Proton transfer pathways and mechanism in bacterial reaction centers

[摘要]

The focus of this minireview is to discuss the state of knowledge of the pathways and rates of proton transfer in the bacterial reaction center (RC) from Rhodobacter sphaeroides. Protons involved in the light driven catalytic reduction of a quinone molecule Q_B to quinol Q_BH₂ travel from the aqueous solution through well defined proton transfer pathways to the oxygen atoms of the quinone. Three main topics are discussed: (1) the pathways for proton transfer involving the residues: His-H126, His-H128, Asp-L210, Asp-M17, Asp-L213, Ser-L223 and Glu-L212, which were determined by a variety of methods including the use of proton uptake inhibiting metal ions (e.g. Zn²⁺ and Cd²⁺); (2) the rate constants for proton transfer, obtained from a ‘chemical rescue’ study was determined to be 2×10⁵ s⁻¹ and 2×10⁴ s⁻¹ for the proton uptake to Glu-L212 and Q_B ⁻ , respectively; (3) structural studies of altered proton transfer pathways in revertant RCs that lack the key amino acid Asp-L213 show a series of structural changes that propagate toward L213 potentially allowing Glu-H173 to participate in the proton transfer processes.

[学科分类] 生物化学/生物物理

[关键词] Buffer effect;General acid catalysis;Brönsted plot;Proton-coupled electron transfer;Hydrogen ion;Proton transport;Site-directed mutations;Bacterial photosynthesis;Rhodobacter sphaeroides;RC;reaction center;D;primary electron donor;QA and QB;the primary and secondary quinone electron acceptor molecules;A−;intermediate proton acceptor group;k (2);apparent second order rate constant;K D;dissociation constant of the acid;k ON;k OFF;the on and off rate constants for binding of the acid;k et;the overall electron transfer rate constant;k H+;the overall proton transfer rate constant