Induced conformational changes upon Cd2+ binding at photosynthetic reaction centers

Area of Science:

• Biochemistry
• Structural Biology
• Photosynthesis Research

Background:

• Bacterial photosynthetic reaction centers (bRC) are crucial for light energy conversion.
• Proton transfer (PT) to the secondary quinone Q(B) is essential for bRC function.
• Cd(2+) binding to bRC is known to inhibit this proton transfer process.

Purpose of the Study:

• To elucidate the mechanism by which Cd(2+) binding inhibits proton transfer in bRC.
• To calculate pK(a) values for residues involved in proton transfer pathways.
• To understand the role of Cd(2+) in altering the bRC's proton transfer dynamics.

Main Methods:

• Utilized crystal structures of wild-type (WT) and Cd(2+)-bound bRC from Rhodobacter sphaeroides.
• Calculated pK(a) values for amino acid residues along the water channels connecting Q(B) to the stromal side.
• Analyzed electrostatic interactions and conformational changes induced by Cd(2+) binding.

Main Results:

• Cd(2+) binding caused proton release from His-H126/128 and significant pK(a) shifts in residues along PT pathways.
• Asp-L213, near Q(B), showed a decreased pK(a) upon Cd(2+) binding.
• Conformational side chain reorientations, not direct charge effects, were the primary drivers of pK(a) shifts.

Conclusions:

• Cd(2+) binding inhibits bRC proton transfer via a long-range electrostatic mechanism.
• Successive side chain reorientations, akin to a domino effect, propagate along PT pathways.
• This mechanism explains how Cd(2+) binding alters the proton transfer efficiency in bRC.