Until recently, the fascinating properties that distinguish tryptases among the serine proteinases, particularly their activity as a heparin-stabilized tetramer, resistance to most proteinaceous inhibitors, and preference for peptidergic over macromolecular substrates presented a riddle. This review solves this riddle with the help of the crystal structure of the human beta(2)-tryptase tetramer, but also indicates controversies between the unique quaternary architecture and some experimental data.

Refined three-dimensional structures of two cyanobacterial C-phycocyanins at 2.1 and 2.5 A resolution. A common principle of phycobilin-protein interaction.

The crystal structure of the light-harvesting protein-pigment complex C-phycocyanin (C-PC) from Mastigocladus laminosus (at 2.1 A resolution (1 A = 0.1 nm] has been refined by energy-restrained least-squares methods to a conventional R-factor of 21.7%. In the same way, the crystal structure of C-PC from Agmenellum quadruplicatum has been refined further (2.5 A, R = 18.4%); pyrrole rings C and D of the chromophore at position A84 have been corrected with respect to the previously reported structure. The two C-PC structures are very similar, 213 C alpha positions have a root-mean-square deviation of 0.49 A. Polar and ionic side-chain interactions are discussed in detail and the two subunits of C-PC from M. laminosus are compared to each other. All three chromophores are completely defined and their tetrapyrroles exhibit very similar geometry. The structure of a C-PC chromophore resembles a cleaved porphyrin which has been twisted roughly 180 degrees around the C-5-C-6 and C-14-C-15 bonds. Accordingly, the configuration/conformation of the chromophores is Z-anti, Z-syn, Z-anti (with the exception of the "configuration" of C-15 of chromophore B155, which is almost midway between Z and E). The three chromophores interact similarly with the protein. They arch around aspartate residues (A87, B87 and B39), and the nitrogens of pyrroles B and C are within hydrogen-bonding distance of one of the carboxylate oxygens. Most of the propionic side-chains of the chromophores form salt bridges with arginine and lysine residues. The updated relative chromophore distances and orientations confirm our conclusion that hexameric aggregates are probably the basic functional units, and that inter-hexameric energy transfer takes place preferentially via the central B84 chromophores.

Bovine chymotrypsinogen A X-ray crystal structure analysis and refinement of a new crystal form at 1.8 A resolution.

The X-ray structure of a new crystal form of chymotrypsinogen A grown from ethanol/water has been determined at 1.8 A resolution using Patterson search techniques. The crystals are of orthorhombic space group P212121 and contain two molecules in the asymmetric unit. Both independent molecules (referred to as A and B) have been crystallographically refined to a final R value of 0.173 with reflection data to 1.8 A resolution. Owing to different crystal contacts, both independent molecules show at various sites conformational differences, especially in segments 33-38, 142-153 and 215-222. If these three loops are omitted in a comparison, the root-mean-square (r.m.s.) deviation of the main-chain atoms of molecules A and B is 0.32 A. If segments 70-79, 143-152 and 215-221 are omitted, a comparison of either molecule A or molecule B with the chymotrypsinogen model of Freer et al. (1970) reveals an r.m.s. deviation of the alpha-carbon atoms of about 0.7 A. Compared with the active enzyme, four spatially adjacent peptide segments, in particular, are differently organized in the zymogen: the amino-terminal segment 11-19 runs in a rigid but strained conformation along the molecular surface due to the covalent linkage through Cys1; also segment 184-194 is in a rigid unique conformation due to several mutually stabilizing interactions with the amino-terminal segment; segment 216-222, which also lines the specificity pocket, adapts to different crystal contacts and exists in both chymotrypsinogen molecules in different, but defined conformations; in particular, disulfide bridge 191-220, which covalently links both latter segments, has opposite handedness in molecules A and B; finally, the autolysis loop 142 to 153 is organized in a variety of ways and in its terminal part is completely disordered. Thus, the allosteric activation domain (Huber & Bode, 1978) is organized in defined although different conformations in chymotrypsinogen molecules A and B, in contrast to trypsinogen, where all four homologous segments of the activation domain are disordered. This reflects the structural variability and deformability of the activation domain in serine proteinase proenzymes. If the aforementioned peptide segments are omitted, a comparison of our chymotrypsinogen models with gamma-chymotrypsin (Cohen et al., 1981) yields an r.m.s. deviation for alpha-carbon atoms of about 0.5 A. The residues of the "active site triad" are arranged similarly, but the oxyanion hole is lacking in chymotrypsinogen.(ABSTRACT TRUNCATED AT 400 WORDS)