Superfamily chymotrypsin

A closer examination of these essential residues, including the catalytic triad, reveals that they are all part of the same two loop regions in the two domains (Figure 11.10). The domains are oriented so that the ends of the two barrels that contain the Greek key crossover connection (described in Chapter 5) between p strands 3 and 4 face each other along the active site. The essential residues in the active site are in these two crossover connections and in the adjacent hairpin loops between p strands 5 and 6. Most of these essential residues are conserved between different members of the chymotrypsin superfamily. They are, of course, surrounded by other parts of the polypeptide chains, which provide minor modifications of the active site, specific for each particular serine proteinase. 

This is nicely illustrated by members of the chymotrypsin superfamily the enzymes chymotrypsin, trypsin, and elastase have very similar three-dimensional structures but different specificity. They preferentially cleave adjacent to bulky aromatic side chains, positively charged side chains, and small uncharged side chains, respectively. Three residues, numbers 189, 216, and 226, are responsible for these preferences (Figure 11.11). Residues 216... 

A family of cysteine proteinase inhibitors different from the cystatin superfamily was isolated from pineapple stem acetone powder. These inhibitors have a Mr of about 5800 and are composed of a longer (41 amino acids) and a shorter (11 amino acids) peptide chain connected with disulfide bonds [29]. The conserved sequence Gln-Val-Val-Ala-Gly of the cystatins is not present in these inhibitors, indicating a different mechanism of interaction. The bromelain inhibitor VI was found to share similar folding and disulfide bond connectivities with the Bowman-Birk trypsin/chymotrypsin inhibitor from soybean [30,31]. The physiological role of these inhibitors remains unclear. 

The enzyme chymotrypsin provides a good example of the strategies and amino acid side chains used by enzymes to lower the amount of activation energy required. Chymotrypsin is a digestive enzyme released into the intestine that catalyzes the hydrolysis of specific peptide bonds in denatured proteins. It is a member of the serine protease superfamily, enzymes that use a serine in the active site to form a covalent intermediate during proteolysis. In the overall hydrolysis reaction, an OH from water is added to the carbonyl carbon of the peptide bond, and an to the N, tha-eby cleaving the bond (Fig. 8.8). The bond that is cleaved is called the scissile bond. 

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