Implications of the effects of viscosity, macromolecular crowding, and temperature for the transient interaction between cytochrome f and plastocyanin from the cyanobacterium Phormidium laminosum

The reaction between cytochrome f and plastocyanin is a central feature of the photosynthetic electron-transport system of all oxygenic organisms. We have studied the reaction in solution to understand how the very weak binding between the two proteins from Phormidium laminosum can nevertheless lead to fast rates of electron transfer. In a previous publication [Schlarb-Ridley, B. G., et al. (2003) Biochemistry 42, 4057-4063], we suggested that the reaction is diffusion-controlled because of a strong effect of viscosity of the medium. The effects of viscosity and temperature have now been examined in detail. High molecular mass viscogens (Ficoll 70 and Dextran 70), which might mimic in vivo conditions, had little effect up to a relative viscosity of 4. Low molecular mass viscogens (ethane diol, glycerol, and sucrose) strongly decreased the bimolecular rate constant (k(2)) over a similar viscosity range. The effects correlated well with the viscosities of the solutions of the three reagents but not with their dielectric constants or molalities. A power law dependence of k(2) on viscosity suggested that k(2) depends on two viscosity-sensitive reactions in series, while the reverse reactions are little affected by viscosity. The results were incompatible with diffusion control of the overall reaction. Determination of the effect of temperature on k(2) gave an activation enthalpy, Delta H-double dagger = 45 kJ mol(-1), which is also incompatible with diffusion control. The results were interpreted in terms of a model in which the stable form of the protein-protein complex requires further thermal activation to be competent for electron transfer.