Analogues that form a covalent bond

1 Active-site-directed irreversible inhibitors (ASDIIs) 

This inhibitor can even discriminate between isoenzymes. Thus, whereas it irreversibly inhibits the lactic dehydrogenase of skeletal muscle, it does not affect that of the heart. A related compound, 5-(phenoxycarbonylamino)salicylic acid, irreverisbly inhibits the heart (but not the skeletal) isoenzyme, whereas its 4-isomer irreversibly inhibits the skeletal (but not the heart) isoenzyme (Baker and Patel, 1964 Baker, 1967). 

For further reading on active-site-directed irreversible inhibitors, see Baker (1967). This is an idea which may lead to useful new agents, but has yet to produce a widely used drug. 

A successful example of an IMBI of this kind is 2-difluoromethylomithine 

Difluoromethylornithine (9.88) has a high therapeutic index and is proving curative in clinical trials of leukaemia and malaria. Unfortunately it has a very short half-life in the human body and that leads to difficulty in maintaining adequate doses. 

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Affinity Labels. Active site-directed, irreversible inhibitors or affinity labels are usually substrate analogues that contain a reactive electrophilic functional group. In the first step, they bind to the active site of the target enzyme in a reversible fashion. Subsequentiy, an active site nucleophile in close proximity reacts with the electrophilic group on the substrate to form a covalent bond between the enzyme and the inhibitor, typically via S 2 alkylation or acylation. Affinity labels do not require activation by the catalysis of the enzyme, as in the case of a mechanism-based inhibitor. 

More striking is the binding of the lactone III to /TV-acetyl-D-glucosaminidase. The dissociation constant of 5 X 10 7 M is 4000 times smaller than the KM of 2 X 10-3 M for the pyranoside substrate V.12 However, it is possible in this example that the enzyme forms a covalent bond with the analogue so that the tight binding does not result solely from noncovalent binding.13... 

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