CALCULATION OF THE REDOX POTENTIALS OF IRON-SULFUR PROTEINS - THE 2- 3--COUPLE OF [FE4S4-ASTERISK-CYS(4)] CLUSTERS IN PEPTOCOCCUS-AEROGENESFERREDOXIN, AZOTOBACTER-VINELANDII FERREDOXIN-I, AND CHROMATIUM-VINOSUM HIGH-POTENTIAL IRON PROTEIN/

Calculations of the redox potentials of the 2-/3- couples of [Fe4S4Cy s(4)] clusters in the iron-sulfur proteins Peptococcus aerogenes ferre doxin (PaFd), Azotobacter vinelandii ferredoxin I (AvFdI) and Chromati um vinosum high potential iron protein (C nu HiPIP) based on the Prote in Dipoles Langevin Dipoles (PDLD) method are reported. The structures of these proteins have been determined by X-ray crystallography; in t he case of PaFd the structure has recently been revised due to a chang e in the sequence close to Cluster II. The large differences between t he potentials of the [Fe4S4Cys(4)] clusters of PaFd and A nu FdI and the potential of the [Fe4S4CYs(4)] cluster of CUHiPIP are successfull y modeled and originate principally in differences in the configuratio n of main-chain amide groups near the clusters. The small difference b etween the potentials of PaFd and AvFdI is also satisfactorily modeled in the case of Cluster I of PaFd. Solvent dipoles close to the cluste r in PaFd are an important contributor to its higher potential. The tw o X-ray structures of PaFd yield similar results for Cluster I of PaFd . In contrast, the results for Cluster II differ substantially; for re asons not yet clear, the recently revised structure leads to results i n worse agreement with experiment.