New stereochemical analogies between iron-sulfur electron transport proteins

Active sites of Chromatium high potential iron protein (HiPIP) and Pseudomonas Aerogenes ferredoxin can be brought into equivalent orientations by assuming that their Fe4S4Sgamma4 clusters have the effective symmetry of the non-axial molecular point group Cs. Previously undetected analogies between the two proteins emerge as a result of selecting a common orientation in this mammer. Polypeptide segments connecting Cys 46 to Cys 63 in HiPIP and Cys 18 to Cys 35 in ferredoxin are analogous in the sense that they are the same length, they connect equivalent cysteinyl sulfur atoms, and they have similar, twisted antiparalled beta conformations. Tyrosine residues 19 (HiPIP) and 2 (ferredoxin) are analogous in the sense that they interact closely with equivalent inorganic sulfur atoms. To a good approximation, interactions with the polypeptide backbone and with tyrosine side chains in the two proteins place their Fe4S4Sgamma4 moieties into diastereomeric environments, which would be expected to induce different physical and chemical behavior. Circular dichroism spectra of native and super reducible HiPIP (Cammack, R. (1973) Biochem. Biophys. Res. Commun. 54, 548-554) suggest that this relationship can help to explain the contrasting oxidoreduction properties of the two proteins.