Role of configurational gating in intracomplex electron transfer from cytochrome c to the radical cation in cytochrome c peroxidase

Electron transfer within complexes of cytochrome c (Cc) and cytochrome c peroxidase (CcP) was studied to determine whether the reactions are gated by fluctuations in configuration. Electron transfer in the physiological complex of yeast Cc (yCc) and CcP was studied using the Ru-39-Cc derivative, in which the H39C/C102T variant of yeast iso-1-cytochrome c is labeled at the single cysteine residue on the back surface with trisbipyridylruthenium(II). Laser excitation of the 1:1 Ru-39-Cc-CcP compound I complex at low ionic strength results in rapid electron transfer from Ru-II* to heme c Fe-III, followed by electron transfer from heme c Fe-II to the Trp-191 indolyl radical cation with a rate constant k(eta) of 2 x 10(6) s(-1) at 20 degrees C. k(eta) is not changed by increasing the viscosity up to 40 cP with glycerol and is independent of temperature. These results suggest that this reaction is not gated by fluctuations in the configuration of the complex, but may represent the elementary electron transfer step. The value of k(eta) is consistent with the efficient pathway for electron transfer in the crystalline yCc-CcP complex, which has a distance of 16 Angstrom between the edge of heme c and the Trp-191 indole [Pelletier, H., and Kraut, J. (1992) Science 258, 1748-1755]. Electron transfer in the complex of horse Cc (hCc) and CcP was examined using Ru-27-Cc, in which hCc is labeled with trisbipyridylruthenium(II) at Lys-27. Laser excitation of the Ru-27-Cc-CcP complex results in electron transfer from Ru-II* to heme c Fe-II with a rate constant k(1) of 2.3 x 10(7) s(-1), followed by oxidation of the Trp-191 indole to a radical cation by Ru-III with a rate constant k(3) of 7 x 10(6) s(-1). The cycle is completed by electron transfer from heme c Fe-II to the Trp-191 radical cation with a rate constant k(4) of 6.1 x 10(4) s(-1). The rate constant k(4) decreases to 3.4 x 10(3) s(-1) as the viscosity is increased to 84 cP, but the rate constants k(1) and k(3) remain the same. The results are consistent with a gating mechanism in which the Ru-27-Cc-CcP complex undergoes fluctuations between a major state A with the configuration of the hCc-CcP crystalline complex and a minor state B with the configuration of the yCc-CcP complex. The hCc-CcP complex, state A, has an inefficient pathway for electron transfer from heme c to the Trp-191 indolyl radical cation with a distance of 20.5 Angstrom and a predicted value of 5 x 10(2) s(-1) for k(4A). The observed rate constant k(4) is thus gated by the rate constant k(a) for conversion of state A to state B, where the rate of electron transfer k(4B) is expected to be 2 x 10(6) s(-1). The temperature dependence of k(4) provides activation parameters that are consistent with the proposed gating mechanism. These studies provide evidence that configurational gating does not control electron transfer in the physiological yCc-CcP complex, but is required in the nonphysiological hCc-CcP complex.