High-potential iron-sulfur proteins and their possible site of electron transfer

The electron-transfer mechanism of the Fe4S4 high-potential iron-sulfur proteins (HiPIP's) was explored via a stopped-flow spectrophotometric kinetic study of the reduction of Chromatium vinosum and Rhodopseudomonas gelatinosa HiPIP's by both native and trinitrophenyllysine-13 horse cytochrome c. The influence of electrostatic effects was also effectively partitioned from the redox process per se. The corrected rates were 12.3 X 10(4) and 3.8 X 10(4) M-1 s-1 for native with C. vinosum and R. gelatinosa HiPIP, respectively, and 17.5 X 10(4) and 5.46 X 10(4) M-1 s-1 for TNP-cytochrome c with the two HiPIP's, respectively. The faster rates of TNP-cytochrome c with the HiPIP's are unexpected in terms of possible steric interaction since lysine-13 is at the top of the heme crevice. In understanding the somewhat faster rates of the TNP-cytochrome c over native cytochrome c it is possible that (1) TNP-cytochrome c reacts more quickly since modification of the lysine-13 residue destabilizes somewhat the heme crevice or (2) in light of the hydrophobic nature of the trinitrophenyl group and the X-ray crystallographic structure of HiPIP, the TNP group facilitates electron transfer by interacting with a hydrophobic region on the HiPIP molecular surface. The region about the S4 sulfur atom is the most exposed and accessible hydrophobic region on the HiPIP surface, in addition to being the point of closest approach of the S4 to the external environment.