Weak temperature dependence of P (+) H A (-) recombination in mutant Rhodobacter sphaeroides reaction centers

• Published in: Photosynthesis research Vol. 128; no. 3; pp. 243 - 258
• Main Authors: Gibasiewicz, Krzysztof, Białek, Rafał, Pajzderska, Maria, Karolczak, Jerzy, Burdziński, Gotard, Jones, Michael R, Brettel, Klaus
• Format: Journal Article
• Language: English
• Published: Netherlands Springer Verlag 01-06-2016
• Subjects: Absorption, Physicochemical; Bacterial Proteins - chemistry; Bacterial Proteins - genetics; Bacterial Proteins - metabolism; Electron Transport; Kinetics; Life Sciences; Models, Molecular; Mutation; Photosynthetic Reaction Center Complex Proteins - chemistry; Photosynthetic Reaction Center Complex Proteins - genetics; Photosynthetic Reaction Center Complex Proteins - metabolism; Rhodobacter sphaeroides - genetics; Rhodobacter sphaeroides - metabolism; Temperature; Thermodynamics; Reaction centers; Charge recombination; Electron transfer; Protein dynamics; Rhodobacter sphaeroides; Transient absorption
• ISSN: 0166-8595; 1573-5079
• DOI: 10.1007/s11120-016-0239-9

In contrast with findings on the wild-type Rhodobacter sphaeroides reaction center, biexponential P (+) H A (-)  → PH A charge recombination is shown to be weakly dependent on temperature between 78 and 298 K in three variants with single amino acids exchanged in the vicinity of primary electron acceptors. These mutated reaction centers have diverse overall kinetics of charge recombination, spanning an average lifetime from ~2 to ~20 ns. Despite these differences a protein relaxation model applied previously to wild-type reaction centers was successfully used to relate the observed kinetics to the temporal evolution of the free energy level of the state P (+) H A (-) relative to P (+) B A (-) . We conclude that the observed variety in the kinetics of charge recombination, together with their weak temperature dependence, is caused by a combination of factors that are each affected to a different extent by the point mutations in a particular mutant complex. These are as follows: (1) the initial free energy gap between the states P (+) B A (-) and P (+) H A (-) , (2) the intrinsic rate of P (+) B A (-)  → PB A charge recombination, and (3) the rate of protein relaxation in response to the appearance of the charge separated states. In the case of a mutant which displays rapid P (+) H A (-) recombination (ELL), most of this recombination occurs in an unrelaxed protein in which P (+) B A (-) and P (+) H A (-) are almost isoenergetic. In contrast, in a mutant in which P (+) H A (-) recombination is relatively slow (GML), most of the recombination occurs in a relaxed protein in which P (+) H A (-) is much lower in energy than P (+) H A (-) . The weak temperature dependence in the ELL reaction center and a YLH mutant was modeled in two ways: (1) by assuming that the initial P (+) B A (-) and P (+) H A (-) states in an unrelaxed protein are isoenergetic, whereas the final free energy gap between these states following the protein relaxation is large (~250 meV or more), independent of temperature and (2) by assuming that the initial and final free energy gaps between P (+) B A (-) and P (+) H A (-) are moderate and temperature dependent. In the case of the GML mutant, it was concluded that the free energy gap between P (+) B A (-) and P (+) H A (-) is large at all times.