The rate of an enzyme-catalysed reaction (v) evolves as a function of time. According to Figure 4.2, the reaction rate is initially high (the steep linear segment corresponding to the initial rate) and decreases. In the evolution of such a reaction rate several factors are involved; among them it is assumed that reduction in the substrate concentration concomitant with an increase in product concentration could favour the reverse reaction, PS; the same effect is caused by enzyme denaturation. In view of the above, it is important to stress the recommendation that the rate of such a reaction be determined before any of the phenomena mentioned above intervenes. In other words, the initial rate represents the most correct experimental datum relating to the amount of active enzyme present in the reaction. If the enzyme itself is present in sufficiently high amounts, the rate of the reaction is determined by the concentration of the substrate present. As the substrate level increases, the enzyme reaction rate also increases. The reaction velocity, v0 is a function of the substrate concentration [S] for the enzyme-catalysed reaction. At high substrate concentrations the reaction velocity reaches a limiting value, Vm; Km is the substrate concentration at which the rate is at the half-maximum value.

It is assumed that when the rate attains the value Vm, the enzyme is ‘saturated’ with substrate. The active site concept provides a simple explanation of what is taking place. A certain number of available active sites are present. When adding a low concentration of substrate, each substrate molecule can eventually bind to the active site of an enzyme molecule. If the substrate concentration is increased, the probability of substrate molecules colliding with enzyme molecules yielding the [ES] complex also increases, thus increasing reaction rate. The maximum number of molecules of substrate that can be converted to product each second per active site is known as the ‘turnover number’ of the particular enzyme involved, designated kcat (the catalytic constant). Because the maximum rate is obtained at high substrate concentrations, when all the active sites are occupied with substrate, the turnover number is a measure of how rapidly an enzyme can operate once the active site is filled.

However, since enzymes usually do not operate at saturating substrate concentrations under physiological conditions, another parameter needs to be introduced, namely the specificity constant. This is the kcat/Km ratio, representing a measure of how rapidly an enzyme can function at low substrate concentrations. The specificity constant is useful for comparing the relative abilities of different compounds to serve as substrates for the same enzyme. If the substrate is present in sufficiently high amounts, the rate of reaction will become a function of the enzyme concentration. As the enzyme level increases, for a defined volume of body fluid, the rate will increase as well. In some specific situations, other reaction conditions should also be taken into account.

Best Regards,
Nancy Ella
Associate Managing Editor
Drug Designing: Open Access