On the role of conserved proline residues in the structure and function of Clostridium pasteurianum 2[4Fe-4S] ferredoxin.

The widespread occurrence of Pro residues adjacent to Cys ligands in the sequences of [4Fe-4S] electron transfer proteins has not yet found a functional basis. The two such Pro of Clostridium pasteurianum 2[4Fe-4S] ferredoxin have been probed by site-directed mutagenesis. Any one of them, but not both simultaneously, can be substituted without impairing the proper folding of the protein. The reduction potentials of the ferredoxin variants fall in a narrow range of < 20 mV above the potential of the native protein. The biological activities with C. pasteurianum hydrogenase and pyruvate-ferredoxin oxidoreductase do not change significantly, except when Lys replaces Pro. In these cases, the data suggest that the two clusters of 2[4Fe-4S] ferredoxin may not always be equivalent in the interaction with the redox partners. Destabilization of the structure has been observed as the consequence of the Pro19 or Pro48 substitutions. Using 2-D NMR, this effect has been associated with perturbations of both the hydrogen bond network and one amino acid side chain around the [4Fe-4S] clusters. Thus, the conserved Pro found in the binding motif of [4Fe-4S] clusters in proteins strongly stabilizes the active site but does not play an essential role in the mechanism of electron transfer.