Inhibitor binding ordered mechanism

Bimolecular processes are very common in biological systems. The binding of a hormone to a receptor is a bimolecular reaction, as is substrate and inhibitor binding to an enzyme. The term bimolecular mechanism applies to those reactions having a rate-limiting step that is bimolecular. See Chemical Kinetics Molecularity Reaction Order Elementary Reaction Transition-State Theory... 

Research in this field is ongoing aiming to understand the mechanism of action of kinetic inhibitors. Lee and Englezos (2005) showed that inclusion of polyethylene oxide (PEO) to a kinetic inhibitor solution was found to enhance by an order of magnitude the performance of the hydrate inhibitor. Binding of inhibitor molecules to the surface of hydrate crystals was considered to be the key aspect of the mechanism of kinetic inhibition (Anderson et al.,... 

In practice, uncompetitive and mixed inhibition are observed only for enzymes with two or more substrates—say, Sj and S2—and are very important in the experimental analysis of such enzymes. If an inhibitor binds to the site normally occupied by it may act as a competitive inhibitor in experiments in which [SJ is varied. If an inhibitor binds to the site normally occupied by S2, it may act as a mixed or uncompetitive inhibitor of Si. The actual inhibition patterns observed depend on whether the and S2-binding events are ordered or random, and thus the order in which substrates bind and products leave the active site can be determined. Use of one of the reaction products as an inhibitor is often particularly informative. If only one of two reaction products is present, no reverse reaction can take place. However, a product generally binds to some part of the active site, thus serving as an inhibitor. Enzymologists can use elaborate kinetic studies involving different combinations and amounts of products and inhibitors to develop a detailed picture of the mechanism of a bisubstrate reaction. 

Noncompetitive inhibitions result from combination of the inhibitor with an enzyme form other than the one the substrate combines with, and one that is present at both high and low levels of the substrate. An example is a dead-end inhibitor resembling the first substrate in an ordered mechanism. It is competitive versus A, but noncompetitive versus B, because B cannot prevent the binding of the inhibitor to free enzyme. In a random mechanism, an inhibitor binding at one site is noncompetitive versus a substrate binding at another site. 

It is difficult to deconvolute how kinase inhibitor binding mode has implications in the clinic. In general, selectivity tends to increase in the order (i) purine site, (ii) selectivity pocket, and (iii) allosteric site. The ability to overcome resistance mutations in cancer therapy tends to be inversely related to this selectivity pattern. Accordingly, allosteric kinase inhibition may become a preferred mechanism for clinical indications other than cancer. 

Inhibition by substrates that also act as dead-end inhibitors was discussed in Volume II. In an ordered mechanism, however, addition of an inhibitor that mimics the first substrate may permit binding of the second substrate. In such a situation the presence of the inhibitor will induce substrate inhibition by the second substrate, as long as the first substrate addition is not in rapid equilibrium ... 

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