The Quinone Connection

Abstract

The concept that the long-chain quinones in energy-transducing membranes (Fig. 5.1 A and B) act as mobile carriers of electrons and protons is partly based on the high concentration of quinone in all energy-transducing membranes (Table 5.1). This quinone can be readily extracted with organic solvent. In spite of the apparent excess of quinone in all these systems, most of it is needed to maintain a normal respiratory flow, because partial extraction of mitochondrial quinone results in loss of activity (Schneider et al., 1985). Quinone analogue inhibitors block electron transfer by competing for the quinone binding sites located on electron transport protein complexes (Fig. 5.1C). The compounds 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) and atrazine inhibit plant and algal photosynthetic electron transport at the site (the quinone-binding D1 polypeptide) of reduction of the plastoquinone pool by photosystem II (PS II). Piericidin and rotenone inhibit at the site of reduction of the ubiquinone (UQ) pool on the NADH dehydrogenase (Fig. 4.25), and several inhibit effectively on the p-side of the membrane (toward which protons are translocated, positive <inline>1</inline>) at the cytochrome b ₆ f (2,5-dibromo-3-methyl-6-isopropylbenzoquinone, stigmatellin) or bc ₁ (stigmatellin, 5-n-undecyl-6-hydroxy-4,7-dioxobenzothiazole, myxathiazol) complexes, preventing oxidation of the plasto- or ubiquinone pool. Antimycin A inhibits the bc ₁ complex on the n-side of the membrane.