Quinone reduction sites and the role of heme in succinate:quinone reductase. Studies in Bacillus subtilis and Paracoccus denitrificans

Succinate:quinone reductase (SQR) is an enzyme in the respiratory system of aerobic cells. SQR catalyzes two reactions, the oxidation of succinate to fumarate and the reduction of quinone to quinol. These reactions are coupled by electron transfer within the enzyme from the site of succinate oxidation to the site of quinone reduction. Some enzymes reduce ubiquinone (exergonic reaction) whereas others reduce menaquinone (endergonic reaction). SQR is composed of one membrane-peripheral domain and one membrane-anchor domain. Succinate is oxidized on the peripheral domain which is attached to the electronegative side of the respiratory membrane. This domain consists of two polypeptides, one covalently bound FAD group, one [2Fe-2S] cluster, one [4Fe-4S] cluster and one [3Fe-4S] cluster. The anchor domain harbors the quinone reduction site(s) and is, depending on the organism, composed of one or two polypeptides and one or two b-type hemes. Succinate:menaquinone reductases contain two heme groups, one proximal heme (heme bP) located close to the negative side of the membrane and one distal heme (heme bD) located close to the positive side of the membrane. Succinate:ubiquinone reductases contain only heme bP. Electron transfer from succinate to quinone occurs in a linear pathway including the FAD, the [2Fe-2S] cluster, the [4Fe-4S] cluster, the [3Fe-4S] cluster and sometimes heme as electron carriers in the given order. I have studied SQR from two evolutionary very distant bacteria, Bacillus subtilis and Paracoccus denitrificans. B. subtilis SQR reduces menaquinone whereas P. denitrificans SQR reduces ubiquinone. The combined experimental results support the following model. In succinate:ubiquinone reductases the quinone reduction site is located close to the [3Fe-4S] cluster on the negative side of the membrane. Heme is not required as an electron carrier in these enzymes. Ubiquinone reduction by succinate is not dependent on the transmembrane electrochemical potential. In succinate:menaquinone reductases the quinone reduction site is located close to the positive side of the membrane. Heme bP and heme bD mediate transmembrane electron transfer from the [3Fe-4S] cluster to menaquinone. The transmembrane electrochemical potential provides driving force for the endergonic reduction of menaquinone by succinate