Kinase substrate-competitive inhibition

Substrate-competitive inhibition is a well known strategy for targeting enzymes, which has been applied successfully in enzyme classes such as the proteases. Nevertheless, its use for kinase inhibition has met with little success. One of the reasons is the rather stretched substrate pocket of kinases. Kinases are likely to use additional binding pockets, which are not located in the immediate environment of the active site [16, 17]. Therefore, kinases lack the specific hydrophobic pockets that could serve as targets for peptidomimetics, as occurs with HIV protease or thrombin. 

AG 1024 has been extensively studied as an IGFR inhibitor [70] and is a substrate competitive inhibitor of this kinase [71]. AG1024 also inhibits other kinases including c-Kit [72]. Additional studies will be needed, including a direct measurement of Abl activity and possible subsequent testing against the imatinib resistant Abl point mutations, to ascertain the possible therapeutic utility of AG 1024. 

The antiviral mechanism of action of triflnridine involves inhibition of viral DNA synthesis. Triflnridine monophosphate irreversibly inhibits thymidylate synthetase, and trifluridine triphosphate is a competitive inhibitor of thymidine triphosphate incorporation into DNA by DNA polymerases. Trifluridine is incorporated into viral and ceUnlar DNA. Trifluridine-resistant HSV with altered thymidine kinase substrate specificity can be selected in vitro, and resistance in clinical isolates has been described. 

What is the mechanism of this drug induced interference with pyrimidine metabolism The finding of orotic acid and orotidine in the urine is similar to that seen after therapy with the drug 6-azauridine. This compound is converted by a pyrimidine kinase to 6-azauridylic acid which competitively inhibits ODC (Handschumacher, 1960). By analogy a nucleotide derivative of allopurinol, oxipurinol or a metabolite may be a competitive inhibitor of ODC. In addition, a purine or pyrazolopyrimidine base could act as an inhibitor of OPRT or both mechanisms could work in concert. Finally, depletion of an essential substrate such as PP-ribose-P could also contribute. 

Both isoforms showed very high levels of CKase activity when expressed in these heterologous systems. Unlike the CKase enzymes of yeast and animals, the two soybean CKase isoforms demonstrated negligible ethanolamine kinase activity. Other potential substrates for the CKase enzyme include monomethylethanolamine (MME) and dimethylethanolamine (DME). A combination of competitive inhibition studies and direct enzyme assays revealed that neither isoform can effectively utilize MME as a substrate, and DME can act as a substrate only for the GmCK2 encoded enzyme. 

Competitive inhibitors are inhibitors which have an effect on the but not on the V of an enzyme-catalysed reaction. The V is unchanged because the number of functional active sites is not altered but a greater substrate concentration is required to achieve the maximum utilization of the sites. Consequently, the for the substrate increases. Competitive inhibition may be overcome by the addition of more substrate to the enzyme reaction mixture. Competitive inhibitors often bear a structural similarity to the substrate and compete with the substrate for the active sites of the enzyme, i.e. they are isosteric. However, competitive inhibitors are not necessarily structurally analogous to the substrate, e.g. salicylate inhibition of 3-phospho-glycerate kinase, and may bind to a site distinct from the active site, e.g. L-isoleucine inhibition of threonine deaminase from Escherichia coli. The classical example of competitive inhibition is the action of malonate on succinate dehydrogenase (Figure 6.9) which advanced the elucidation of the... 

Phosphatidylcholine is preferentially synthesized in lactating mammary tissue (Kinsella, 1973), possibly regulated by the differential activities of choline kinase and ethanolamine kinase. Choline kinase has a lower Km and a higher Vmax with its substrate than does ethanolamine kinase. Also, choline kinase is inhibited slightly by ethanolamine, whereas choline is a potent competitive inhibitor of ethanolamine kinase. Thus, the intracellular concentration of choline probably regulates the synthesis of these two phos-phoglycerides (Infante and Kinsella, 1976). 

Thus far, no potent ATP competitive inhibitors of ERKl/2 have been reported. Because of the likely cross-reactivity of ATP competitive inhibitors, compounds that inhibit through interactions outside the ATP binding pocket are attractive. Substrates bind to MAPKs on the opposite face from the active site in a region, which are sometimes called the common docking or CD site this site can also influence kinase conformation (21). Small molecule inhibitors that bind to the CD site of ERK2 have been identified recently but have not received extensive testing (23). 

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