Tertiary structure, carboxypeptidase

The shape of a large protein is influenced by many factors including of course Its primary and secondary structure The disulfide bond shown m Figure 27 18 links Cys 138 of carboxypeptidase A to Cys 161 and contributes to the tertiary structure Car boxypeptidase A contains a Zn " ion which is essential to the catalytic activity of the enzyme and its presence influences the tertiary structure The Zn ion lies near the cen ter of the enzyme where it is coordinated to the imidazole nitrogens of two histidine residues (His 69 His 196) and to the carboxylate side chain of Glu 72... 

The shape of a large protein is influenced by many factors, including, of course, its primary and secondary structure. The disulfide bond shown in Figure 27.18 links Cys-138 of carboxypeptidase A to Cys-161 and contributes to the tertiary structure. Car-... 

Fig. 1.28. Tertiary structures (schematic spiral a-helix, arrow pleated sheet) of the p-chain of hemoglobin (a), of triosephosphate isomerase (b) and carboxypeptidase (c). (according to Walton, 1981)...

The conformation of a number of enzymes is changed by the binding of the substrate. An example is carboxypeptidase A, in which the Try located in the active site moves approximately 12 A towards the substrate, glycyl-L-phenylalanine, to establish contact. This and other observations support the dynamic induced-fit model proposed by Koshland (1964). Here, only the substrate has the power to induce a change in the tertiary structure to the active form of the enzyme. Thus, as the substrate molecule approaches the enzyme surface, the amino acid... 

The formation of tertiary structure buries a certain amount of secondary structure surfaces which varies from 32% to 60%. The proportion of the surface which is buried varies for each protein and increases with the molecular weight of the protein. In fact, it is the proportion of the nonpolar surface that becomes buried during folding, which increases with the molecular weight. From 60% for pancreatic trypsin inhibitor, it varies up to 79% in carboxypeptidase among the six proteins considered by Chothia... 

Many enzymes contain metal ions as an integral part of their structures (e.g., zinc in ALP and carboxypeptidase A). The function of the metal may be to stabilize tertiary and quaternary protein structures. Removal of divalent metal ions by treatment with an appropriate concentration of EDTA solution is accompanied by conformational changes with inactivation of the enzyme. The enzyme can often be reactivated by dialysis against a solution of the appropriate metal ion or simply by adding the ion to the reaction mixture. Reactivation may take some time, because rearrangement of the polypeptide chains into the active conformation is not instantaneous. 

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