Multiple binding sites, enzyme inhibitors

It is also predicted that an allosteric inhibitor should bind noncooperatively to an enzyme that binds its substrates cooperatively, since the inhibitor binds to the predominant T state. The converse should be true for activators binding to multiple binding sites in the R state. 

There are many indications that enzymes exist possessing multiple binding sites, usually in pairs, for the substrates, activators and inhibitors. These sites are usually carried by paired polypeptide subunits (or protomers) of the active molecule they may be identical or different. The oligomer must have at least one point of symmetry (cf. Fig. 18). Such enzymes are known as allosteric enzymes. 

Protein kinases can be classified according to the amino acid residue that is phosphorylated in the cellular process. Consequently, there are tyrosine-specific kinases and serine/threonine kinases. Tyrosine kinases are a family of tightly regulated enzymes, and the aberrant activation of various members of this family is one of the hallmarks of cancer. Tyrosine phosphorylation has been linked to multiple cell growth and differentiation pathways. Imatinib mesylate (1) is a tyrosine kinase inhibitor (TKI). An important characteristic of imatinib mesylate (1) is that it is an ATP-competitive inhibitor. It binds at the ATP binding site and blocks ATP binding thereby inhibiting kinase activities. 

The other extreme is when a compound binds only to the E S complex but not to the free enzyme, in which case uncompetitive inhibition occurs (Scheme 2). Although it is rare in single substrate reactions, it is common in multiple substrate systems. An inhibitor of a two-substrate enzyme that is competitive against one of the substrates often is found to give uncompetitive inhibition when the other substrate is varied. The inhibitor binds at the active site but only prevents the binding of one of the substrates. 

An extremely important aspect of bisubstrate inhibitors is that they usually utilize multiple binding points to achieve their high affinities for enzymes. The current catechol analog COMT inhibitors rely on the nitro-substituent to provide potency. When uncertainty about the cause of occurrences of hepatoxicity in Parkinson s disease patients on the cocktail therapy was directed at the nitro-substituent, the question quickly arose as to whether the nitro-substituent was required for bisubstrate analog potency. It was shown that utilizing a 4-methyl-phenyl group in place of the nitro-substituent (16) resulted in an inhibitor with only twofold less potency (IC50 value of 2 3 versus 9 nmol When the new inhibitor was modeled into the COMT active site,... 

Such effects are not unique to carboxypeptidase A. The rate of substrate polysaccharide hydrolysis by lysozyme is remarkably dependent on the polysaccharide chain length 153, 154). Both steady-state kinetic studies and X-ray crystallographic studies on enzyme-inhibitor complexes for chymotrypsin 156) trypsin 156), elastase 157), and subtilisin 158) are indicative of the existence of multiple-loci substrate binding sites. Furthermore, the dependence of Acat on substrate chain length for all these enzymes strongly implies that the filling... 

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