SUBSTRATE AND COFACTOR REACTIVITY OF A CARBON-MONOXIDE DEHYDROGENASE CORRINOID ENZYME COMPLEX - STEPWISE REDUCTION OF IRON-SULFUR AND CORRINOID CENTERS, THE CORRINOID CO2+ 1+ REDOX MIDPOINT POTENTIAL, AND OVERALL SYNTHESIS OF ACETYL-COA/

Cleavage of the acetyl carbon-carbon bond of acetyl-CoA in Methanosarc ina barkeri is catalyzed by a high molecular mass multienzyme complex. The complex contains a corrinoid protein and carbon monoxide dehydrog enase and requires tetrahydrosarcinapterin (H4SPt) as methyl group acc eptor. Reactions of the enzyme complex with carbon monoxide and with t he methyl group donor N5-methyltetrahydrosarcinapterin (CH3-H4SPt) hav e been analyzed by UV-visible spectroscopy. Reduction of the enzyme co mplex by CO occurred in two steps. In the first step, difference spect ra exhibited peaks of maximal absorbance decrease at 426 nm (major) an d 324 nm (minor), characteristic of Fe-S cluster reduction. In the sec ond step, corrinoid reduction to the Co1+ level was indicated by a pro minent peak of increased absorbance at 394 nm. Spectrophotometric anal yses of the corrinoid redox state were performed on the intact complex at potentials poised by equilibration with gas mixtures containing di fferent [CO2]/[CO] ratios or by variation of the [H+]/[H-2] ratio. The corrinoid Co2+/1+ midpoint potential was -426 mV (+/-4 mV, n = 1.16 e lectrons, 24-degrees-C), independent of pH (pH 6.4-8.0). The results i ndicated a significant fraction of Co1+ corrinoid at potentials existi ng in vivo. The reduced corrinoid reacted very rapidly with CH3-H4SPt. Reaction with methyl iodide was slow, and methylation by S-adenosylme thionine was not observed. The rate of methyl group transfer from CH3- H4SPt greatly exceeded the rate of CO reduction of enzyme centers. The enzyme complex catalyzed efficient synthesis of acetyl-CoA from coenz yme A, CO, and CH3-H4SPt. During acetyl-CoA synthesis, demethylation o f CH3-H4SPt was monitored by the absorbance increase at 312 nm. Concom itant appearance of the 394-nm enzyme peak indicated Co1+ corrinoid re generation from Co3+-methyl-methyl corrinoid. Results support the prop osed function of the corrinoid as methyl group carrier during acetyl-C oA synthesis and decomposition.