Interactions between cytochrome c2 and photosynthetic reaction center from Rhodobacter sphaeroides:: Changes in binding affinity and electron transfer rate due to mutation of interfacial hydrophobic residues are strongly correlated

The structure of the complex between cytochrome c(2) (cyt) and the photosynthetic reaction center (RC) from Rhodobacter sphaeroides shows contacts between hydrophobic residues Tyr L162, Leu M191, and Val M192 on the RC and the surface of the cyt [Axelrod et al. (2002) J. Mol. Biol. 319, 501-515]. The role of these hydrophobic residues in binding and electron transfer was investigated by replacing them with Ala and other residues. Mutations of the hydrophobic residues generally resulted in relatively small changes in the second-order electron-transfer rate k(2) (Bronsted coefficient, alpha = 0.15 +/- 0.05) indicating that the transition state for association occurs before short-range hydrophobic contacts are established. Larger changes in k(2), found in some cases, were attributed to a change in the second-order mechanism from a diffusion controlled regime to a rapidly reversible binding regime. The association constant, K-A, of the cyt and the rate of electron transfer from the bound cyt, k(e), were both decreased by mutation. Replacement of Tyr L162, Leu M191, or Val M192 by Ala decreased K-A and k(e) by factors of 130, 10, 0.6, and 120, 9, 0.6, respectively. The largest changes were obtained by mutation of Tyr L162, showing that this residue plays a key role in both binding and electron transfer. The binding affinity, K-A, and electron-transfer rate, k(e) were strongly correlated, showing that changes of hydrophobic residues affect both binding and electron transfer. This correlation suggests that changes in distance across hydrophobic interprotein contacts have similar effects on both electron tunneling and binding interactions.