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

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Abstract
The electron-transfer mechanism of the Fe4S4* high-potential iron-sulfur proteins (HiPIP) was explored via a stopped-flow spectrophotometric kinetic study of the reduction of Chromatium vinosum and Rhodopseudomonas gelatinosa HiPIP by native and (TNP) (trinitrophenyl[TNP]-Lys-13) horse cytochrome c. The influence of electrostatic effects was effectively partitioned from the redox process per se. The corrected rates were 12.3 .times. 104 and 3.8 .times. 104 M-1 s-1 for native cytochrome c with C. vinosum and R. gelatinosa HiPIP, respectively, and 17.5 .times. 104 and 5.46 .times. 104 M-1 s-1 for TNP-cytochrome c with the 2 HiPIP, respectively. The faster rates of TNP-cytochrome c with the HiPIP are unexpected in terms of possible steric interaction since Lys 13 is at the top of the heme crevice. The somewhat faster rates of the TNP-cytochrome c over native cytochrome c may be explained by the hypothesis that TNP-cytochrome c reacts more quickly because modification of the Lys-13 residue somewhat destabilizes the heme crevice. Alternately, in the light of the hydrophobic nature of the trinitrophenyl group and the X-ray crystallographic structure of HiPIP, the TNP group may facilitate electron transfer by interacting with a hydrophobic region on the HiPIP molecular surface. The region about the S*4 sulfur atom is the most exposed and accessible hydrophobic region on the HiPIP surface and is the point of closest approach of the S*4 to the external environment.