Convergence of active center geometries.

Comparisons have been made between the active center geometries of lactate dehydrogenase and glyceraldehyde-3-phosphate dehydrogenase, chymotrypsin and papain, and glyceraldehyde-3-phosphate dehydrogenase and papain. In the dehydrogenases, orientation of the nicotinamide ring about the glycosidic bond is determined by the substrate stereochemistry. The proper positioning of the carboxyamide moiety allows for the close approach of the C4 atom on the nicotinamide and the reactive carbon of the substrate. It follows that, once the conformation of the substrate or substrate intermediate has been established with respect to the functional groups in the enzyme, the A- or B-side specificity of the nicotinamide ring is predetermined. Hence, dehydrogenases which are divergently evolving from a common precursor must maintain the nicotinamide specificity if the protein fold of the catalytic domain is conserved. The tetrahedral intermediates produced during acylation of chymotrypsin and papain are found to be of opposite hand, while those of papain and glyceraldehyde-3-phosphate dehydrogenase can be regarded to be of the same hand. Thus the serine proteases, subtilisin and those of the chymotrypsin family, are of one hand while the cysteine enzymes, glyceraldehyde-3-phosphate dehydrogenase and papain, are of the other.