Chapter 6 - Enzymatic kinetics

Abstract

Michaelis and Menten assumed the formation of an enzyme-substrate complex (ES complex) is in rapid equilibrium with free enzyme and breakdown of ES to form products is assumed to be slower than: formation of ES and breakdown of ES to re-form E and S. This assumption is equivalent to assumption of the quasi-equilibria for the first step. In fact in order to derive a kinetic expression for enzymatic kinetics this assumption is not needed. Not all enzymes obey Michaelis-Menten kinetics. The experimental dependence of the reaction rate on substrate concentration is different from Michaelis-Menten type of concentration - substrate curve and follows a sigmoidal, or S-shaped, curve. The rate of enzyme catalyzed reactions is also affected by the presence of inhibitors, which are compounds that lower the enzyme efficiency. Inhibitors do not necessarily destroy all of the enzymes activity. The inhibitors can be divided into two groups on the basis of the kind of interaction, which occurs between the enzyme and the inhibitor. The essential catalytic groups in the active-site are often ionizable and frequently act as acid, base, nucleophile or electrophile catalysts, and thus are only functional in one of their ionization states. The activity of enzymes can therefore have a very strong dependence on pH, leading to sigmoidal shape or bell shape behavior.