Proton release due to manganese binding and oxidation in modified bacterial reaction centers.
The pH dependence of binding and oxidation of Mn2+ in highly oxidizing reaction centers with designed metal-binding sites was characterized by light-minus-dark optical difference spectroscopy and direct measurements of proton uptake/release. These mutants bind a Mn2+ ion that can efficiently transfer an electron to the oxidized bacteriochlorophyll dimer, as described earlier [Thielges et al. (2005) Biochemistry 44, 7389-7394]. The dissociation constant, KD, significantly increased with decreasing pH. The pH dependence of KD between pH 7 and pH 8 was consistent with the binding of Mn2+ being stabilized by the electrostatic release of two protons. The strong pH dependence of proton release upon Mn2+ binding, with a maximal release of 1.4 H+ per reaction center, was interpreted as being a result of a shift in the pKa values of the coordinating residues and possibly other nearby residues. A small amount of proton release associated with Mn2+ oxidation was observed upon illumination. These results show that functional metal-binding sites can be incorporated into proteins upon consideration of both the metal coordination and protonation states of the ligands.
Kálmán
L
L
Department of Physics, Concordia University, Montreal, Quebec H4B 1R6, Canada.
Thielges
M C
MC
Williams
J C
JC
Allen
J P
JP
D001665:Binding Sites; D004583:Electrons; D006863:Hydrogen-Ion Concentration; D007477:Ions; D007700:Kinetics; D008024:Ligands; D008027:Light; D008345:Manganese / Q000737:chemistry*; D008958:Models, Molecular; D009154:Mutation; D010100:Oxygen / Q000737:chemistry*; D045322:Photosynthetic Reaction Center Complex Proteins / Q000737:chemistry; D011485:Protein Binding; D011487:Protein Conformation; D011522:Protons; D012242:Rhodobacter sphaeroides / Q000378:metabolism*; D013053:Spectrophotometry; D055672:Static Electricity
D007477:Ions; D008024:Ligands; D045322:Photosynthetic Reaction Center Complex Proteins; D011522:Protons; D008345:Manganese; D010100:Oxygen