Enzymology: Michaelis-Menten

This rule has found a number of uses in enzymology. A simple example applies to the standard Michaelis-Menten equation, v = Lmax[S]/(ii m + [S]). Here, /(x) = Lmax[S] and g(x) = + [S]. Hence, limit... 

As in the ferrocene case, the reaction is first order in both HRP and a metal electron donor suggesting kinetic insignificance of the enzyme-electron donor intermediate (181). It should be mentioned that the first order in Mn is observed for complexes of moderate reactivity. Very reactive complexes such as [0s(bpy)2(py)(H20)]+ in Table VIII obey the Michaelis-Menten kinetics because the formation of Compound I [Eq. (5)] starts to slow down the rate (see below). The dependence of the rate of reaction 44 on the H202 concentration shown in Fig. 20 resembles the ferrocene case (119). The decline in rate after reaching a maximum has routinely been rationalized in terms of an inactivation of HRP. The true nature of this phenomenon has recently been underscored in the course of detailed spectroscopic and electrochemical studies of the HRP-catalyzed oxidation of [OsCl(bpy)2(py)]+ (121). Thus, transition metal electron donors are convenient probes for solving fundamental problems of enzymology. 

This form is recognized as identical to that of the Michaelis-Menten equation. Clearly, k is equivalent to the enzymological Vmax PA—the reactant pressure in a gas phase reaction—is equivalent to [S] and UK is equivalent to Km. 

From the results presented in this section, we conclude that the postulates of the Michaelis-Menten Formalism and the canons of good enzymological practice in vitro are not appropriate for characterizing the behavior of integrated biochemical systems. The very conditions that may have made it possible to identify important qualitative features of an enzymatic mechanism and produce a rate law in vitro tend to make the quantitative characterization of the reaction rate in vivo by this rate law invalid. 

Ifrrnover Refers to the ability of a catalytic species to be used many times over in a catalytic cycle. In enzymology, the catalytic constant cat, obtained from a Michaelis-Menten kinetic analysis, is often called the turnover number because it represents the number of times the enzyme turns over per unit time, although other uses of the term have arisen. The more precise term is turnover frequency. 

Since significant meaning is placed on these measured constants and parameters, it is important that they be determined accurately and unambiguously. It is also important that the reader or practitioner in the field of enzymology be able to assess if the measurement of these parameters is reliable. Furthermore, since enzyme behavior is often modeled as Michaelis-Menten (hyperbolic) kinetics, it seems reasonable that interpretations of observations should be made in the context of the Michaelis-Menten model. In some cases, alternative explanations for enzyme kinetic behavior may be appropriate and one may be inclined to select one interpretation over another (preferably based on a kinetic analysis, although too often this is done on intuition). 

Proteases, when solubilized in the aqueous core of reverse micelles, can catalyze the hydrolysis of various small model peptides. In most of the cases, the substrates are partitioned between the micelles and the external solvent while the hydrolytic reaction is taking place within the dispersed aqueous domains. Since the beginning of micellar enzymology, a-chymotrypsin and trypsin have been extensively studied in different reverse micellar systems, employing various model peptides as substrates. In almost all cases the reactions followed Michaelis-Menten kinetics. 

R. Grima, N.G. Walter, S. SchneU, Single-molecule enzymology a la Michaelis—Menten, FEES... 

Bertrand, in 1897, observed that certain enzymes required dialysable substances to exert catalytic activity. He named these substances coenzymes . Sorensen pointed out the dependence of enzyme activity on pH in 1909 (Sumner and Somers, 1953). An important step entering physico-chemistry, and hence extending the theoretical basis of enzymology, were the kinetic investigations and their interpretation by Michaelis and Menten. They postulated that enzymatic action is due to the formation of an intermediate compound between enzyme and substrate, and they presented a mathematical form still used today (Sumner and Myrback, 1950). 

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