Maximum velocity Vmax

The true concentration of enzyme is difficult to measure especially in terms of molar concentration but if the substrate concentration is large compared with that of the enzyme, all of the enzyme will be present as the ES complex and the reaction will proceed at maximum velocity. Under these conditions of excess substrate and maximum velocity (Vmax) ... 

The equation gives a measure of the Michaelis constant (Km) in terms of the measured velocity of the reaction (v) which results from a substrate concentration ([S]) and the maximum velocity (Vmax) which can be achieved using very high concentrations of substrate. 

The maximum velocity, Vmax = /c3[E]t, is attained only when all of the enzyme is converted into ES. Under other conditions v = /c3[ES] and Eq. 9-19 holds. 

In the oxidation of secondary alcohols by DADH, the coenzyme is the leading substrate, the release of NADH from the enzyme-NADH complex is the rate-limiting step, and the maximum velocity vmax is independent of the chemical nature of the alcohol. In the case of primary alcohols, as vmax is much lower and depends on the nature of the alcohol, Theorell-Chance kinetics (Figure 9.9) are not observed and the rate-limiting step is the chemical interconversion from alcohol to aldehyde. With all this biochemical information it is possible to delineate a catalytic reaction mechanism that is in agreement with the crystal structures and the steps of alcohol oxidation observed in the kinetic analysis of the DADH reaction. 

This combination changes either or both the maximum velocity, Vmax, and Michaelis constant, Kg, for both solutes. 

The inverse of the maximum velocity Vmax is obtained at the intersection of the plot with the y-axis (Figure 3.3). This yields a value of V ax equal to... 

When the substrate concentration is so high that all the enzyme is present in the form of ES, that is, [ES] = [Er], we reach the maximum velocity, Vmax, given by a form of Equation (5.13),... 

The maximum velocity, Vmax, will be attained when all the enzyme is forced into the ES form by an excess of substrate. The effect of substrate and enzyme concentrations on the observed velocity is given by the Michaelis-Menten expression (6, 7) ... 

The activity of Thermus PEPC was enhanced by acetyl-CoA (CoASAc), as is the case with the E. coli enzyme. Acetyl-CoA affected not only maximum velocity (Vmax) but also halfsaturation concentrations (5o.5) of PEP and Mg2+. Thermus PEPC was inhibited by aspartate and malate. Unexpectedly, it was also inhibited by various phosphorylated compounds such as fructose 1,6-bisphosphate and GTP which are known to be activators of E. coli PEPC. The inhibition by these compounds showed tendencies to be released by increasing CoASAc. These properties were essentially the same with those of native Thermus PEPC. 

Simultaneous to the graph creation, kinetic properties in each vRxn are used to create the appropriate reaction rate equations (ordinary differential equations, ODE). These properties include rate constants (e.g., Michaelis constant, Km, and maximum velocity, Vmax, for enzyme-catalyzed reactions, and k for nonenzymatic reactions), inhibitor constants, A) and modes of inhibition or allosterism. The total set of rate equations and specified initial conditions forms an initial value problem that is solved by a stiff ODE equation solver for the concentrations of all species as a function of time. The constituent transforms for the each virtual enzyme are compiled by carefully culling the literature for data on enzymes known to act on the chemicals and chemical metabolites of interest. 

It is usually difficult to measure [ES] in a reaction mixture. Consequently, Equation (6.3) is not useful experimentally. On the other hand, the velocity (v) and the maximum velocity (Vmax) are readily determined by a variety of methods. 

They found other peculiarities when studying the transport of L-tryptophan in human freshly sampled red cells uptake could be resolved into linear and saturable components but upon infection or storage of red cells the linear component was substantially increased whereas the Kt and maximum velocity (Vmax) remained constant. (They also contended that the presence of parasitized red cells altered the permselectivity of uninfected red cells.) Further, the changes in the permselectivity of the P. falciparum-infected red cells was unaffected by p-chloromercuribenzoate (PCMB) and cytochalasin B, inhibitors for glucose transport, as well as DIDS and DNDS for anion transport, but was inhibited by phloretin, a modifier of the membrane dipole potential shown to block a variety of mediated and non-mediated transport mechanisms. Phloretin also inhibited the in vitro growth of P. falciparum. This work is reviewed in Ginsburg and Kirk, 1998. 

For enzymes that obey the Michaelis-Menten relationship, a plot of Mr versus 1/[S] the so-called double-reciprocal plot yields a straight line, from which one can obtain more accurate values for the Michaelis constant, Km, and the maximum velocity, Vmax (Figure 6.4). 

FIGURE 4.1 Representative hyperbolic plot indicative of a reaction following Michaelis-Menten kinetics. = substrate concentration necessary to achieve one-half maximum velocity. Vmax = maximum velocity. 

Michaelis constant, which leads to Eq. 9.52, which is called the Michaelis-Menten equation. This equation predicts a kinetic scenario that will show saturation behavior when [S] Km-Under this condition, the rate of the reaction is equal to /Ccai[E]o/ which is called the maximum velocity (Vmax)- If is the fastest that the catalytic reaction can occur, because all the catalyst has been converted to the catalyst-substrate complex (E S). The catalyst/enzyme is considered to be saturated with the substrate. 

By applying shear stress, a laminar shear flow is generated between the two plates. The uppermost layer moves at the maximum velocity Vmax, while the lowermost layer remains at rest. The shear rates of typical actions are summarized in Table 1. 

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