Covalently binding enzyme inhibitors

Rational Design of Covalently Binding Enzyme Inhibitors, 754... 

RATIONAL DESIGN OF COVALENTLY BINDING ENZYME INHIBITORS... 

The first group of covalently binding enzyme inhibitors, the chemical modifiers, are small organic molecules, generally electrophiles, that are used to modify the enzyme s side chains in such a way as to produce a stable covalent bond. These are often used to study enzyme inactivation and to identify residues potentially involved in binding and catalysis. Some of the commonly used reagents are... 

The inherent complexity of the inactivation mechanisms of covalently binding enzyme inhibitors makes it necessary to evaluate their proposed modes of action carefully. An overview of the criteria for the study of irreversible inhibitors is provided below. 

Pseudoirreversible inhibitors are the least common of the covalently binding enzyme inhibitors. They have some features in common with both affinity labels (Section 3.2) and mechanism-based inhibitors (Section3.3) but they have one distinguishing feature that is. 

The various kinds of reversible inhibition that have been identified all depend on non-covalent binding, but inhibitors differ in how they act, with consequent differences in their kinetic effects. Figure 8-6 depicts a general scheme for enzyme inhibition of a simple single substrate-single product reaction. 

Elucidating Mechanisms for the Inhibition of Enzyme Catalysis An inhibitor interacts with an enzyme in a manner that decreases the enzyme s catalytic efficiency. Examples of inhibitors include some drugs and poisons. Irreversible inhibitors covalently bind to the enzyme s active site, producing a permanent loss in catalytic efficiency even when the inhibitor s concentration is decreased. Reversible inhibitors form noncovalent complexes with the enzyme, thereby causing a temporary de-... 

This class of inhibitors usually acts irreversibly by permanently blocking the active site of an enzyme upon covalent bond formation with an amino acid residue. Very tight-binding, noncovalent inhibitors often also act in an irreversible fashion with half-Hves of the enzyme-inhibitor complex on the order of days or weeks. At these limits, distinction between covalent and noncovalent becomes functionally irrelevant. The mode of inactivation of this class of inhibitors can be divided into two phases the inhibitors first bind to the enzyme in a noncovalent fashion, and then undergo subsequent covalent bond formation. 

Usually, a rapid binding step of the inhibitor I to the enzyme E leads to the formation of the initial noncovalent enzyme-inhibitor complex E-I. This is usually followed by a rate determining catalytic step, leading to the formation of a highly reactive species [E—I ]. This species can either undergo reaction with an active site amino acid residue of the enzyme to form the covalent enzyme-inhibitor adduct E—I", or be released into the medium to form product P and free active enzyme E. 

The often fast binding step of the inhibitor I to the enzyme E, forming the enzyme inhibitor complex E-I, is followed by a rate-determining inactivation step to form a covalent bond. The evaluation of affinity labels is based on the fulfillment of the following criteria (/) irreversible, active site-directed inactivation of the enzyme upon the formation of a stable covalent linkage with the activated form of the inhibitor, (2) time- and concentration-dependent inactivation showing saturation kinetics, and (3) a binding stoichiometry of 1 1 of inhibitor to the enzyme s active site (34). 

Irreversible metabolic inhibition caused by covalent binding of the inhibitor to the enzyme after being metabolized by the same enzyme. The inhibitory effect remains after elimination of the inhibitor from the body. 

Not all enzyme inhibitors bind through reversible interactions. In some cases enzymes are inactivated by formation of covalent complexes with inhibitory molecules. 

Most poly(HA) depolymerases are inhibited by reducing agents, e.g., dithio-erythritol (DTT), which indicates the presence of essential disulfide bonds, and by serine hydrolase inhibitors such as diisopropyl-fluoryl phosphate (DFP) or acylsulfonyl derivates. The latter compounds covalently bind to the active site serine of serine hydrolases and irreversibly inhibit enzyme activity [48]. 

Competitive AIs are chemical compounds that compete with the substrate androstenedione for non-covalent binding to the active site of the enzyme to decrease the amount of product formed. Initially, research and development of AIs started with the synthesis and biochemical characterisation of competitive inhibitors. 

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