Substrate specificity chymotrypsin family

This endopeptidase [EC 3.4.21.20], a member of the peptidase family SI, has substrate specificity similar to that of chymotrypsin C. 

The properties and spatial arrangement of the amino acid residues forming the active site of an enzyme will determine which molecules can bind and be substrates for that enzyme. Substrate specificity is often determined by changes in relatively few amino acids in the active site. This is clearly seen in the three digestive enzymes trypsin, chymotrypsin and elastase (see Topic C5). These three enzymes belong to a family of enzymes called the serine proteases - serine because they have a serine residue in the active site that is critically involved in catalysis and proteases because they catalyze the hydrolysis of peptide bonds in proteins. The three enzymes cleave peptide bonds in protein substrates on the carboxyl side of certain amino acid residues. 

Trypsin and factor Xa (fXa) are two members of the chymotrypsin family that have 38% sequence identity on the amino acid level and have distinguishable substrate specificities. Recently, the N-terminal 13-barrel of fXa and the C-terminal /3-barrel of trypsin were fused at a rationally designed site in the linker region between the two domains in order to create a hybrid fXa-trypsin protease (Hopfner et al., 1998). The fXa-trypsin hybrid was highly active and more active than either parent on three of the ten substrates assayed, as determined by k /Km. For most substrates, the activity of fXa-trypsin was an admixture of the two parents, probably because trypsin had higher activity than fXa for all the substrates tested. 

Within each protease family, individual members will differ in their substrate specificity. Most proteases have extended substrate binding sites and will bind to and recognize several amino acid residues of a polypeptide substrate (see Figure 2). Usually one of these will be the primary binding site. For example, in the serine proteases chymotrypsin, trypsin, and elastase, the primary substrate binding site is the Si subsite... 

Phage libraries have also been used to study the substrate specificity of enzymes by finding an improved artificial substrate. Coombs et al. (69) reported the detailed assessment of specificity for a serine protease belonging to the a-chymotrypsin family, the prostate specific antigen (PSA). They used both substrate optimization by singlepoint mutations and phage display libraries. The sequence of the 14-member substrate 10.2 (70) was used to start the iterative optimization process (Fig. 10.11) in which substitution or exchange of the PI, P2, or P2 residues increased the substrate affinity... 

Chymotrypsin a family of structurally and cataly-tically homologous serine proteases (see Proteases) whose precursors (zymogens) are formed and stored in the pancreas Chymotrypsinogen A (pi 9.1, 245 amino acids, M, 25,670) is cationic at pH 8, whereas chymotrypsinogen B (pi 5.2, 248 amino acids M 25,760) is anionic at pH 8. The latter is lacking in swine pancreas which contains chymotrypsinogen C (281 amino acids lip-rich, M, 31,800). Ihe activiated forms have different substrate specificities All C hydrolyse preferentially tyrosyl and tiyptophanyl pep-... 

The serine endopeptidases include the chymotrypsin family (EC 3.4.21.1), trypsin (EC 3.4.21.4), elastase (EC 3.4.21.37), thrombin (EC 3.4.21.5), subtilisin (EC 3.4.21.62) and a-lytic proteases (EC 3.4.21.12). The enzymes are all endopeptidases. The substrate specificities of the individual members of this group are often quite different, which is attributed to different structures of the binding pockets. 

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