Succinate:Quinone Oxidoreductases

Abstract

Succinate:quinone oxidoreductases encompass the succinate:quinone reductase (SQR) enzymes of aerobic respiration (respiratory complex II) and the quinol:fumarate reductase (QFR) enzymes of anaerobic respiration. These enzymes couple the two‐electron oxidation of succinate to fumarate to the two‐electron reduction of quinone to hydroquinone (quinol), and also catalyse the reverse reaction, the reduction of fumarate by quinol. Members of the superfamily are composed of two hydrophilic subunits, a large flavoprotein subunit A and a smaller subunit B containing three iron–sulfur centers, and one large subunit C or two small subunits C and D hydrophobic polypeptides that are integral membrane proteins and bind two, one, or no heme b groups. On the basis of the available crystal structures of members of the superfamily, structural and functional aspects of succinate:quinone oxidoreductases are comprehensively discussed.

Substrate usage determines carbon flux via the citrate cycle in Helicobacter pylori

“…The formation of this isotopologue starting from either [U‐ 13 C 5 ]glutamate or [U‐ 13 C 4 ]succinate provided strong evidence for fluxes via a closed oxidative TCA cycle as the complete 13 C‐backbone of the respective tracers remained intact throughout the cycle. Photometric measurements in H. pylori cell extracts also support the activity of a succinate oxidizing enzyme (Chen, 1999; Lancaster & Simon, 2002). However, it is still unclear whether the fumarate reductase in H. pylori can act in a reversible fashion, as proposed earlier (Ge et al., 2000), or whether another unidentified enzyme is capable of succinate oxidation.…”

“…Finally, malate oxidation to oxaloacetate uses quinone as the electron‐acceptor (enzyme 4) (Kather et al., 2000). Although no designated succinate dehydrogenase is found in the genome, this reaction could be carried out by the respective fumarate reductase, whose bidirectionality was already observed, although with a strong preference for fumarate reduction (Lancaster et al., 2002). It is also noteworthy that genes encoding malate synthase or isocitrate lyase have not been annotated in the genome of H. pylori .…”