Abstract
The dimeric iron–sulfur flavoprotein (Isf) from Methanosarcina thermophila contains one 4Fe–4S center and one FMN per monomer, and is the prototype of a family widely distributed among strictly anaerobic prokaryotes. Although Isf is able to oxidize ferredoxin, the physiological electron acceptor is unknown; thus, the ability of Isf to reduce O 2 and H 2O 2 was investigated. The product of O 2 or H 2O 2 reduction by Isf was determined to be water. The kinetic parameters of the oxidative half-reactions with O 2 and H 2O 2 as electron acceptors were consistent with a role for Isf in combating oxidative stress. Isf depleted of the 4Fe–4S cluster was unable to oxidize ferredoxin and reduce the FMN cofactor, supporting a role for the cluster in transfer of electrons from ferredoxin to the cofactor. The implications of these properties on the possible function and mechanism of Isf are discussed.
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