FACTORS INFLUENCING REDOX THERMODYNAMICS AND ELECTRON SELF-EXCHANGE FOR THE [FE4S4] CLUSTER IN CHROMATIUM-VINOSUM HIGH-POTENTIAL IRON PROTEIN - THE ROLE OF CORE AROMATIC RESIDUES IN DEFINING CLUSTER REDOX CHEMISTRY

The roles of aromatic core residues in regulating the reduction potent ial, the enthalpy and entropy of reduction, and the self-exchange rate constants for electron-transfer reactions for the prosthetic [Fe4S4]( 3+/2+) cluster of Chromatium vinosum high potential iron protein (HiPI P) have been addressed by a combination of site-directed mutagenesis, high field NMR (EXSY) experiments, and variable temperature spectroche mical redox titration measurements. Minimal changes are observed follo wing nonconservative mutation of residues Tyr19, Phe48, and Phe66. App arently these hydrophobic residues play only a minor role in defining the electronic properties of the cluster. These data support a model, first defined from results obtained on Tyr19 mutant HiPIP's [Agarwal, A., Li, D., & Cowan, J. A. (1995) Proc. Natl. Acad. Sci. U.S.A. 92, 94 40-9444], in which the aromatic core restricts solvent accessibility a nd thereby stabilizes the oxidized [Fe4S4](3+) cluster.