Sites of inhibition

The unique properties and actions of an inhibitory substance can often help to identify aspects of an enzyme mechanism. Many details of electron transport and oxidative phosphorylation mechanisms have been gained from studying the effects of particular inhibitors. Figure 21.29 presents the structures of some electron transport and oxidative phosphorylation inhibitors. The sites of inhibition by these agents are indicated in Figure 21.30. 

Scheme 1. Sites of inhibition of cholesterol biosynthesis by Mevacor and zaragozic acid A.

Barbiturates produce CNS depression, which ranges from sedation to general anesthesia. Action is through suppression of the mesencephalic reticular activating system. Barbiturates enhance GABA-induced inhibition the site of inhibition may be presynaptic in the spinal cord or postsynaptic in the cortical... 

Figure 12-7. Proposed sites of inhibition (0) of the respiratory chain by specific drugs, chemicals, and antibiotics. The sites that appear to support phosphorylation are indicated. BAL, dimercaprol. TTFA, an Fe-chelating agent. Complex I, NADHiubiquinone oxidoreductase complex II, succinate ubiquinone oxidoreductase complex III, ubiquinohferricytochrome c oxidoreductase complex IV, ferrocytochrome ctoxygen oxidoreductase. Other abbreviations as in Figure 12-4.

In the first group of studies, involving kinetic inhibition studies, comparisons of the uilibrium (K ), phosphorylation (IC), and inhibition constant (K.) for the inhibition of electric eel and human erythrocyte AChE by ANTX-A(S) and DFP were done (Table II). From Table II it is seen that ANTX- A(S) has a higher affinity for human erythrocyte AChE (K =0.253 fiM) than electric eel AChE (K j=3.67 aM). AN DC-A(S) also shows greater affinity for AChE than DFP (K =300 fiM). And finally the bimolecular rate constant, Kj, which indicates the overall rate of reaction, shows AChE is more sensitive toward inhibition by ANTX-A(S) (Kj=1.36 pM- min- ) than DFP (K, = 0.033 /iM- min ). These studies add information to the comparative activity of ANTX-A(S) and other irreversible AChE inhibitors but do not show the site of inhibition. 

Figure 13.12 The protonmotive Q cycle. Electron transfer reactions are numbered and circled. Dashed arrows designate movement of ubiquinol or ubiquinone between centres N and P and of the ISP between cytochrome b and cytochrome c,. Solid black bars indicate sites of inhibition by antimycin, UHDTB and stigmatellin. (From Hunte et al., 2003. Copyright 2003, with permission from Elsevier.)...

The key to unraveling the toxicity of fluoracetate came from observations of Buffa and Peters (1949) that in animals treated with FAc, considerable quantities of citrate accumulated in some tissues. Oxygen uptake was also diminished. The citric acid cycle was thus implicated as the site of inhibition. Fluorcitrate was then isolated from the affected tissues. It was found to be a powerful competitive inhibitor of aconitase, thus blocking citrate oxidation. The suggestion was therefore made that fluoracetate was toxic not in itself, but because it was metabolized in cells via fluoracetyl CoA to give a toxic derivative, an example of lethal synthesis —the capacity of organisms to metabolize nontoxic compounds and convert them to potentially lethal products. 

The schematic presentation of ergosterol biosynthesis and EBI sites of inhibition is available in references (Siegel [5] Kato [6]) and only that segment directly related to the interests of this report will be presented here (Figure 4). 

Selected entries from Methods in Enzymology [vol, page(s)] Electron-transport chain [components, 69, 205, 206 sites of inhibition, 69, 676, 677] chloroplast [autoxidizable carriers, 69, 416, 417 DBMIB, 69, 422, 423 dichlorophenolindophenol and related carriers, 69, 418 ferricyanide, 69, 417, 418 isolated, 69,... 

The objectives of the studies reported herein were to (a) compare the effects of a series of phenolic acids, coumarins, and flavonoids on whole chain electron transport and phosphorylation in Isolated plant chloroplasts and mitochondria and (b) identify specific sites of inhibition with polarographic and enzymatic techniques. Exploratory studies were conducted with the 20 compounds listed in Table I. The three glycosides are shown indented below the corresponding aglycones. Detailed studies were conducted with the six compounds, one representative member from each chemical family, designated with an asterisk. 

The site of inhibition is in case of hexamethonium in the sodium channel, whereas trimethaphan blocks the acetylcholine binding site of the receptor. 

Granger, D. L., and Lehninger, A. L. (1982). Sites of inhibition of mitochondrial electron transport in macrophage-injured neoplastic cells. J. Cell Biol. 95, 521-535. 

Figure 4.6 Glycerophospholipid biosynthesis and fungicide mode of action (solid bars indicate sites of inhibition)...

FIGURE 19-11 Cytochrome be, complex (Complex III). The complex is a dimer of identical monomers, each with 11 different subunits. (a) Structure of a monomer. The functional core is three subunits cytochrome b (green) with its two hemes (bH and foL, light red) the Rieske iron-sulfur protein (purple) with its 2Fe-2S centers (yellow) and cytochrome ci (blue) with its heme (red) (PDB ID 1BGY). (b) The dimeric functional unit. Cytochrome c, and the Rieske iron-sulfur protein project from the P surface and can interact with cytochrome c (not part of the functional complex) in the intermembrane space. The complex has two distinct binding sites for ubiquinone, QN and QP, which correspond to the sites of inhibition by two drugs that block oxidative phosphorylation. Antimycin A, which blocks electron flow from heme bH to Q, binds at QN, close to heme bH on the N (matrix) side of the membrane. Myxothiazol, which prevents electron flow from... 

Figure 20-6 Biosynthesis of the dolichol diphosphate-linked oligosaccharide precursor to glycoproteins. The site of inhibition by tunicamycin is indicated.

The poly (ribitol phosphate) synthetase and poly (glycerol phosphate) synthetase are inhibited by vancomycin, novobiocin, and Crystal Violet. Other antibiotic substances which interfere with cell-wall synthesis (such as bacitracin, ristocetin, and streptomycin) are almost without effect on the isolated synthetases, and penicillin is inhibitory at high concentrations only. Moreover, penicillin, vancomycin, and bacitracin do not markedly inhibit synthesis of cell-wall glycosaminopeptide in vitro, although the synthetical activity of extracts of cells which have been pretreated with these antibiotics is lowered.Convincing evidence that the primary site of inhibition by antibiotics is the biosynthesis of cell-wall material has been obtained only for the penicillins and cycloserine, and it appears that the action of even those antibiotics may be more complex than was originally supposed. 

Figure 10. Biosynthetic pathway of ergosterol in Pyricularia oryzae indicating the site of inhibition by azoles and pyrimidines.

Chrysayi Tokousbalides, M. Sisler, H.D. (1979) Site of Inhibition by Tricyclazole in the Melanin Biosynthetic Pathway of Ver-ticillium dahliae. Pesticide Biochemistry and Physiology 11, 64-73. 

The inhibition of chlorophyll biosynthesis by metals was described for higher plants (Bazinsky et al., 1980 Prassad and Prassad, 1987) as well as for algae (De Filippis and Pallaghy, 1976 De Filippis et al., 1981 a). Sensitivity to metals was found for two enzymes of this pathway 8-aminolaevulinic acid (ALA)-dehydratase (EC 4.2.1.24) and protochlorophyllide reductase. Stobart et al. (1985) reported that the synthesis of 8-ALA is also an important site of inhibition in barley (Hordeum vulgare). 

Figure 17.4 The electron transport chain of mitochondria. Triangles indicate sites of inhibition by various compounds. Cyt, cytochrome ETF, electron transfer flavoprotein. (Reproduced with permission from Moreadith RW, Batshaw ML, Ohnishi T, Kerr D, Knox B, Jackson D, Hruben R, Olson J, Reynafarje B, Lehninger AL. Deficiency of the iron-sulfur clusters of mitochondrial reduced nicotinamide-adenine dinucleotide-ubiquinone oxidoreductase (complex I) in an infant with congenital lactic acidosis J Clin Invest 74 685-697, 1984.)...

Sites of inhibition (-) or excitation (+) in the striatum and substantia nigra... 

It has been shown that the accumulation of 5-amino-iV-D-ribosyl-4-imidazolecarboxamide in a purine-requiring mutant of Escherichia coli ceases when the bacteria are supplied with an excess of purine. The site of inhibition is, apparently, before the formation of the imidazole ring occurs, but after the formation of the D-ribosyl moiety, since adenine has only a... 

Is a specific Inhibitor of type II fatty acid synthetase In higher plants and . coll 12401. The acetyl-CoA ACP S-acety1-transferase Is the apparent specific site of Inhibition 12411. Another antibiotic, cerulenin (structure not shown) Inhbits -ketococy1-ACP synthetase I In bacteria, fungi, and plants, but also Is Inhibitory to other sites such as polyketide and sterol biosynthesis 1242-2441. Cerulenin and thiolactomycin Inhibited CQ14W-acetate Incorporation Into fatty acids at 150 values of 50 and 4 uM, respectively 12451. Recently cydohexanedlone herbicides have been shown to Inhibit lipid biosynthesis by Inhibition of acetyl-CoA carboxylase 12461. 

SERTRALINE PIMOZIDE plasma concentrations of these drugs and potential risk of dangerous arrhythmias Sertraline inhibits metabolism of pimozide. Precise site of inhibition uncertain Avoid co-administration... 

This hypothesis was tested by measuring the inhibitory activity of cis- and trans- 2-hydroxy-1,4-cineole. The cis-form was more than an order of magnitude more effective against AS than 1,4-cineole [16] (Figure 2). The addition of the alcohol to the molecule renders it less volatile, therefore allowing more of the compound to react with the site of inhibition. The trans-form of 2-hydroxy-1,4-cineole was less active than either the cis-diastereomer or 1,4-cineole. Greenhouse studies had demonstrated that the cis-form of... 

Allen, NE, Nonclassical targets for antibacterial agents, Annu. Rep. Med. Chem., 20,155-162,1985. Ray, P H, Kelsey, J E, Bigham, E C, Benedict, C D, Miller, T A, Synthesis and use of 3-deoxy-D-manno-2-octulosonate (KDO) in Escherichia coli. Potential sites of inhibition, ACS Symp. Ser., 231, 141-169, 1983. 

Figure 8-10 Folate pathw in humans and bacteria and sites of inhibition by sulf amides and trimethoprim.

Fig. 12.9 Pathway for synthesis of the essential fungal sterol ergosteroland the sites of inhibition by the antifungal agents terbinafine, imidazoles and triazoles.

Figure 3 The sites of inhibition by azole compounds in the ergosterol and gibberellin biosynthesis.

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