Crystal structure of carboxypeptidase T from Thermoactinomyces vulgaris.

The crystal structure of carboxypeptidase T from Thermoactinomyces vulgaris has been determined at 0.235-nm resolution by X-ray diffraction. Carboxypeptidase T is a remote homologue of mammalian Zn-carboxypeptidases. In spite of the low degree of amino acid sequence identity, the three-dimensional structure of carboxypeptidase T is very similar to that of pancreatic carboxypeptidases A and B. The core of the protein molecule is formed by an eight-stranded mixed β sheet. The active site is located at the C-edge of the central (parallel) part of the β sheet. The structural organization of the active centre appears to be essentially the same in the three carboxypeptidases. Amino acid residues directly involved in catalysis and binding of the C-terminal carboxyl of a substrate are strictly conserved. This suggests that the catalytic mechanism proposed for the pancreatic enzymes is applicable to carboxypeptidase T and to the whole family of Zn-carboxypeptidases. Comparison of the amino acid replacements at the primary specificity pocket of carboxypeptidases A, B and T provides an explanation of the unusual ‘A + B’ type of specificity of carboxypeptidase T. Four calcium-binding sites localized in the crystal structure of carboxypeptidase T could account for the high thermostability of the protein.