Chymotrypsin three chains

Fio. 7. Stepwise formation of the three chains of a-chymotrypsin (A ) (64). ChTg, bovine chymotrypsinogen A. ChT-jr, -i and -a, respectively, ir-chymotrypsin (Ai), S-chymotrypsin (Aj), and -chymotrypsin (Ad. Neo-ChTg, neochymotrypsinogens (degraded and still activatable forms of chymotrypsinogen A). + and —, NHs- and COOH-terminal residues, respectively. T.C. and C.C., hydrolysis catalyzed by trypsin and chymotrypsin, respectively. Asp, asparagine residue. A, B, and C, chains A, B, and C of a-chymotrypsin (Ad. 

Figure 9.6. Three-Dimensional Structure of Chymotrypsin. The three chains are shown in ribbon form in orange, blue, and green. The side chains of the catalytic triad residues, including serine 195, are shown as ball-and-stick representations, as are two intrastrand and interstrand disulfide bonds.

Figure 10.32. Proteolytic Activation of Chymotrypsinogen. The three chains of a-chymotrypsin are linked hy two interchain disulfide bonds (A to B, and B to C).

The determination of the three-dimensional structure of chymotrypsin by David Blow in 1967 was a source of further insight into its mechanism of action. Overall, chymotrypsin is roughly spherical and comprises three polypeptide chains, linked by disulfide bonds. It is synthesized as a single polypeptide, termed chymotrypsinogen, which is activated by the proteolytic cleavage of the polypeptide to yield the three chains. The active site of chymotrypsin, marked by serine 195, lies in a cleft on the surface of the enzyme (Figure 9.6). 

The polypeptide chain of chymotrypsinogen, the inactive precursor of chymotrypsin, comprises 245 amino acids. During activation of chymotrypsinogen residues 14-15 and 147-148 are excised. The remaining three polypeptide chains are held together by disulfide bridges to form the active chymotrypsin molecule. 

Figure 11.7 Schematic diagram of the structure of chymotrypsin, which is folded into two antiparallel p domains. The six p strands of each domain are red, the side chains of the catalytic triad are dark blue, and the disulfide bridges that join the three polypeptide chains are marked in violet. Chain A (green, residues 1-13) is linked to chain B (blue, residues 16-146) by a disulfide bridge between Cys 1 and Cys 122. Chain B is in turn linked to chain C (yellow, residues 149-245) by a disulfide bridge between Cys 136 and Cys 201. Dotted lines indicate residues 14-15 and 147-148 in the inactive precursor, chmotrypsinogen. These residues are excised during the conversion of chymotrypsinogen to the active enzyme chymotrypsin.

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... 

Figure 9-6. Selective proteolysis and associated conformational changes form the active site of chymotrypsin, which includes the Aspl 02-His57-Ser195 catalytic triad. Successive proteolysis forms prochymotrypsin (pro-CT), Jt-chymotrypsin (jt-CT),and ultimately a-chymotrypsin (a-CT), an active protease whose three peptides remain associated by covalent inter-chain disulfide bonds.

Fig. 42. Five superimposed examples of classic /3 bulges, in stereo chymotrypsin Phe-41, Cys-42 opposite Leu-33 chymotrypsin Ala-86, Lys-87 opposite Lys-107 con-canavalin A Leu-107, Ser-108 opposite Ala-196 carbonic anhydrase C Ile-90, Gln-91 opposite Val-120 and staphylococcal nuclease lie-15, Lys-16 opposite Lys-24. Here and in Fig. 43 side chains are shown (out to Cy) only for the three positions within the bulge. At the very bottom of this figure only the backbone is shown, where the two strands overlap in this projection.

For example, chymotrypsin cleaves peptides on the C-terminal side of aromatic amino acid residues phenylalanine, tyrosine, and tryptophan, and to a lesser extent some other residues with bulky side-chains, e.g. Leu, Met, Asn, Gin. On the other hand, trypsin cleaves peptides on the C-terminal side of the basic residues arginine and lysine. Elastase usually catalyses hydrolysis of peptide bonds on the C-terminal side of neutral aliphatic amino acids, especially glycine or alanine. These three pancreatic enzymes are about 40% identical in their amino acid sequences, and their catalytic mechanisms are nearly identical. 

Proteasomes of Thermoplasma contain a single type of p subunit but eukaryotic proteasomes contain subunits with at least three distinct substrate preferences.347 M9c They all appear to use the same hydrolytic mechanism but in their substrate specificities they are chymotrypsin-like, peptidylglutamyl-peptide hydrolyzing, branched chain amino acid preferring, and small neutral amino acid preferring based on the P, amino acid residue. In the spleen some of the P subunits of the proteasomes appear to have been replaced by proteins encoded by the major histocompatibility complex of the immune system (Chapter 31).347 This may alter the properties of the proteasome to favor their function in antigen processing. Proteasomes are also ATP- and ubiquitin-dependent, as discussed in Section 6. 

Trypsin, chymotrypsin, and elastase—three members of the serine protease family—catalyze the hydrolysis of proteins at internal peptide bonds adjacent to different types of amino acids. Trypsin prefers lysine or arginine residues chymotrypsin, aromatic side chains and elastase, small, nonpolar residues. Carboxypeptidases A and B, which are not serine proteases, cut the peptide bond at the carboxyl-terminal end of the chain. Carboxypeptidase A preferentially removes aromatic residues carboxypeptidase B, basic residues. (Illustration copyright by Irving Geis. Reprinted by permission.)... 

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