Oxidation of Carbon Monoxide in Cell Extracts of Pseudomonas carboxydovorans

Abstract

Extracts of aerobically, CO-autotrophically grown cells of Pseudomonas carboxydovorans were shown to catalyze the oxidation of CO to CO 2 in the presence of methylene blue, pyocyanine, thionine, phenazine methosulfate, or toluylene blue under strictly anaerobic conditions. Viologen dyes and NAD(P) + were ineffective as electron acceptors. The same extracts catalyzed the oxidation of formate and of hydrogen gas; the spectrum of electron acceptors was identical for the three substrates, CO, formate, and H 2 . The CO- and the formate-oxidizing activities were found to be soluble enzymes, whereas hydrogenase was membrane bound exclusively. The rates of oxidation of CO, formate, and H 2 were measured spectrophotometrically following the reduction of methylene blue. The rate of carbon monoxide oxidation followed simple Michaelis-Menten kinetics; the apparent K m for CO was 45 μM. The reaction rate was maximal at pH 7.0, and the temperature dependence followed the Arrhenius equation with an activation energy (ΔH 0 ) of 35.9 kJ/mol (8.6 kcal/mol). Neither free formate nor hydrogen gas is an intermediate of the CO oxidation reaction. This conclusion is based on the differential sensitivity of the activities of formate dehydrogenase, hydrogenase, and CO dehydrogenase to heat, hypophosphite, chlorate, cyanide, azide, and fluoride as well as on the failure to trap free formate or hydrogen gas in coupled optical assays. These results support the following equation for CO oxidation in P. carboxydovorans : CO + H 2 O → CO 2 + 2 H + + 2e − The CO-oxidizing activity of P. carboxydovorans differed from that of Clostridium pasteurianum by not reducing viologen dyes and by a pH optimum curve that did not show an inflection point.