Carbon Monoxide Oxidation by Clostridium thermoaceticum and Clostridium formicoaceticum

Abstract

Cultures of C. formicoaceticum and C. thermoaceticum growing on fructose and glucose, respectively, rapidly oxidized CO to CO2. Rates up to 0.4 .mu.mol min-1 mg of wet cells-1 were observed. CO oxidation by cell suspensions was dependent on pyruvate, inhibited by alkyl halides and arsenate and stimulated CO2 reduction to acetate. Cell extracts catalyzed the oxidation of CO with methyl viologen at specific rates up to 10 .mu.mol min-1 mg of protein-1 (35.degree. C, pH 7.2). NAD, NADP and ferredoxin from C. pasteurianum were ineffective as electron acceptors. The catalytic mechanism of CO oxidation was ping-pong, indicating that the enzyme catalyzing CO oxidation can be present in an oxidized and a reduced form. The oxidized form reacted reversibly with cyanide, and the reduced form reacted reversibly with alkyl halides: cyanide inactivated the enzyme only in the absence of CO, and alkyl halides inactivated it only in the presence of CO. Extracts inactivated by alkyl halides were reactivated by photolysis. Apparently, CO oxidation in the 2 bacteria is catalyzed by a corrinoid enzyme and in vivo the reaction is coupled with the reduction of CO2 to acetate. Cultures of C. acidiurici and C. cylindrosporum growing on hypoxanthine did not oxidize CO, indicating that clostridia mediating a corrinoid-independent total synthesis of acetate from CO2 do not possess a CO-oxidizing system.