Complex II Is Complex Too

摘要

In aerobic eukaryotic cells, the generation of energy in the form of adenosine triphosphate (ATP) is mainly driven by the activity of the respiratory chain enzymes of the mitochondrial inner mem- brane. The respiratory chain―composed of enzyme complexes Ⅰ to Ⅳ, ubiquinone, cyto- chrome c, and ATP synthase (complex Ⅴ)―transfers electrons from NADH and succinate at one end to molecular oxygen at the other (1). On page 700 of this issue, Yankovskaya et al. (2) present the x-ray crystal structure at 2.6 A resolution of the bacterial version of eukaryotic complex Ⅱ. This enzyme, called succinate:quinone oxidoreductase (SQR) in Escherichia coli, couples the two-electron oxidation of succinate forming fumarate, to the reduction of ubiquinone (3). SQR, which is also the succinate dehydrogenase of the Krebs cycle, is crucial for intermediary metabolism and energy production in eukaryotic cells and bacteria under aerobic conditions.