Simultaneous true, gated, and coupled electron-transfer reactions and energetics of protein rearrangement.

Cytochromes c(6) and f react by three et mechanisms under similar conditions. We report temperature and viscosity effects on the protein docking and kinetics of (3)Zncyt c(6)+cyt f(III)→Zncyt c(6)(+)+cyt f(II). At 0.5-40.0°C, this reaction occurs within the persistent (associated) diprotein complex with the rate constant k(pr) and within the transient (collision) complex with the rate constant k(tr). The viscosity independence of k(pr), the donor-acceptor coupling H(ab)=(0.5±0.1)cm(-1), and reorganizational energy λ=(2.14±0.02) eV indicate true et within the persistent complex. The viscosity dependence of k(tr) and a break at 30°C in the Eyring plot for k(tr) reveal mechanisms within the transient complex that are reversibly switched by temperature change. Kramers protein friction parameters differ much for the reactions below (σ=0.3±0.1, δ=0.85±0.07) and above (σ=4.0±0.9, δ=0.40±0.06) 30°C. The transient complex(es) undergo(es) coupled et below ca. 30°C and gated et above ca. 30°C. Brownian dynamics simulations reveal two broad, dynamic ensembles of configurations "bridged" by few intermediate configurations through which the interconversion presumably occurs.