Maximal reaction velocity

Initial maximal reaction velocity with NADPH,, ... 

We highlight several advantages of the generalized parameters, as compared to the more usual description in terms of Michaelis constants and maximal reaction velocities ... 

V = maximal reaction velocity attained when all enzyme active sites are filled with substrate molecules... 

When v0 = Tmax/2 (i.e. at half-maximal initial velocity), this equation reduces to Am = [S], that is, as defined in the initial graphical description, Am is the substrate concentration giving half-maximal reaction velocity. 

It has been shown that the relationship between substrate concentration and reaction velocity is hyperbolic and that the parameters of this curve are given by two fiictors, and (see Fig. 3). is the concentration at which half-maximal reaction velocity is achieved. The smaller value is for... 

In other words, the rate of reaction is proportional to the product of the maximal reaction velocity, Vmax. and the initial substrate concentration, divided by the sum of the MM constant. Km, and the initial substrate concentration. Under the MM assumptions, equation (3) defines the MM constant, which can be determined computationally given the magnitude of k and k-, which can be approximated using the free-energy difference between the ES complex and the enzyme and substrate alone (Fig. 7-1), as well as ki, which can be determined by computing the energy barrier to the biochemical transformation catalyzed by a given enzyme, as in... 

In this scenario, enzyme affinity for its substrate suffers, and therefore increases to m(1 + Pl/ i). while the maximal reaction velocity is unaffected. 

Because metabolic studies of pesticides have been extensively conducted in animals in vitro and in vivo, in vitrcj-in vivo correlation.s have been established in animals. By comparing in vitro data between humans and animals and assuming the in vifra-in vivo correlations observed in animals also exist in humans in a similar manner, one may be able to predict in vivo human metabolism of pesticides using in vitro human pesticide metabolism data. Therefore, the parameters obtained from the in vitro studies, such as /ffii (Michadis-Menten constant), (maximal reaction velocity), intrinsic clearance (inhibition... 

The step catalyzed by will be rate-limiting because the actual substrate concentration (10 M) is much less than that needed to achieve half-maximal reaction velocity (10 M) for this step. 

PRPP synthetase superactivity, diversity in the kinetic mechanisms underlying increased PRPP synthesis has been identified. This diversity has important implications for the design of methods for detection of abnormalities of the enzyme. The four categories of kinetic alteration thus far associated with PRPP synthetase superactivity in man are abnormal catalytic properties (increased maximal reaction veloc-city) 2) defective regulatory properties (purine nucleotide feedback resistance) 3) increased affinity for the substrate ribose-5-P and 4) combined alterations of catalytic and regulatory properties. 

PRPP synthetases. A. Enzyme activities in dialyzed hemolysates. Sigmoidal activation is seen for both enzymes. Activity of the mutant enzyme with an increased maximal reaction velocity is, however, increased by a constant proportion at all Pi concentrations. B. Enzyme activities in undialyzed (crude) hemolysates. The mutant enzyme, deficient in inhibitor responsiveness, shows hyperbolic activation with increased activity only at Pi concentrations below 2 mM. C. Enzyme activities in partially purified erythrocyte preparations from a normal individual and the patient with the feedback-resistant PRPP synthetase studied in B. Note hyperbolic activation of the purified normal enzyme and the resulting similarity of the Pi activation curves. D. Enzyme activities in dialyzed fibroblast extracts. The mutant enzyme, with combined increased maximal reaction velocity and diminished nucleotide responsiveness, shows both hyperbolic activation and increased enzyme activity at all Pi concentrations. 

A final category of functional abnormality underlying PRPP synthetase superactivity is represented by the enzyme characterized from the fibroblasts of the child with purine overproduction and deafness referred to earlier. Both increased maximal reaction velocity and... 

We have recently examined the responsiveness to Mg + and MgATP of the subunit aggregates from a superactive form of PRPP synthetase with normal substrate and inhibitor binding properties but an increased maximal reaction velocity. The distribution of forms of the mutant enzyme on sucrose gradient after the dilution and incubation procedures described above was indistinguishable from that of normal PRPP synthetase. In several respects, however, the mutant enzyme behaved in an abnormal fashion. First, under conditions of suboptimal Mg2+... 

The Michaelis constant is a fundamental value in enzyme chemistry not only can it be used to estimate the [S] needed to give maximal reaction velocity but it can also be used in the identification of enzymes, to examine the conditions under which an enzyme will operate within the cell and to give information on the mode of action of the enzyme and of substances that affect its activity. 

To express this in words That substrate concentration at which half-maximal reaction velocity is reached equals the dissociation constant of the enzyme-substrate complex. This constant is named the Michaelis constant, K , after the originator of the theory. Its dimensions are those of the substrate concentration (moles/liter) when [S] = the enzyme is only half saturated. 

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