Role of carbon monoxide dehydrogenase in the autotrophic pathway used by acetogenic bacteria.

Abstract

Anaerobic acetogenic bacteria utilize a pathway of autotrophic growth that differs from any previously described. One part of the pathway involves the reduction of CO2 formate and its subsequent conversion to the methylmoiety of methyltetrahydrofolate. The 2nd part involves the formation of a C1 intermediate from CO, CO2 and H2, or the carboxyl or pyruvate and combination of the intermediate with CoA and methyltetrahydrofolate mediated by a corrinoid enzyme to yield acetyl-CoA. Based on studies concerned with this latter portion of the pathway, and it was proposed that a C1 intermediate is formed via CO dehydrogenase. It remained possible, however, that the function of the CO dehydrogenase is to reduce the C of the corrinoid enzyme to Co+, which is required for it to act as a methyl acceptor, and that the dehydrogenase is not involved directly in the formation of a C1 intermediate. All the enzymes required for the synthesis of acetyl-CoA from CO and methyltetrahydrofolate or from methyltetrahydrofolate and the carboxyl of pyruvate have now been purified. With these purified enzymes, it was possible to show that CO dehydrogenase is essential for acetyl-CoA synthesis with CO as the substrate under conditions in which the C of the corrinoid is reduced by other means. Using pyruvate ferredoxin oxidoreductase, it was also shown that a 14C1-CO dehydrogenase complex is formed from [1-14C]pyruvate. [1-14C]acetyl-CoA was synthesized using the 14C1-CO dehydrogenase complex. Thus, the evidence appears conclusive that CO dehydrogenase has a direct role in the formation of the carboxyl of acetyl-CoA.