The autotrophic pathway of acetogenic bacteria. Role of CO dehydrogenase disulfide reductase.

An enzyme from Clostridium thermoaceticum has been isolated which reduces disulfides of carbon monoxide dehydrogenase and it has been named CO dehydrogenase disulfide reductase. The enzyme is a tetramer of molecular weight 225,000 made up of four apparently identical monomers. It does not contain methionine or tryptophan and contains 2 calcium and 1 zinc/monomer. NADP or ferredoxin serves as an electron carrier. This enzyme is part of the system that permits certain bacteria to grow with CO or CO2 and H2 as the source of carbon and energy. The portion of the pathway which is being investigated is the conversion of methyltetrahydrofolate, CO, and CoASH to acetyl-CoA. All the enzymes required for this synthesis have now been purified. In combination with CO dehydrogenase, CO dehydrogenase disulfide reductase with NADP or ferredoxin catalyzes a reversible exchange of [3H]CoASH with acetyl-CoA. The disulfide reductase apparently is involved in the portion of the pathway in which CoASH is introduced into the acetyl-CoA. In addition, the reductase activates CO dehydrogenase in the overall synthesis of acetyl-CoA from methyltetrahydrofolate, CO, and CoASH by reducing about one disulfide group/monomer of the alpha 3 beta 3 CO dehydrogenase. The above exchange reaction in combination with the observation that [14C]acetate is formed from CO and the 14CH3-[Co]corrinoid enzyme in the absence of CoASH have permitted ordering of the sequence of reactions by which CO dehydrogenase plays a central role in the autotrophic synthesis of acetyl-CoA.