EVIDENCE THAT CARBON-MONOXIDE IS AN OBLIGATORY INTERMEDIATE IN ANAEROBIC ACETYL-COA SYNTHESIS

Carbon monoxide is produced by several biological reactions. It is pro posed to act as an intracellular signaling molecule and can serve as t he carbon and electron source for certain bacteria. Direct evidence fo r a new biological role for CO is presented here. The results strongly indicate that CO is produced as an obligatory intermediate during gro wth of the acetogenic bacterium Clostridium thermoaceticum on glucose, H-2/CO2, or aromatic carboxylic acids. Our results are consistent wit h earlier hypotheses of the intermediacy of CO during growth of acetog enic bacteria on CO2 and hexoses [Diekert, G., & Ritter, ICI. (1983) F EMS Microbiol. Lett. 17, 299-302] and methanogenic Archaea on CO2 [Stu pperich, E., Hammel, K. E., Fuchs, G., & Thauer, R. K. (1983) FEES Let t. 152, 21-23]. Therefore, CO production is a key step in the Wood-Lju ngdahl pathway of acetyl-CoA synthesis. The carbonyl group of acetyl-C oA is shown to be formed from the carboxyl group of pyruvate by the fo llowing steps. (i) Pyruvate undergoes decarboxylation by pyruvate:ferr edoxin oxidoreductase to form acetyl-CoA and CO2. (ii) CO2 is reduced to CO by the CODH site of the bifunctional enzyme CO dehydrogenase/ace tyl-CoA synthase (CODH/ACS). (iii) CO generated in situ combines with the ACS active site to form a paramagnetic adduct that has been called the NiFeC species, and (iv) the bound carbonyl group combines with a bound methyl group and CoA to generate acetyl-CoA. To our knowledge, t his paper represents the first demonstration of a pathway in which CO is produced and then used as a metabolic intermediate.