Inhibition of enzyme

The efficiency of an enzyme can be reduced or can even become negligible in the presence of certain substances, known as inhibitors. Many inhibitors have structural resemblances with the substrates and compete with them for the formation of complexes with the enzyme. This is the case of the inactivation of cytochrome c oxidase by the cyanide ion, which blocks the mitochondrial electron-transport chain to oxygen. Similarly, the inactivation of the succinate dehydrogenase by malonate involves its inhibition of the conversion of succinate to fumarate in the citric acid cycle. In the latter case, the mechanism for competitive inhibition is 

When the concentrations of the enzyme-inhibitor complex and of the free enzyme, which can be obtained form eqs. (14.22) and (14.23), are substituted in the expression for the conservation of the total enzyme concentration 

SO that the initial reaction rate defined by eq. (14.5), is expressed as 

This shows that in competitive catalysis the maximum rate V is not changed. Rather, the presence of I has the effect of making [S] appear more dilute than it actually is, and the effect of the inhibitor can be overcome for sufficiently high substrate concentrations. 

In uncompetitive inhibition, the inhitritor binds to the enzyme, but not to its active site, and an ESI complex can be formed. 

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Elucidating Mechanisms for the Inhibition of Enzyme Catalysis An inhibitor interacts with an enzyme in a manner that decreases the enzyme s catalytic efficiency. Examples of inhibitors include some drugs and poisons. Irreversible inhibitors covalently bind to the enzyme s active site, producing a permanent loss in catalytic efficiency even when the inhibitor s concentration is decreased. Reversible inhibitors form noncovalent complexes with the enzyme, thereby causing a temporary de-... 

Herbicidal Inhibition of Enzymes. The Hst of known en2yme inhibitors contains five principal categories group-specific reagents substrate or ground-state analogues, ie, rapidly reversible inhibitors affinity and photo-affinity labels suicide substrate, or inhibitors and transition-state, or reaction-intermediate, analogues, ie, slowly reversible inhibitors (106). 

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...

A homogeneous electrochemical enzyme immunoassay for 2,4-dinitrophenol-aminocaproic acid (DNP-ACA), has been developed based on antibody inhibition of enzyme conversion from the apo- to the holo- form Apoglucose oxidase was used as the enzyme label. This enzyme is inactive until binding of flavin adenine dinucleotide (FAD) to form the holoenzyme which is active. Hydrogen peroxide is the enzymatic product which is detected electrochemically. Because antibody bound apoenzyme cannot bind FAD, the production of HjOj is a measure of the concentration of free DNP-ACA in the sample. 

The two reactions are achieved in only one step without altering the product quality. The inhibition of enzymes by excess of intermediate components is reduced in this system. The presence of an organic solvent in the medium allows a high solubility of acylglycerols and a well-controlled partition of the components in the reactor. 

If the inhibitor is found to bind rapidly (linear progress curves) and dissociate rapidly (rapid recovery of activity upon dilution) from its target enzyme, then one can proceed to analyze its inhibition modality and affinity by classical methods. The modes of reversible inhibition of enzymes were described in Chapter 3. In the next section of this chapter we will describe convenient methods for determining reversible inhibition modality of lead compounds and lead analogues during compound optimization (i.e., SAR) studies. 

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. 

Showdomycin inhibits synthesis of nucleic acid. Thiols, such as cysteine and glutathione (among other compounds), reverse this inhibition, and it is considered that the interaction of the maleimide moiety with sulfhydryl groups within the cell or in the membrane may be responsible for the selective inhibition of enzymes by show-... 

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... 

The renal tubular dysfunction of galactosemia is very similar to that caused by, e.g., heavy metal poisoning in both cases it seems likely that the inhibition of enzyme systems prevents the cells of the renal tubule... 

Lithium(I) ions are small but strongly hydrated and could interfere with Mg(II) biochemistry. However, the favored mode of action is interference with Ca(II) metabolism via inhibition of enzymes in the inositol phosphate pathways (470-472). Inositol phosphates are responsible for mobilizing Ca(II) inside cells in response to external stimnlii. Lithium also stimulates glutamate release presumably via activation of the AT-methyl-D-asparate receptor and leads to Ca(II) entry (473). The increased influx of intracellular Ca(II) may activate phospholipase C and stimulate accumulation of inositol 1,4,5-triphosphate (473). 

Allenic amino acids belong to the classical suicide substrates for the irreversible mechanism-based inhibition of enzymes [5], Among the different types of allenic substrates used for enzyme inhibition [128, 129], the deactivation of vitamin B6 (pyr-idoxal phosphate)-dependent decarboxylases by a-allenic a-amino acids plays an important role (Scheme 18.45). In analogy with the corresponding activity of other /3,y-unsaturated amino acids [102,130], it is assumed that the allenic amino acid 139 reacts with the decarboxylase 138 to furnish the imine 140, which is transformed into a Michael acceptor of type 141 by decarboxylation or deprotonation. Subsequent attack of a suitable nucleophilic group of the active site then leads to inhibition of the decarboxylase by irreversible formation of the adduct 142 [131,132]. 

A substance that decreases the rate of an enzyme-catalysed reaction is known as an inhibitor and its effects may be permanent or transient. The inhibition of some reactions by substances which may be products of either that reaction or a subsequent reaction provides a control mechanism for cellular metabolism, while the selective inhibition of enzymes is the basis of many aspects of pharmacology and chemotherapy. 

J. H. Weisburger, C. M. Goodall, Steric Inhibition of Enzyme Reactions. Lack of Enzymatic Hydrolysis of 2, 4, 6 -Trimethyl-Acetanilide , Life Sci. 1968, 7, 263-267. 

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