Enzyme quiescent affinity labels

Figure 8.2 Mechanisms of irreversible enzyme inactivation. (A) Nonspecific affinity labeling, (B) quiescent affinity labeling, and (C) mechanism-based inactivation.

However, the conversion of omeprazole to the active sulphenamide does not result in formation of a reversible enzyme inhibitor, but rather results in in situ formation of a powerful affinity label. Hence we can consider omeprazole to be a unique example of quiescent affinity labeling in which selectivity results from the unique environment of the target enzyme. 

More recently attempts to generate highly selective quiescent affinity labels have been made for a number of protease and kinase targets. As examples, inhibitors of the Rhinovirus 3C protease (Mathews et al 1999) and of the epidermal growth factor receptors (Boschelli, 2002), both incorporating Michael acceptors to covalently inactivate cysteine residues in their target enzymes (Lowry and Richardson, 1981 Figure 8.6), have entered human clinical trials for the treatment of rhinovirus infection and cancer, respectively. 

Suicide substrates and quiescent affinity labels, unlike the other types of inhibitors discussed in this chapter, form covalent bonds with active site nucleophiles and thereby irreversibly inactivate their target enzymes. A suicide substrate,191 also described by Silverman in a comprehensive review1101 as a mechanism-based inactivator, is a molecule that resembles its target enzyme s true substrate but contains a latent (relatively unreactive) electrophile. When the target enzyme attempts to turn over the... 

CONTENTS Introduction to the Series An Editor s Foreword, Albert Padwa. Preface, Bruce E. Maryanoff and Cynthia A. Maryanoff. Computer Assisted Molecular Design Related to the Protein Kinase C Receptor, Paul A. Wenderand Cynthia M. Cribbs. Chemistry and Biology of the Immunosuppressant (-)-FK-506, Ichiro Shinkai and Nolan H. Sigal. The Development of Ketorolac Impact on Pyrrole Chemistry and on Pain Therapy, Joseph M. Muchowski. Application of Silicon Chemistry in the Corticosteroid Field, Douglas A. Livingston. Hu-perzine A-A Possible Lead Structure in the Treatment of Alzheimers Disease, Alan P. Kozikowski, X.C, Tang and Israel Hanin. Mechanism-Based-Dual-Action Cephalosporins, Harry A. Albrecht and James G. Christenson. Some Thoughts on Enzyme Inhibitors and the Quiescent Affinity Label Concept, Mien Krantz Index. 

The best definition of the third type of irreversible enzyme inhibitor, mechanism-based inactivators, is provided by Dr Richard Silverman, a leading authority on the subject. A mechanism-based inactivator (sometimes, much to my dismay, called a suicide substrate), he writes, is an unreactive compound that has a structural similarity to a substrate or product for an enzyme. Once at the active site of the enzyme, it is converted into a species that generally forms a covalent bond to the enzyme, producing inactivation. Although both quiescent affinity labels and mechanism-based inactivators can be mistaken for... 

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