The catalytic mechanism of 2-oxoacid:ferredoxin oxidoreductases from Halobacterium halobium. One-electron transfer at two distinct steps of the catalytic cycle.

The catalytic cycle of the 2-oxoacid:ferredoxin oxidoreductases from Halobacterium halobium was investigated. The first step is binding of the 2-oxoacid to the enzyme followed by decarboxylation and transfer of one electron to the [4Fe-4S] cluster of the functional unit. The cluster is then reoxidized by ferredoxin or, in the absence of the physiological electron acceptor, by oxygen. In the resulting stable enzyme-intermediate radical the decarboxylation product of the 2-oxoacid remains tightly bound until reaction with coenzyme A caused formation of acyl-CoA and concomitant transfer of the second electron to the cluster, which again is reoxidized by ferredoxin or oxygen. After purification, part of the enzyme molecules still contain the intermediate radical. Enzyme preparations either free of radical or containing enhanced amounts are obtained by treatment with coenzyme A or 2-oxoacid, respectively. Whenever the radical is present in an enzyme molecule the respective binding site for the 2-oxoacid is blocked.