Enzyme classical inhibitor

Hydrazides are formed by the acylation of hydrazines, and have a C-N bond of rather low chemical stability toward hydrolysis. It is, therefore, not surprising that the cleavage of this bond represents a major metabolic pathway for most hydrazides. The reaction is catalyzed by amidases since it can be inhibited by O-ethyl 0-(4-nitrophenyl) phenyl phosphothionate or bis(4-nitrophenyl) phosphate, which are classical inhibitors of this enzyme. 

HDACs are zinc metalloproteases involved in the acetylation of histone. Inhibition of HDACs represents a new strategy in human cancer therapy since these enzymes play a fundamental role in regulating gene expression and chromatin assembly. Along this line, inhibition of HD AC by fluoroketones has been studied. The inhibition power of fluoroketones toward HDACs is comparable to that of hydroxamates, which are the classical inhibitors of metalloproteases. These fluoroketones exhibit antiproliferative activities on tumor cell lines (Figure 7.44). ... 

The classic example of competitive inhibition is inhibition of succinate dehydrogenase, an enzyme, by the compound malonate. Hans Krebs first elucidated the details of the citric acid cycle by adding malonate to minced pigeon muscle tissue and observing which intermediates accumulated after incubation of the mixture with various substrates. The structure of malonate is very similar to that of succinate (see Figure 1). The enzyme will bind malonate but cannot act further on it. That is, the enzyme and inhibitor form a nonproductive complex. We call this competitive inhibition, as succinate and malonate appear to compete for the same site on the enzyme. With competitive inhibition, the percent of inhibition is a function of the ratio between inhibitor and substrate, not the absolute concentration of inhibitor. 

Classical NSAIDs and COX-2 inhibitors are time-dependent, irreversible inhibitors of hCOX-2, which is consistent with a two-step process, involving an initial rapid equilibrium binding of enzyme and inhibitor, followed by a slow formation of a tightly bound enzyme-inhibitor complex. COX-2 inhibitors show a time-independent inhibition of hCOX-1, consistent with the formation of a reversible enzyme-inhibitor complex (Ouellet and Percival 1995 Riendeau et al. 2001). 

Enzyme inhibitors are broadly classified as irreversible and reversible. Inhibitors of the first class usually cause an inactivating, covalent modification of enzyme structure. Cyanide is a classic example of an irreversible enzyme inhibitor. The kinetic effect of irreversible inhibitors is to decrease the concentration of active enzyme. Irreversible inhibitors are usually considered to be poisons and are generally unsuitable for therapeutic purposes. 

Noncompetitive inhibitors. Classical noncompetitive inhibitors have no effect on substrate binding and vice versa, given that they bind randomly and reversibly to different sites on the enzyme. They also bind with the same affinity to the free enzyme and to the enzyme-substrate complex. Both the enzyme- inhibitor complex E. I and the enzyme-substrate-inhibitor complex E. S. I are catalytically inactive. The equilibria are outlined in Equation 17.20. 

However, because malonic acid has only one methylene group, it is obvious that no oxidation-reduction can take place. Only association of the enzyme and inhibitor, and dissociation of the I complex, can occur. Another classical example of a competitive inhibitor is the sulfa drug sulfanilamide, which interferes with the biosynthesis of folic acid from the precursor -aminobenzoic acid (PABA). 

The term should be used for enzymes that display Michaelis-Menten kinetics. Thus, it is not used with allosteric enzymes. Technically, competitive and noncompetitive inhibition are also terms that are restricted to Michaelis-Menten enzymes, although the concepts are applicable to any enzyme. An inhibitor that binds to an allosteric enzyme at the same site as the substrate is similar to a classical competitive inhibitor. One that binds at a different site is similar to a noncompetitive inhibitor, but the equations and the graphs characteristic of competitive and noncompetitive inhibition don t work the same way with an allosteric enzyme. 

To date no drugs preferentially target leishmanial components of the pyrimidine biosynthetic pathway. This is likely due to similarities in enzymatic activities shared between Leishmania and humans. For example, acivicin and PALA inhibit both human and leishmanial pyrimidine biosynthesis. Similarly, classical inhibitors such as methotrexate, which target leishmanial DHFR-TS, an essential component of thymidylate synthesis, are poor anti-leishmanial drugs because they also inhibit the human enzyme. ... 

The a-(AO-heterocyclic carboxaldehyde thiosemicarbazones are primarily inhibitors of the synthesis of DNA in neoplastic cells [57,70] therefore, they exert their effect in the S phase of the cell cycle [71,72]. Inhibition of the biosynthesis of RNA and protein is also produced by agents of this class however, these metabolic processes are considerably less sensitive than is the replication of DNA [57,70,73-75]. Interference with the biosynthesis of DNA by these agents was shown to be due to inhibition of the enzyme ribonucleoside diphosphate reductase [70,73,74,76]. The a-(AO-heterocyclic carboxaldehyde thiosemicarbazones constitute, as a class, the most potent known inhibitors of ribonucleoside diphosphate reductase, being 80-5000 times more effective than the classical inhibitor of this enzyme, hydroxyurea [for appropriate references see 77]. 

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. 

---