Undulating Free\nEnergy Landscapes Buffer Redox Chains\nfrom Environmental Fluctuations

Roller-coaster or undulating free\nenergy landscapes,\nwith alternating\nhigh and low potential cofactors, occur frequently in biological redox\nchains. Yet, there is little understanding of the possible advantages\ncreated by these landscapes. We examined the tetraheme subunit associated\nwith Blastochloris viridis reaction\ncenters, comparing the dynamics of the native protein and of hypothetical\n(in silico) mutants. We computed the variation in the total number\nof electrons in wild type (WT) and mutant tetrahemes connected to\nan electron reservoir in the presence of a time-varying potential,\nas a model for a fluctuating redox environment. We found that roller-coaster\nfree energy landscapes buffer the redox cofactor populations from\nthese fluctuations. The WT roller-coaster landscape slows forward\nand backward electron transfer in the face of fluctuations, and may\noffer the advantage of sustaining the reduction of essential cofactors,\nsuch as the chlorophyll special pair in photosynthesis, even though\nan undulating landscape introduces thermodynamically uphill steps\nin multistep redox chains.