A Neutral Mutation Changes the Ionic Strength Dependence of the Rate of Electron Transfer between Cyt c2 and RCs from Rb. sphaeroides

Abstract

The contact between the electron transfer proteins cytochrome c2 (cyt) and Reaction Center (RC) is centered on the hydrophobic residue Tyr L162. In the YAL162 mutant a greater ionic strength dependence of the second order electron transfer rate constant k2 was found even though no charge changes were made. We explain this result by a transition state model (figure). For Native RC, k2 is diffusion limited (electron transfer occurs before dissociation) and decreases with ionic strength due to increasing energy of the transition state for association. For YAL162 RCs, mutation of Tyr L162 increases the dissociation rate and decreases the electron transfer rate so that k2 is in the fast exchange (pre-equilibration) regime (dissociation occurs before electron transfer). Here ionic strength effects are due to changes in binding energy, which are greater than changes in transition state energy (by ∼2-fold) accounting for the steeper slope for the mutant. The decreased electron transfer rate due to this mutation demonstrates the importance of hydrophobic interactions in binding and electron transfer. (1)Gong et al. (2003) Biochem. 42, 14492. ∗Supported by NIH(GM41637).