Pyruvate-Ferredoxin Oxidoreductase

The sequence of the reaction catalyzed by pyruvate-ferredoxin oxidoreductase was investigated. The pyruvate14CO2 exchange reaction and acetoin formation are inhibited by electron acceptors such as ferredoxin, FAD, FMN, and dyes, but not by DPN. The enzyme catalyzes the formation of acetoin in the presence of pyruvate and acetaldehyde, which suggests that hydroxyethyl thiamine pyrophosphate is an intermediate of the reaction. When substrate levels of the enzyme reacted with pyruvate, a stoichiometric amount of HCO3- was formed but the enzyme chromophore is not reduced. The addition of coenzyme A to the reaction mixture in the absence of an electron acceptor leads to bleaching of the enzyme chromophore and formation of a stoichiometric amount of acetyl-CoA. The enzyme chromophore reduced by addition of pyruvate and CoA is reoxidized by FAD or ferredoxin. The chromophore is also reduced by reduced ferredoxin formed by the hydrogenase system. Based on these observations a reaction sequence is proposed and evidence in support of the mechanism is discussed. It is concluded that the enzyme chromophore, rather than ferredoxin, serves as a primary electron acceptor.