Elastase catalytic activity

Schematic diagrams of the amino acid sequences of chymotrypsin, trypsin, and elastase. Each circle represents one amino acid. Amino acid residues that are identical in all three proteins are in solid color. The three proteins are of different lengths but have been aligned to maximize the correspondence of the amino acid sequences. All of the sequences are numbered according to the sequence in chymotrypsin. Long connections between nonadjacent residues represent disulfide bonds. Locations of the catalytically important histidine, aspartate, and serine residues are marked. The links that are cleaved to transform the inactive zymogens to the active enzymes are indicated by parenthesis marks. After chymotrypsinogen is cut between residues 15 and 16 by trypsin and is thus transformed into an active protease, it proceeds to digest itself at the additional sites that are indicated these secondary cuts have only minor effects on the enzymes s catalytic activity. (Illustration copyright by Irving Geis. Reprinted by permission.)...

In particular, excessive proteolysis of elastin by HLE has been implicated in pulmonary emphysema [19]. In this case, the imbalance appears to result from reduced levels of active extracellular alpha,-proteinase inhibitor (a,-PI), the primary plasma inhibitor of HLE. This decrease is caused either by a genetic disorder (PiZZ phenotype individuals) or by reduction in the elastase inhibitory capacity (EIC) of ai-PI due to its oxidative inactivation by tobacco smoke [20]. The detailed evidence supporting the potential role of elastase in the development of emphysema has been extensively reviewed [21] and will not be repeated here. The fact that HLE is also a potent secretagogue [22] may play a role in several disease states, including cystic fibrosis [23], chronic bronchitis [24], and acute respiratory distress syndrome (ARDS) [25]. The mechanism of the secretagogue activity is not known, but, since the HLE-induced secretion can be blocked by specific HLE inhibitors, it appears to require catalytic activity by the enzyme [26]. 

Studies with ab initio types of hybrid potential include the early work of Weiner et al. on the nature of catalysis in trypsin and the studies of the catalytic activity of phospholipase A2 by Hillier et al. Investigations with semiempirical hybrid potentials are more extensive and include calculations of the reactions in triosephosphate isomerase by Bash et al. and in chorismate mutase by Lyne et al. and a study of the proton jump in the catalytic triad of human neutrophil elastase. The study of the chorismate mutase reaction was especially interesting because the enzyme is the only known one that catalyzes a pericyclic reaction that also occurs readily in solution. The results of the hybrid study were particularly lucid in this case because the enzyme works, not by chemically catalyzing the reaction, but by preferentially binding a distorted form of the substrate and stabilizing the transition state. 

FIGURE 16.16 Comparison of the amino acid sequences of chymotrypsinogen, trypsino-gen, and elastase. Each circle represents one amino acid. Nmnbering is based on the sequence of chymotrypsinogen. Filled circles indicate residues that are identical in all three proteins. Disnlfide bonds are indicated in yellow. The positions of the three catalytically important active-site residues (His, Asp °-, and Ser ) are indicated. 

This is a 29-kDa protein that has NH 2-terminal sequence homology with elastase and cathepsin G. However, it contains glycine and not serine at the predicted catalytic site, and so lacks protease and peptidase activity. Purified azurocidin kills a range of organisms (e.g. E. coli, S.faecalis, and C. albicans) in vitro. It functions optimally at pH 5.5 and in conditions of low ionic strength. 

The presence of a covalent acyl-enzyme intermediate in the catalytic reaction of the serine proteases made this class of enzymes an attractive candidate for the initial attempt at using subzero temperatures to study an enzymatic mechanism. Elastase was chosen because it is easy to crystallize, diffracts to high resolution, has an active site which is accessible to small molecules diffusing through the crystal lattice, and is stable in high concentrations of cryoprotective solvents. The strategy used in the elastase experiment was to first determine in solution the exact conditions of temperature, organic solvent, and proton activity needed to stabilize an acyl-enzyme intermediate for sufficient time for X-ray data collection, and then to prepare the complex in the preformed, cooled crystal. Solution studies were carried out in the laboratory of Professor A. L. Fink, and were summarized in Section II,A,3. Briefly, it was shown that the chromophoric substrate -carbobenzoxy-L-alanyl-/>-nitrophenyl ester would react with elastase in both solution and in crystals in 70 30 methanol-water at pH 5.2 to form a productive covalent complex. These... 

Fig. 18. The active site region of the electron density difference map between N-carbobenzoxy-L-alanine-elastase at —SS C and native elastase at the same temperature. The resolution is 3.5 A. The bilobed feature is consistent with the binding of the alanyi portion of the substrate to the oxygen of the catalytic serine, with weak interaction of the carbobenzoxy group to the surface of the enzyme.

Several classes of (3-lactamases, often encoded in transmissible plasmids, have spread worldwide rapidly among bacteria, seriously decreasing the effectivenss of penicillins and other (3-lactam anti-biotics.t y Most (3-lactamases (classes A and C) contain an active site serine and are thought to have evolved from the dd transpeptidases, but the B typey has a catalytic Zn2+. The latter, as well as a recently discovered type A enzyme,2 hydrolyze imipenem, currently one of the antibiotics of last resort used to treat infections by penicillin-resistant bacteria. Some (3-lactam antibiotics are also powerful inhibitors of (3-lactamases.U/aa/bb These antibiotics may also have uses in inhibition of serine proteasesCC/dd such as elastase. Some antibiotic-resistant staphylococci produce an extra penicillin-binding protein that protects them from beta lactams.ee Because of antibiotic resistance the isolation of antibiotics from mixed populations of microbes from soil, swamps, and lakes continues. Renewed efforts are being... 

The charge relay system is found at the active site of a group of enzymes called serine proteases. They include chymotrypsin, trypsin, a-lytic protease, elastase, and subtilisin. It is interesting that the charge relay system was found in enzymes belonging to different branches of diemical evolution (chymotrypsin and subtilisin). This suggests that this system is a hydrolytic catalytic system of general importance which is derived solely from amino acid residues. 

A final group of covalent small-molecule inhibitors of proteases are mechanism-based inhibitors. These inhibitors are enzyme-activated irreversible inhibitors, and they involve a two-hif mechanism that completely inhibits the protease. Some isocoumarins and -lactam derivatives have been shown to be mechanistic inhibitors of serine proteases. A classic example is the inhibition of elastase by several cephalosporin derivatives developed at Merck (Fig. 8). The catalytic serine attacks and opens the -lactam ring of the cephalosporin, which through various isomerization steps, allows for a Michael addition to the active site histidine and the formation of a stable enzyme-inhibitor complex (34). These mechanism-based inhibitors require an initial acylation event to take place before the irreversible inhibitory event. In this way, these small molecules have an analogous mechanism of inhibition to the naturally occurring serpins and a-2-macroglobin, which also act as suicide substrates. 

Other proteases employ the same catalytic strategy. Some of these proteases, such as trypsin and elastase, are homologs of chymotrypsin. In other proteases, such as subtilisin, a very similar catalytic triad has arisen by convergent evolution. Active-site structures that differ from the catalytic triad are present in a number of other classes of proteases. These classes employ a range of catalytic strategies but, in each case, a nucleophile is generated that is sufficiently powerful to attack the peptide carbonyl group. In some enzymes, the nucleophile is derived from a side chain whereas, in others, an activated water molecule attacks the peptide carbonyl directly. 

Understanding how HLE inhibitors work and/or designing new inhibitors requires a model of HLE s active-site and an understanding of its mechanism of action. All serine proteinases share a similar catalytic region and mechanism of action but differ in several amino acids in the extended substrate-binding region. These changes are responsible for the specificity differences between HLE and other serine proteinases. In some cases analysis of the enzyme-inhibitor interactions has only been carried out with other related enzymes, and those results are referenced as appropriate. One closely related enzyme, porcine pancreatic elastase (PPE, EC 3.4.21.36) has... 

Figure 5. Variation of the protein MEP along the active sites of some enzymes. It a-chymotiypsin, 2t p-tiypsin, 3 porcine pancreatic elastase, 4 Streptomyces Griseus hydrolase, Si a-lytic protease, 6t subtilisin NOVO, 7i acetylcholinesterase, 8> lipase A, 9 lysozyme, lOi D-xyloie isomerase. Point A is at OG of die active serine in 1-8, at the bisector ofODl and OD2 of Asp-52 in 9, at Ol of the cyclic xylose m 10. Point B is atNE2 of the catalytic histidine in 1-8, in the first trisector of points A and Din 9, atNEl ofHis-S4in 10. Point C is at ND1 of the catalytic histidine in 1-8 and 10, at the second trisector of points A and D in 9. Point D is at the bisector of the carboxylate oxygens of the catalytic Asp or Glu side chains.

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