Inhibitors and artificial electron acceptors

See also Difference Spectra, Inhibitors and Artificial Electron Acceptors, FMN, FMNH2, NAD" ", NADH, Rotenone, Amytal, Coenzyme Q, Cytochromes, Antimycin A, Cyanide, Azide, Carbon Monoxide. 

Figure 15.9 Sites of action of some respiratory inhibitors and artificial electron acceptors. 

Inhibitors and Artificial Electron Acceptors Shuttling Electron Carriers into the Mitochondrion... 

See also NADH, Coenzyme Q, Inhibitors and Artificial Electron Acceptors... 

The development by Chance of a dual wavelength spectrophotometer permitted easy observation of the state of oxidation or reduction of a given carrier within mitochondria.60 This technique, together with the study of specific inhibitors (some of which are indicated in Fig. 18-5 and Table 18-4), allowed some electron transport sequences to be assigned. For example, blockage with rotenone and amytal prevented reduction of the cytochrome system by NADH but allowed reduction by succinate and by other substrates having their own flavoprotein components in the chain. Artificial electron acceptors, some of which are shown in Table 18-5,... 

Artificial electron acceptors, such as ferricyanide, can be substituted for NADP these give rise to oxygen evolution but involve only a short segment of the oxidation chain. This partial reaction is known as the Hill reaction and compounds that disrupt it are known as Hill inhibitors. Herbicides that inhibit the Hill reaction, by blocking electron transport, prevent the production of ATP and NADPH required for carbon dioxide fixation. 

The movement of electrons through the electron carrying proteins of the inner mitochondrial membrane is shown in Figure 15.9. Also shown are inhibitors of electron movement at their point of action and the sites where artificial electron acceptors can accept electrons from the electron transport system. Specific inhibitors shown in Figure 15.9 are rotenone, amytal, antimycin A, cyanide, azide, and carbon monoxide. The artificial electron acceptors are methylene blue, phenazine methosulfate, 2,6-indophenol, tetramethyl-p-phenylene diamine, and ferricyanide. 

Artificial electron acceptors have the opposite effect of inhibitors. That is they relieve the build-up of electrons at a specific point arising from an inhibitor. For example, if mitochondria were treated with both antimycin A (an inhibitor) and methylene blue (an artificial electron acceptor). Complex I would be oxidized relative to CoQ, due to release of electrons from complex I to methylene blue. CoQ would remain reduced, however, because it would be blocked from transferring its electrons to the next carrier. Complex III. 

See also Electron Transport, Cytochrome Oxidase, Inhibitors and Artificial CyOfliCl Electron Acceptors, Azide, Carbon Monoxide, Complex IV... 

As shown in Fig. 2 (lanes B, D) DCMU (3-(3 ,4 -dichlorophenyl)-l,l-dimethylauria) and PpBQ (phenyl-p-benzoquinone) completely abolish PPi dependent protein phosphorylation. Inhibition by the PSII inhibitor DCMU confirms that this phosphorylation is electron transport dependent. Inhibition by PpBQ is consistent with its role as an artificial electron acceptor, leaving the plastoquinone in an oxidized state. This indicates that PQ has to be reduced for the activation of the PPi dependent protein kinase, as is known to be the case with ATP. 

It should be noted that the dye-linked assay system is artificial in every way the electron acceptor may also be an inhibitor it has a high pH optimum (about pH 9) it requires ammonia as activator, but this may also inhibit cyanide is a competitive inhibitor and may be used as a protective agent in the absence of added substrate a high rate of dye reduction occurs which may or may not be taken into account when calculating rates of reaction. This complexity and confusion is mentioned here as an explanation for the length and complexity of some the following discussion. 

Schemes of electron transfer interactions of FNR in thylakoid membranes are deduced mainly from experimental results obtained in model systems (reviewed in 1). Pioneering works by Bouges-Bocquet (4), who studied flash-induced transient of FNR in algal cells, has not tDeen followed by systematic investigations in isolated chloroplasts and thylakoid membranes. In algal cells, ambiguity arises from intense light scattering (5). Low permeability of the cell wall also restricts the use of inhibitors, ionophores, artificial acceptors and substrates. It is consequently necessary to confirm and extend these earlier studies using isolated thylakoid membranes and/or subchloroplast particles.

Barbiturates, rotenone and piericidin block electron transfer from the FeS centres to ubiquinone-10, but do not block the reduction of artificial acceptors by NADH in Type II NADH dehydrogenase. Both rotenone [282,286] and piericidin [307] bind to the enzyme with an apparent 1 1 stoicheiometry to the content of FMN [39]. The binding of these inhibitors is non-covalent which has so far prevented identification of the binding site(s). All EPR- or optically detectable redox centres (possibly with the exception of centre N-la) are reducible by NADH in the presence of these inhibitors [308,309]. The structural analogy between piericidin and Q-10 provides further evidence that these inhibitors interact at the point where the enzyme delivers reducing equivalents to ubiquinone (Fig. 3.14). 

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