Specific effects of the coupling of protein reactions to slow protein structure dynamics are studied. We focus on accumulation of structural changes produced in consecutive protein cycles and eventually modifying the cycle itself. We showed previously [Christophorov et al., Chem. Phys. 256, 45 (2000); Goushcha et al., Biophys. J. 79, 1273 (2000)] that such an effective interaction between cycles can cause the thresholdlike emergence of a new stable functional state of the protein macromolecule. To elucidate this mechanism, we have performed numerical modeling of the reaction kinetics in a two-state system coupled to diffusion in the corresponding conformational potentials. Specifically, the model is related to the charge separation and recombination processes in photosynthetic reaction centers (RCs). It is shown that the percentage of RCs remaining structurally deformed after recombination, until the next photoexcitation event (“memory-bearing” centers), can be quite low. Nonetheless, under prolonged photoexcitation it is sufficient for driving eventually all the RCs to a state of high charge-separation efficiency. The dependence of this efficiency on quasistationary photoexcitation intensity is pronouncedly hysteretic. The conformation potentials anharmonicity extends the bistability range noticeably, thereby improving RC adaptation properties. Experimental protocols to detect the memory-bearing centers in the RC ensemble are proposed, simulated and tested, disqualifying the electron escape to hypothetical redox traps. The technique proposed can be used in the studies of cooperative effects under repeated cycling of biomolecules.

Experimental and theoretical results in support of nonlinear dynamic behavior of photosynthetic reaction centers under light-activated conditions are presented. Different conditions of light adaptation allow for preparation of reaction centers in either of two different conformational states. These states were detected both by short actinic flashes and by the switching of the actinic illumination level between different stationary state values. In the second method, the equilibration kinetics of reaction centers isolated from Rhodobacter sphaeroides were shown to be inherently biphasic. The fast and slow equilibration kinetics are shown to correspond to electron transfer (charge separation) at a fixed structure and to combined electron-conformational transitions governed by the bounded diffusion along the potential surface, respectively. The primary donor recovery kinetics after an actinic flash revealed a pronounced dependence on the time interval (deltat) between cessation of a lengthy preillumination of a sample and the actinic flash. A pronounced slow relaxation component with a decay half time of more than 50 s was measured for deltat > 10 s. This component corresponds to charge recombination in reaction centers for which light-induced structural changes have not relaxed completely before the flash. The amplitude of this component depended on the conditions of the sample preparation, specifically on the type of detergent used in the preparation. The redox potential parameters as well as the structural diffusion constants were estimated for samples prepared in different ways.

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