Probing ligands to reaction centers to limit the photocycle in photosynthetic bacterium Rubrivivax gelatinosus

Light-induced electron flow between reaction center and cytochrome bc₁ complexes is mediated by quinones and electron donors in purple photosynthetic bacteria. Upon high-intensity excitation, the contribution of the cytochrome bc₁ complex is limited kinetically and the electron supply should be provided by the pool of reduced electron donors. The kinetic limitation of electron shuttle between reaction center and cytochrome bc₁ complex and its consequences on the photocycle were studied by tracking the redox changes of the primary electron donor (BChl dimer) via absorption change and the opening of the closed reaction center via relaxation of the bacteriochlorophyll fluorescence in intact cells of wild type and pufC mutant strains of Rubrivivax gelatinosus. The results were simulated by a minimum model of reversible binding of different ligands (internal and external electron donors and inhibitors) to donor and acceptor sides of the reaction center. The calculated binding and kinetic parameters revealed that control of the rate of the photocycle is primarily due to 1) the light intensity, 2) the size and redox state of the donor pool, and 3) the unbinding rates of the oxidized donor and inhibitor from the reaction center. The similar kinetics of strains WT and pufC lacking the tetraheme cytochrome subunit attached to the reaction center raise the issue of the physiological importance of this subunit discussed from different points of view.

Significance

A crucial factor for the efficacy of electron donors in photosynthetic photocycle is not just the substantial size of the pool and large binding affinity (small dissociation constant K_D = k_off/k_on) to the RC, but also the mean residence time (k_off)⁻¹ in the binding pocket. This is an important parameter that regulates the time of re-activation of the RC during multiple turnovers. The determination of k_off has proven challenging and was performed by simulation of widespread experimental data on the kinetics of P⁺ and relaxation of fluorescence. This work is a step towards better understanding the complex pathways of electron transfer in proteins and simulation-based design of more effective electron transfer components in natural and artificial systems.