The kinetics of electron transfer from the triplet-excited Zn-porphyrin to a Ru(NH3)5(His-33)3+ complex have been measured in Zn-substituted ruthenium-modified cytochrome c under denaturing conditions. In the folded protein, the electron-tunneling rate constant is 7.5 × 105 s-1. As the protein is denatured with guanidine hydrochloride, a faster adiabatic electron-transfer reaction appears (4.0 × 106 s-1, [guanidine hydrochloride] = 5.4 M) that is limited by the rate of intrachain diffusion to bring the Zn-porphyrin and Ru complex into contact. The 250-ns contact time for formation of a 15-residue loop in denatured cytochrome c is in accord with a statistical model developed by Camacho and Thirumalai [Camacho, C. J. & Thirumalai, D. (1995) Proc. Natl. Acad. Sci. USA 92, 1277-1281] that predicts that the most probable transient loops formed in denatured proteins are comprised of 10 amino acids. Extrapolation of the cytochrome c contact time to a 10-residue loop sets the folding speed limit at ≈107 s-1.
|Number of pages||3|
|Journal||Proceedings of the National Academy of Sciences of the United States of America|
|Publication status||Published - 2003 Apr 1|
ASJC Scopus subject areas