Enzymes inhibition constant

Dixon, M. (1952) The determination of enzyme inhibition constants. Biochemical Journal, 55, 170-171. 

Many lines of evidence indicate that the first committed step in de novo purine nucleotide biosynthesis, production of 5-phosphoribosylamine by glutamine PRPP amidotransfer-ase, is rate-limiting for the entire sequence. Consequently, regulation of this enzyme is probably the most important factor in control of purine synthesis de novo (fig. 23.24). The enzyme is inhibited by purine-5 -nucleotides, but the most inhibitory nucleotides vary with the source of the enzyme. Inhibition constants (A, ) are usually in the range 10-3-10-5 M. The maximum effect of this end-product inhibition is produced by certain combinations of nucleotides (e.g., AMP and GMP) in optimum concentrations and ratios, indicating two kinds of inhibitor binding sites. This is an example of a concerted feedback inhibition. 

In these models, Ki is the enzyme inhibition constant and IC90 is the concentration of inhibitor resulting in 90% inhibition of viral RNA production in HIV-infected MT-2 cells. The presence of bilinear hydrophobic terms in QSAR 2 3, where there are ahphatic variations at the P2/P2 position of 16 indicate that hydrophobic interactions are important up to a rather high value Ki 6.53, IC90 6.96). Steric interaction of the X-substituent was also found to... 

Chelation is a feature of much research on the development and mechanism of action of catalysts. For example, enzyme chemistry is aided by the study of reactions of simpler chelates that are models of enzyme reactions. Certain enzymes, coenzymes, and vitamins possess chelate stmctures that must be involved in the mechanism of their action. The activation of many enzymes by metal ions most likely involves chelation, probably bridging the enzyme and substrate through the metal atom. Enzyme inhibition may often result from the formation by the inhibitor of a chelate with a greater stabiUty constant than that of the substrate or the enzyme for a necessary metal ion. 

Substrate and product inhibitions analyses involved considerations of competitive, uncompetitive, non-competitive and mixed inhibition models. The kinetic studies of the enantiomeric hydrolysis reaction in the membrane reactor included inhibition effects by substrate (ibuprofen ester) and product (2-ethoxyethanol) while varying substrate concentration (5-50 mmol-I ). The initial reaction rate obtained from experimental data was used in the primary (Hanes-Woolf plot) and secondary plots (1/Vmax versus inhibitor concentration), which gave estimates of substrate inhibition (K[s) and product inhibition constants (A jp). The inhibitor constant (K[s or K[v) is a measure of enzyme-inhibitor affinity. It is the dissociation constant of the enzyme-inhibitor complex. 

Because of the complexity of biological systems, Eq. (1) as the differential form of Michaelis-Menten kinetics is often analyzed using the initial rate method. Due to the restriction of the initial range of conversion, unwanted influences such as reversible product formation, effects due to enzyme inhibition, or side reactions are reduced to a minimum. The major disadvantage of this procedure is that a relatively large number of experiments must be conducted in order to determine the desired rate constants. 

The GIPF technique has been used to establish quantitative representations of more than 20 liquid, solid and solution properties,31 34 including boiling points and critical constants, heats of phase transitions, surface tensions, enzyme inhibition, liquid and solid densities, etc. Our focus here shall be only upon those that involve solute-solvent interactions. 

In this case, [I] represents the inhibitor concentration at the enzyme-active site and K is the inhibition constant for a single enzyme ... 

The inhibition constant Kt for the complex between the enzyme (E) and the inhibitor (7) is the dissociation constant for the enzyme inhibitor complex (El) ... 

Much earlier, Wolfenden (Westerick and Wolfenden, 1972) and Thompson (1973), established a criterion for enzyme inhibitors working as TSAs. Iliey proposed that such activity should be reflected by a linear relationship between the inhibition constant for the enzyme K and its inverse second-... 

The inhibition constant for the process in which the enzyme glutathione reductase is inhibited by G6P was determined by measuring the initial rate of the oxidation of NADPH at various concentrations of GSSG. This procedure was repeated four times, each time with a separate concentration of the inhibitor G6P All the solutions were prepeared in 0.1 M Tris buffer pH 8 containing 10 mM MgCl2 and 0.94 mM EDTA. 

Figure 4.11 Effect of inhibition of enzyme 2 by cofactor A and of enzyme 1 by cofactor B (i.e., product inhibition) on the concentration of B in the basic system when operated as a fed-batch reactor. For the central and right panels the inhibition constants are indicated on top of each section. In the left panel, inhibition by products was not considered, and—indicates that the parameter is not applicable. Data presented in the left panel are taken from Figure 4.4. The values used for all other parameters ares given in Table 4.1, set I.

Based on the results of the preceding section, potential inhibitors to the enzymes involved, G6PDH and GR, were searched for among the compounds that participate in the experimental system considered. It was found that GR is inhibited by G6P, the substrate of the other reaction. To determine the inhibition constant, G6P was considered as a dead-end inhibitor [146] that... 

Competitive, 249, 123, 146, 190 [partial, 249, 124 progress curve equations for, 249, 176, 180 for three-substrate systems, 249, 133, 136] competitive-uncompetitive, 249, 138 concave-up hyperbolic, 249, 143 dead-end, 249, 124 [for bireactant kinetic mechanism determination, 249, 130-133 definition of kinetic constants, 249, 220-221 effects on enzyme progress curves, nonlinear regression analysis, 249, 71-72 inhibition constant evaluation, 249, 134-135 kinetic analysis with, 249, 123-143 one-substrate systems, 249, 124-126 unireactant systems, theory,... 

---