Catalysis of Acetyl-CoA Cleavage and Tetrahydrosarcinapterin Methylation by a Carbon Monoxide Dehydrogenase-corrinoid Enzyme Complex

An enzyme complex containing carbon monoxide dehydrogenase and a corrinoid protein has been isolated from Methanosarcina barkeri. Sodium dodecyl sulfate-gel electrophoresis revealed five polypeptides of molecular masses alpha = 19,700, beta = 84,500, gamma = 63,200, delta = 53,000, and epsilon = 51,400 Da in equimolar amounts. One mol of cobamide cofactor was found per minimal alpha beta gamma delta epsilon unit. The molecular mass of the native complex was 1,600,000 Da by high pressure liquid chromatography (HPLC) gel filtration, which suggested an alpha 6 beta 6 gamma 6 delta 6 epsilon 6 oligomeric structure. Catalysis of a reaction involving cleavage of acetyl-CoA and methylation of tetrahydrosarcinapterin was indicated by spectrophotometric analyses; a time-dependent absorption decrease in the 300-320 nm region was observed in the complete reaction mixture which contained acetyl-CoA, tetrahydrosarcinapterin, and the enzyme complex. In control samples lacking any one of the these components the absorption spectrum remained virtually unaltered. Reversed-phase HPLC analysis confirmed that tetrahydrosarcinapterin was converted to a product that co-eluted with authentic methyltetrahydrosarcinapterin. The product also exhibited the UV-visible absorption spectrum expected for methyltetrahydrosarcinapterin. Free CoA was identified as an additional product of the reaction. The carbonyl group of acetyl-CoA was oxidized to carbon dioxide. Spectral changes indicated concomitant Fe/S center reduction. Production of CoA was essentially stoichiometric with methyltetrahydrosarcinapterin formation and tetrahydrosarcinapterin consumption. Analyses during purification showed that catalytic activity was restricted exclusively to the fractions that contained the carbon monoxide dehydrogenase-corrinoid enzyme complex.