Interactions between the molybdenum cofactor and iron-sulfur clusters of Escherichia coli dimethylsulfoxide reductase

We have used site-directed mutagenesis to study the interactions between the molybdo-bis(molybdopterin guanine dinucleotide) cofactor (Mo-bisMGD) and the other prosthetic groups of Escherichia coli Me2SO reductase (DmsABC). In redox-poised preparations, there is a significant spin-spin interaction between the reduced Em,7 = -120 mV [4Fe-4S] cluster of DmsB and the Mo(V) of the Mo-bisMGD of DmsA. This interaction is significantly modified in a DmsA-C38S mutant that contains a [3Fe-4S] cluster in DmsA, suggesting that the [3Fe-4S] cluster is in close juxtaposition to the vector connecting the Mo(V) and the Em,7 = -120 mV cluster of DmsB. In a DmsA-R77S mutant, the interaction is eliminated, indicating the importance of this residue in defining the interaction pathway. In ferricyanide-oxidized glycerol-inhibited DmsAC38SBC, there is no detectable interaction between the oxidized [3Fe-4S] cluster and the Mo-bisMGD, except for a minor broadening of the Mo(V) spectrum. In a double mutant, DmsAS176ABC102SC, which contains an engineered [3Fe-4S] cluster in DmsB, no significant paramagnetic interaction is detected between the oxidized [3Fe-4S] cluster and the Mo(V). These results have important implications for (i) understanding the magnetic interactions between the Mo(V) and other paramagnetic centers and (ii) delineating the electron transfer pathway from the [4Fe-4S] clusters of DmsB to the Mo-bisMGD of DmsA.