The flavoprotein succinate dehydrogenase (succinate:quinone reductase — SQR) plays a crucial role both in the Krebs cycle and, as the main component of complex II, in mitochondrial and bacterial aerobic respiratory chains. SQR catalyzes the reversible oxidation of succinate to fumarate, coupling it with the reduction of ubiquinone in the membrane. But, the details of the succinate to ubiquinone electron transport pathway, and how the enzyme limits reactive oxygen species formation have been unclear. In the January 30 Science, Victoria Yankovskaya and colleagues at the Veterans Affairs Medical Center, San Francisco, USA, report the structure of SQR from Escherichia coli at 2.6 Å resolution (Science, 299:700-704, January 31, 2003).

Yankovskaya et al. observed that SQR is composed of four subunits containing flavin adenine dincucleotide (FAD), three iron-sulfur clusters and one heme b as prosthetic groups. To understand the mechanisms of reactive oxygen species production they compared...

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