GENERAL OCCURRENCE OF BINDING SYNERGISM IN ZINC PROTEASES AND ITS POSSIBLE SIGNIFICANCE

The observation of binding synergism has been successfully extended to include carboxypeptidases A and B. The behaviour of these two enzymes follows the same pattern previously found for three other Zn-protease s. Thus in all cases examined, the affinity of a suitable Zn-ligand is increased in the presence of a compound (specificity probe) which con tains the key structural features of specific substrates. A bifunction al ligand such as phosphonoacetate is particularly useful for generati ng synergism in both carboxypeptidases. Presumably the carboxylate moi ety binds to the C-terminal recognition site while the other functiona l group interacts with the metal ion. Several basic compounds (e.g. me thyl guanidine) act as effective specificity probes for carboxypeptida se B while phenol and other hydrophobic substances serve this purpose in carboxypeptidase A. The above phenomenon appears to be a mechanism designed to enhance catalytic efficiency through a substrate-induced c onformational change. We postulate that there is a requirement for at least one ionizable group at the active site. The proposed mechanism k eeps this group in the correct ionization state in the presence of wat er and increases its reactivity after exclusion of water by substrate binding. We suggest the term xerophilic shift for this process. Since proton transfer is a common process in enzyme reactions, the xerophili c-shift mechanism may play a similar role in many instances. It should therefore be possibe to detect binding synergism in a wide variety of enzymes.