Transition State Analog Inhibitors and Enzyme Catalysis

Enzymes, like midwives, ease the difficult passage of substances from one metastable situation to another. Catalysis appears to depend on the development of strong forces of attraction between the enzyme and activated forms of the substrate, which subside as the product is formed and the enzyme returns to its free state. It can be shown, on the basis of any rate theory that assumes the existence of an equilibrium of activation, that the substrate in its passage to product acquires a fleeting, but greatly elevated, affinity for the enzyme. So great is this affinity that a stable substance that looks to the enzyme like a substrate in the midst of its conversion to product is expected to be an unusually effective inhibitor ( 1-3). General reviews of some experimental and theoretical aspects of state affinity, and of early efforts to design potential transition state analogs, have appeared (4-7). As of early 1975, almost 60 possible examples had been described. In addition to their possible use as anti metabolites, inhibitors of this kind can provide a useful indication of the particular mechanism by which a substrate is transformed by the enzyme that it inhibits. The structure of an effective inhibitor should reflect and confirm the mechanism on which its design was based. In certain cases it may be difficult to isolate reaction intermediates, and inhibitors can sometimes provide mechanistic information that is not easily accessible by other methods. In the case of adenosine deaminase, for example, inhibitors provided the initial indication that the substrate adenosine is attacked directly by the second substrate, water. A double displacement mechanism, so common among hydrolytic enzymes, seemed improbable in the case of this particular enzyme, and subsequent work appears to support this conclusion. Structural features of strong inhibitors may also provide an indication of binding determinants at the active site, which are important for catalysis. The binding of analogs of activated intermediates in substrate transformation, in conjunction with cxact structural studies, may help to provide a dynamic picture of the succession of events that occur during the catalytic process, and may be useful for examining