Effects of structural memory in protein reactions

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 , Chem. Phys. 256, 45 (2000); Goushcha , 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. (C) 2002 American Institute of Physics.