Activity, inhibition of enzyme

The simplest kind of enz5nne inhibition is competitive inhibition, characterized by a rate equation of the following form  

The opposite extreme from competitive inhibition is uncompetitive inhibition, and is defined by the presence of a factor that affects the variable term in the denominator of the Michaelis-Menten equation instead of the constant term  

The inhibition constant Ki is an uncompetitive inhibition constant. This type of inhibition is not common, but it is important as a component of mixed inhibition, when both competitive and imcompetitive effects occur simultaneously  

There is no particular reason for the two inhibition constants Kic and to be equal, and most of the mechanisms for mixed inhibition suggest that they ought to be different. However, the case where Ki . = Km is often given an undeserved prominence in discussions of inhibition, largely because experiments done many years ago suggested that it was a more common phenomenon than it is. It is called non ompetitive inhibition and its rate equation is the same as equation 17, but vwth both and Km written simply as Kj. 

All of these kinds of inhibition are conveniently discussed in terms of apparent Michaelis-Menten parameters, i.e. the parameters that replace the ordinary para- 

---

It is now believed that many of our useful drugs exert their beneficial action by the inhibition of enzyme activity in bacteria. Some bacteria, such as staphylococcus, require for their growth the simple organic compound poraaminobenzoic... 

Figure 7.2 Biochemical pathways for the synthesis and metabolism of dopamine. (—) indicates drug inhibition of enzyme activity...

The concentration of inhibitor, causing 50% inhibition of enzyme activity (I50/ M) was calculated. In many cases the enzyme-inhibitor rate interaction constant (k2 M 1 min 1) was calculated according to the formula ... 

The ability of flavonoids (quercetin and rutin) to react with superoxide has been shown in both aqueous and aprotic media [59,94]. Then, the inhibitory activity of flavonoids in various enzymatic and nonenzymatic superoxide-producing systems has been studied. It was found that flavonoids may inhibit superoxide production by xanthine oxidase by both the scavenging of superoxide and the inhibition of enzyme activity, with the ratio of these two mechanisms depending on the structures of flavonoids (Table 29.4). As seen from Table 29.4, the data obtained by different authors may significantly differ. For example, in recent work [107] it was found that rutin was ineffective in the inhibition of xanthine oxidase that contradicts the previous results [108,109], The origins of such big differences are unknown. 

Methylenetetrahydrofolate reductase (MTHFR) catalyzes the NAD(P)H-dependent reduction of 5,10-methylenetetrahydrofolate (CH2-THF) to 5-methyltetrahydrofolate (CH3-THF). CH3-THF then serves as a methyl donor for the synthesis of methionine. The MTHFR proteins and genes from mammalian liver and E. coli have been characterized,12"15 and MTHFR genes have been identified in S. cerevisiae16 and other organisms. The MTHFR of E. coli (MetF) is a homotetramer of 33-kDa subunits that prefers NADH as reductant,12 whereas mammalian MTHFRs are homodimers of 77-kDa subunits that prefer NADPH and are allosterically inhibited by AdoMet.13,14 Mammalian MTHFRs have a two-domain structure the amino-terminal domain shows 30% sequence identity to E. coli MetF, and is catalytic the carboxyterminal domain has been implicated in AdoMet-mediated inhibition of enzyme activity.13,14... 

Compounds 13-18 were tested for acetylcholinesterase inhibition activity and it was found that compounds 13 and 14 exhibited acetylchohnesterase inhibition activity with IC values (inhibition of enzyme activity by 50%) of 17 and 13 pM, respectively. Compounds 15-18 showed moderate enzyme inhibition activity with ICj, values of 35, 80, 76, and 100 pM, respectively. This bioactivity data suggested that the higher enzyme inhibition potency of compounds 13 and 14 may hypothesized due to the presence of a tetrahydrofuran ring incorporated in their stractures. Fnrthermore, compounds 1 and 2 exhibited nearly the same bioactivity and this indicated that C-7 hydroxyl group does not play any role in enzyme inhibition activity. 

Binding of a reversible inhibitor to an enzyme is rapidly reversible and thus bound and unbound enzymes are in equilibrium. Binding of the inhibitor can be to the active site, or to a cofactor, or to some other site on the protein leading to allosteric inhibition of enzyme activity. The degree of inhibition caused by a reversible inhibitor is not time-dependent the final level of inhibition is reached almost instantaneously, on addition of inhibitor to an enzyme or enzyme-substrate mixture. 

Inhibition of enzyme activity by a chemical species that binds slowly and is tight-binding as well has a low dissociation constant). Such inhibitors require special kinetic analysis . The most common method of obtaining the inhibition parameters is by nonlinear regression analysis of the progress curves. 

Acid glucan-Lq-a-glucosidase (mouse) Inhibition of enzyme activity by 0.3 mM HA with parallel inhibition of glucose-stimulated insulin release 58... 

Inhibition of enzyme activity was not used to assess the antibody-antigen reaction of lignin-peroxidase, as the addition of control serum to enzyme reaction mixtures increased the pH above the pH specificity of the... 

In contrast, selective inhibition of enzyme activity involves highly specific interactions between the protein and chemical groups on the xenobiotic. An excellent example of this type of inhibition is seen in the toxic effect of fluoroacetate, which is used as a rodenticide. Although fluoroacetate is not directly toxic, it is metabolized to fluoroacetyl-CoA, which enters the citric acid cycle due to its structural similarity to acetyl-CoA (Scheme 3.5). Within the cycle, fluoroacetyl-CoA combines with oxalo-acetate to form fluorocitrate, which inhibits the next enzyme, aconitase, in the cycle [42]. The enzyme is unable to catalyze the dehydration to cis-aconitate, as a consequence of the stronger C-F bond compared with the C-H bond. Therefore, fluorocitrate acts as a pseudosubstrate, which blocks the citric acid cycle and, subsequently, impairs ATP synthesis. 

Considerable DNase but no RNase activity results if Ca-+ is replaced by Sr-+, while Fe-+ and Cu J+ cause minimal activation (3, 40). A number of heavy metal cations inhibit DNase and RNase activities competitively with Ca-+ Hg-+, Zn2+, and Cd-+ are the most potent of these (3). Studies with synthetic substrates, to be discussed below, indicate that Ca2+ is not only required for the proper binding of substrates but also that it is required for the subsequent independent hydrolytic process. Although several divalent cations can substitute for Ca2t in the binding function, as evidenced by their competitive inhibition of enzymic activity (3) and their ability to promote nucleotide binding (62), the catalytic role of Ca2+ appears to be unique. 

The dependence of the kinetic constants (Table I) upon the concentration of Ca2t, pH, and ionic strength is not the same for all the synthetic substrates shown on Table I. In general, however, with all these substrates the maximal velocities are achieved between pH 9 and 11, maximal affinity for substrate occurs between pH 7.5 and 8.5 (Fig. 4), and inhibition of enzymic activity is observed with NaCl concentrations greater than 0.1 N. Similar dependence upon these parameters is seen when activities are measured with DNA and RNA (3). 

In 1954, Beaufay and de Duve (27) first suggested a relationship between microsomal phospholipid and glucose-6-phosphatase. They observed a loss of enzymic activity from phospholipid-rich microsomal preparations concomitant with extraction with such organic solvents as butanol or treatment with lecithinase. Various studies were carried out to demonstrate that the latter effect was not produced through inhibition of enzymic activity by accumulated products of the hydrolysis of phospholipids. On the basis of their observations that deoxycholate treatment labilized microsomes to phospholipase action, they concluded that . . . the detergent did not exert its primary effect on the dissociation of phospholipids from microsomal protein, but that it probably disrupted... 

Proteins Oxidation of SH groups, chemical cross-linking of membrane proteins and lipids, and inhibition of enzymic activities (Farooqui and Horrocks, 2007)... 

This expectation was confirmed by running a set of experiments in which the effect of the butyric acid (BA) butanol (ButOH) molar ratio on the esterification catalyzed by POS-PVA lipase was investigated in the range of 0.5-2.0. As shown in Fig. 2, no inhibition of enzyme activity was... 

Arylhalide 5 -iodo-2 -deoxy- uridine monophosphate Thymidine kinase Both iodine and nucleotide covalently incorporated. Only nucleotide incorporation parallels inhibition of enzyme activity (Chen et al., 1976). 

---