Inhibitor binding modification

Reversible inhibition occurs rapidly in a system which is near its equilibrium point and its extent is dependent on the concentration of enzyme, inhibitor and substrate. It remains constant over the period when the initial reaction velocity studies are performed. In contrast, irreversible inhibition may increase with time. In simple single-substrate enzyme-catalysed reactions there are three main types of inhibition patterns involving reactions following the Michaelis-Menten equation competitive, uncompetitive and non-competitive inhibition. Competitive inhibition occurs when the inhibitor directly competes with the substrate in forming the enzyme complex. Uncompetitive inhibition involves the interaction of the inhibitor with only the enzyme-substrate complex, while non-competitive inhibition occurs when the inhibitor binds to either the enzyme or the enzyme-substrate complex without affecting the binding of the substrate. The kinetic modifications of the Michaelis-Menten equation associated with the various types of inhibition are shown below. The derivation of these equations is shown in Appendix S.S. 

Irreversible inhibition occurs when the inhibitor reacts at or near the active site of the enzyme with covalent modification of the active site or when the inhibitor binds so tightly that, for practical purposes, there is no dissociation of enzyme and inhibitor. The latter situation occurs in the case of proteinase inhibitors (see below). Thus, physical separative processes are ineffective in removing the irreversible inhibitor from the enzyme. Irreversible inhibitor reaction is written... 

In such circumstances, referred to as noncompetitive inhibition, the inhibitor binds to a site other than the active site. Inhibitor binding results in a modification of the enzyme s conformation that prevents product formation (Figure 6.10. Usually, noncompetitive inhibitors do not affect substrate binding and they have little or no structural resemblance to substrate. As with uncompetitive inhibition, noncompetitive inhibition is only partially reversed by increasing the substrate concentration. Analysis of reactions inhibited by noncompetitive inhibitors is often complex for several reasons. As with uncompetitive inhibition, noncompetitive inhibition usually involves two or more substrates. Thus... 

Suicide inhibitors, or mechanism-based inhibitors are modified substrates that provide the most specific means to modify an enzyme active site. The inhibitor binds to the enzyme as a substrate and is initially processed by the normal catalytic mechanism. The mechanism of catalysis then generates a chemically reactive intermediate that inactivates the enzyme through covalent modification. The fact that the enzyme participates in its own irreversible inhibition strongly suggests that the covalently modified group on the enzyme is catalytically vital. One example of such an inhibitor is N,N-dimethylpropargylamine. A flavin prosthetic group of monoamine oxidase... 

Ribosomal Protein Synthesis Inhibitors. Figure 5 Nucleotides at the binding sites of chloramphenicol, erythromycin and clindamycin at the peptidyl transferase center. The nucleotides that are within 4.4 A of the antibiotics chloramphenicol, erythromycin and clindamycin in 50S-antibiotic complexes are indicated with the letters C, E, and L, respectively, on the secondary structure of the peptidyl transferase loop region of 23S rRNA (the sequence shown is that of E. coll). The sites of drug resistance in one or more peptidyl transferase antibiotics due to base changes (solid circles) and lack of modification (solid square) are indicated. Nucleotides that display altered chemical reactivity in the presence of one or more peptidyl transferase antibiotics are boxed. 

---