MECHANISM OF CARBON-MONOXIDE OXIDATION BY THE CARBON-MONOXIDE DEHYDROGENASE ACETYL-COA SYNTHASE FROM CLOSTRIDIUM-THERMOACETICUM - KINETIC CHARACTERIZATION OF THE INTERMEDIATES/

Carbon monoxide dehydrogenase/acetyl-CoA synthase (CODH/ACS) from Clos tridium thermoaceticum catalyzes (i) the synthesis of acetyl-CoA from a methylated corrinoid protein, CO, and coenzyme A and (ii) the oxidat ion of CO to CO2. CO oxidation occurs at a Ni- and FeS-containing cent er known as cluster C. Electrons are transferred from cluster C to a s eparate metal center, cluster B, to external accepters like ferredoxin . In the work described here, we performed reductive titrations of COD H/ACS with CO and sodium dithionite and monitored the reaction by elec tron paramagnetic resonance (EPR) spectroscopy. We also performed pre- steady-state kinetic studies by rapid freeze-quench EPR spectroscopy ( FQ-EPR) and stopped-flow kinetics. Redox titrations of CODH/ACS reveal ed the existence of a UV-visible and EPR-silent electron acceptor deno ted center S that does not appear to be associated with any of the oth er metal centers in the protein. Our results support the previous prop osals [Anderson, M. E., & Lindahl, P. A. (1994) Biochemistry 33, 8702- 8711; Anderson, M. E., & Lindahl, P. A. (1996) Biochemistry 35, 8371-8 380] that the C-red2 form of cluster C is two electrons more reduced t han the C-red1 form. The combined results from titrations and pre-stea dy-state studies were used to formulate a mechanism for CO oxidation, composed of the following steps: (i) CO binding to the [C-red1,B-ox,X- ox] state to yield a C-red1-CO complex; (ii) two-electron reduction of C-red1 to C-red2 concerted with CO2 release; (iii) binding of a secon d CO molecule to the [C-red2,B-ox,X-ox] state to form a C-red2-CO comp lex; (iv) electron transfer from C-red2-CO to cluster B to form [C-red 2,B-red,X-red] with concerted release of the second CO2. Step iii comp etes with internal electron transfer from C-red2 to B-ox and X-ox. At high CO concentrations, step iii is favored, whereas at low concentrat ions, only one CO molecule per turnover binds and undergoes oxidation. Closure of the catalytic cycle involves electron transfer from reduce d enzyme to an electron acceptor protein, like ferredoxin. X-ox is a y et-uncharacterized electron acceptor that may be an intermediate in th e reduction of center S. The C-red2 state appears to be the predominan t state of cluster C during steady-state turnover. The rate-determinin g step for the first half-reaction is step iv, while during steady-sta te turnover, it appears to be electron transfer to external electron a ccepters.