A suicide inhibitor, also known as a suicide substrate, is a structural analogue of a substrate for an enzyme that contains a latent reactive group. Following reversible binding of the inhibitor at the enzyme’s active site, this group is activated by the enzyme during the course of its catalysis of the normal reaction to release a chemically reactive group within the active site. This reactive group can react covalently with an appropriately located active-site residue, inactivating the enzyme. That is, the enzyme catalyzes its own suicide (1). A wide range of functional groups can be catalytically unmasked by their target enzymes to produce electrophiles that cause inactivation. These include acetylenes, olefins, and beta-substituted amino acids. The chemical mechanism of their action has been outlined (2).
Suicide substrates are characterized kinetically by the concomitant formation of product and inactivation of the enzyme. The reactions can be formulated as
After the reversible formation of the enzyme-suicide substrate complex (E.SS), the enzyme catalyzes the reversible formation of an EX complex, where X is an activated intermediate that can either (1) be released as product P or (2) modify the enzyme. The partition ratio that describes the relative rates of the two reactions, k7/k5, remains constant over the time course of the reaction and is, in effect, a measure of enzyme turnover relative to enzyme inactivation.
With a suicide inhibitor, a plot of the concentration of P as a function of time would show a time-dependent decrease in the rate of product formation, long before the equilibrium of the reaction was reached. Further, at infinite time, the enzyme would be completely inactivated, and the kinetic plot would exhibit a horizontal asymptote. In this respect, the behavior of a suicide substrate is similar to that of a slow-binding enzyme inhibitor whose action is, essentially irreversible. At infinite time, with classical, non-allosteric enzymes, a stoichiometric amount of intermediate should be covalently bound to all the enzyme molecules so that PJE, the ratio of the total amount of product formed, Px, to the total amount of enzyme present, Et, equals ky/k^, the partition ratio (3). Partition ratios vary from one for the inactivation of GABA aminotransferase by gabaculine and L-aspartate aminotransferase by vinylglycine, to values in excess of 1000 (1). The lower the partition ratio, the greater is the effectiveness of a suicide inhibitor as an enzyme inactivator.
