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Carbon monoxide dehydrogenase (CODH) from Oligotropha carboxydovorans catalyzes the oxidation of carbon monoxide to carbon dioxide, providing the organism both a carbon source and energy for growth. In the oxidative half of the catalytic cycle, electrons gained from CO are ultimately passed to the electron transport chain of the Gram-negative organism, but the proximal acceptor of reducing equivalents from the enzyme has not been established. Here we investigate the reaction of the reduced enzyme with various quinones and find them to be catalytically competent. Benzoquinone has a k(ox) of 125.1 s(-1) and a K(d) of 48 μM. Ubiquinone-1 has a k(ox)/K(d) value of 2.88 × 10(5) M(-1) s(-1). 1,4-Naphthoquinone has a k(ox) of 38 s(-1) and a K(d) of 140 μM. 1,2-Naphthoquinone-4-sulfonic acid has a k(ox)/K(d) of 1.31 × 10(5) M(-1) s(-1). An extensive effort to identify a cytochrome that could be reduced by CO/CODH was unsuccessful. Steady-state studies with benzoquinone indicate that the rate-limiting step is in the reductive half of the reaction (that is, the reaction of oxidized enzyme with CO). On the basis of the inhibition of CODH by diphenyliodonium chloride, we conclude that quinone substrates interact with CODH at the enzyme's flavin site. Our results strongly suggest that CODH donates reducing equivalents directly to the quinone pool without using a cytochrome as an intermediary.

Reaction of the Molybdenum- and Copper-Containing Carbon Monoxide Dehydrogenase from Oligotropha carboxydovorans with Quinones