Four Cys residues in heterodimeric 2-oxoacid:ferredoxin oxidoreductase are required for CoA-dependent oxidative decarboxylation but not for a non-oxidative decarboxylation

Heterodimeric 2-oxoacid:ferredoxin oxidoreductase (OFOR) from Sulfolobus tokodaii (StOFOR) has only one [4Fe-4S]2 + cluster, ligated by 4 Cys residues, C12, C15, C46, and C197. The enzyme has no other Cys. To elucidate the role of these Cys residues in holding of the iron-sulfur cluster in the course of oxidative decarboxylation of a 2-oxoacid, one or two of these Cys residues was/were substituted with Ala to yield C12A, C15A, C46A, C197A and C12/15A mutants. All the mutants showed the loss of iron-sulfur cluster, except the C197A one which retained some unidentified type of iron-sulfur cluster. On addition of pyruvate to OFOR, the wild type enzyme exhibited a chromophore at 320 nm and a stable large EPR signal corresponding to a hydroxyethyl-ThDP radical, while the mutant enzymes did not show formation of any radical intermediate or production of acetyl-CoA, suggesting that the intact [4Fe-4S] cluster is necessary for these processes. The stable radical intermediate in wild type OFOR was rapidly decomposed upon addition of CoA in the absence of an electron acceptor. Non-oxidative decarboxylation of pyruvate, yielding acetaldehyde, has been reported to require CoA for other OFORs, but StOFOR catalyzed acetaldehyde production from pyruvate independent of CoA, regardless of whether the iron-sulfur cluster is intact [4Fe-4S] type or not. A comprehensive reaction scheme for StOFOR with a single cluster was proposed.