Reaction velocity, enzymatic maximum

Kinetics is the branch of science concerned with the rates of chemical reactions. The study of enzyme kinetics addresses the biological roles of enzymatic catalysts and how they accomplish their remarkable feats. In enzyme kinetics, we seek to determine the maximum reaction velocity that the enzyme can attain and its binding affinities for substrates and inhibitors. Coupled with studies on the structure and chemistry of the enzyme, analysis of the enzymatic rate under different reaction conditions yields insights regarding the enzyme s mechanism of catalytic action. Such information is essential to an overall understanding of metabolism. 

The lag phase is the time for the enzymatic reaction to reach maximum velocity the latter is the rate that is zero-order in substrate, and substrate-saturation kinetics are assumed. At the beginning of the reaction, changes in concentration of the substrate are assumed to be trivial, and the substrate is present in large excess. The lag phase depends on the specimens activity and the complexity of the reaction scheme being used. For example, the determination of LD with the pyruvate-to-lactate reaction has little or no lag phase, even with serum specimens having low activity ... 

Many enzymatic reactions in supercritical fluids follow Michaelis-Menten kinetics. The Michaelis constants and maximum reaction velocities as well as inhibiting substrates have been identified in several enzymatic reactions in... 

The most relevant parameters that can be obtained from a kinetic study of enzymatic catalysis are the dependence of the velocity of catalysis on the concentration of the substrates, in particular in the early stages of reaction, and the maximum rate at which the enzyme can accelerate the reaction. 

According to transition state theory, if the transmission coefficient k = 1, T and ET will be transformed to products at the same rate. Thus, if the mechanisms of the nonenzymatic and enzymatic reactions are assumed the same, the ratio of maximum velocities for first-order transformation of ES and S will be given by Eq. 9-85. For some enzymes the ratio... 

We see that the rate of the enzyme-catalyzed reaction depends linearly on the enzyme concentration, and in a more complicated way on the substrate concentration. Thus, when [S] Km, (Eq. (2.41)) reduces to v = k2[E]0, and the reaction is zero order in [S], This means that there is so much substrate that all of the enzyme s active sites are occupied. It also means that [S] remains effectively unchanged, even though products are formed. This situation is known as saturation kinetics. The value k2[E]0 is also called the maximum velocity of the enzymatic reaction, and written as vmax. 

Vmax. The maximum velocity of an enzymatic reaction when the binding site is saturated with substrate. 

The rate of the enzymatic cleavage can proceed at temperatures as low as 4 °C, although increasing the temperature increases the rate of the reaction (Dalgleish, 1979). Another way of affecting the enzymatic process is to decrease the pH. The pH of maximum velocity is pH 6.0 (Van Hooydonk et al., 1986b). 

When we measure the rate (also called the velocity) of an enzymatic reaction at varying substrate concentrations, we see that the rate depends on the substrate concentration, [S]. We measure the initial rate of the reaction (the rate measured immediately after the enzyme and substrate are mixed) so that we can be certain that the product is not converted to substrate to any appreciable extent. This velocity is sometimes written or Vq to indicate this initial velocity, but it is important to remember that aU the calculations involved in enzyme kinetics assume that the velocity measured is the initial velocity. We can graph our results as in Figure 6.8. In the lower region of the curve (at low levels of substrate), the reaction is first order (Section 6.3), implying that the velocity, V, depends on substrate concentration [S]. In the upper portion of the curve (at higher levels of substrate), the reaction is zero order the rate is independent of concentration. The active sites of aU of the enzyme molecules are saturated. At infinite substrate concentration, the reaction would proceed at its maximum velocity, written kJnax-... 

Substituting the expression for 1 into Equation 6.14 enables us to relate the observed velocity at any substrate concentration to the maximum rate of an enzymatic reaction ... 

On the basis of kinetic studies [75-77], Smith suggested that the major factors governing the choice of two alternative metabohc routes are the first-and zero-order rate constants of the two reactions. The first-order rate constant in conjugation reactions may not be subject to serious species variations, but the zero-order rate constant depends not only on the mobihzation rate of glycine, glucuronic acid, etc., which is different in different species, but also on the compound metabohzed. The compound may affect the zero-order rate constant either by virtue of its tissue level, which depends on dose, or by its effect on the maximum velocity of the enzymatic reaction [23]. 

It can be shown that Km, the Michaelis constant, which has the dimensions of concentration (moles/ .), is in fact the substrate concentration corresponding to a value of half the maximum velocity (Fig. 35). Knowledge of the Michaelis constant for a given enzymatic reaction, allovre v to be calculated for any value of [5], provided that k, is known. 

To evaluate Km two approaches have been used. At the substrate concentration (s. Fig. 2) required for the enzymatic reaction to reach its maximum velocity (Fmax, Fig. 2) practically all the active enzyme present is combined with the substrate. Thus at Fmax (cf. equation 4),... 

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