Substrates simple irreversible enzyme inhibition

Figure 5.2. Initial inhibitor concentration dependence of the inhibition rate constant for simple irreversible enzyme inhibition in the presence of substrate.

Mutagenesis of known enzyme towards a desired activity will be the fastest developing direction. The use of mutants of simple serine-hydrolases, which exhibit the phosphotriesterase activity (in contrast to the native enzymes, which are irreversibly inhibited under such conditions), clearly shows that practically any kind of substrates can be enzymatically transformed. The... 

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. 

Inhibitors can be grouped broadly in two categories, reversible and irreversible. Reversible inhibitors are weakly bonded to the surface and therefore can be removed relatively easily (by dialysis or simple dilution). Irreversible inhibitors are essentially those that cannot be easily removed. Several categories of reversible enzymes are possible the most important are competitive, noncompetitive, and substrate. Given in Table 20.5 are brief descriptions of these types of inhibition along with their effects on the constants of the MM model, as reflected in the original MM equation (see Levenspiel, 1993, for derivations). 

Additionally, global fitting of this equation to the kinetic data was able to confirm that the inhibitor imidazole had an irreversible component to its inhibition of P-galactosidase (Kim et al., 2003) as the simple insertion of modifier terms into equation 35 was unable to describe the effect of the inhibitor on the enzyme. While the hydrolysis of substrate tended towards zero in the absence of imidazole, the introduction of the inhibitor stopped the enzymatic activity in a concentration dependent manner. Therefore it was reasoned that a certain fraction of the inhibitor bound enzyme population was inactivated by this process (Equation 37). 

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