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

Cytochromes c6 and f react by three et mechanisms under similar conditions. We report temperature and viscosity effects on the protein docking and kinetics of 3Zncyt c6 + cyt f(III) → Zncyt c6+ + cyt f(II). At 0.5–40.0 °C, this reaction occurs within the persistent (associated) diprotein complex with the rate constant kpr and within the transient (collision) complex with the rate constant ktr. The viscosity independence of kpr, the donor-acceptor coupling Hab = (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 ktr and a break at 30 °C in the Eyring plot for ktr 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.

Cytochrome c6 and cytochrome f undergo the reaction 3Zncyt c6 + cyt f(III) → Zncyt c6+ + cyt f(II) by three mechanisms. The persistent complex undergoes true electron transfer at all temperatures studied, but transient complex(es) undergo(es) coupled or gated electron transfer at different temperatures. Simulations reveal two ensembles of interconverting diprotein complexesFigure optionsDownload as PowerPoint slide