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Evidence that histidine forms a coordination bond to the A0A and A0B chlorophylls and a second H-bond to the A1A and A1B phylloquinones in M688HPsaA and M668HPsaB variants of Synechocystis sp. PCC 6803
Authors:Junlei Sun  Sijie Hao  Matthew Radle  Wu Xu  Ivan Shelaev  Victor Nadtochenko  Vladimir Shuvalov  Alexey Semenov  Heather Gordon  Art van der Est  John H Golbeck
Institution:1. Department of Biochemistry and Molecular Biology, The Pennsylvania State University, University Park, PA 16802, USA;2. Department of Chemistry, University of Louisiana at Lafayette, Lafayette, LA 70504, USA;3. Department of Molecular and Chemical Physics, Moscow Institute of Physics and Technology, Dolgoprudny, Moscow Region 141700, Russian Federation;4. A.N. Belozersky Institute of Physical-Chemical Biology, Moscow State University, Moscow 119991, Russia;5. Department of Chemistry, Brock University, St. Catharines, ON, Canada L2S 3A1;6. Department of Chemistry, The Pennsylvania State University, University Park, PA 16802, USA
Abstract:The axial ligands of the acceptor chlorophylls, A0A and A0B, in Photosystem I are the Met sulfur atoms of M688PsaA and M668PsaB. To determine the role of the Met, His variants were generated in Synechocystis sp. PCC 6803. Molecular dynamics simulations on M688HPsaA show that there exist low energy conformations with the His coordinated to A0A and possibly H-bonded to A1A. Transient EPR studies on M688HPsaA indicate a more symmetrical electron spin distribution in the A1A phyllosemiquinone ring consistent with the presence of an H-bond to the C1 carbonyl. Ultrafast optical studies on the variants show that the 150 fs charge separation between P700 and A0 remains unaffected. Studies on the ns timescale show that 57% of the electrons are transferred from A0A to A1A in M688HPsaA and 48% from A0B to A1B in M668HPsaB; the remainder recombine with P700+ with 1/e times of 25 ns and 37 ns, respectively. Those electrons that reach A1A and A1B in the branch carrying the mutation are not transferred to FX, but recombine with P700+ with 1/e times of ~ 15 μs and ~ 5 μs, respectively. Hence, the His is coordinated to A0 in all populations, but in a second population, the His may be additionally H-bonded to A1. Electron transfer from A0 to A1 occurs only in the latter, but the higher redox potentials of A0 and A1 as a result of the stronger coordination bond to A0 and the proposed second H-bond to A1 preclude electron transfer to the Fe/S clusters.
Keywords:PS I  Photosystem I  Chl  chlorophyll  RC  reaction center  PhQ  phylloquinone  flv  flavodoxin  DCPIP  2  6-dichlorophenolindophenol  EPR  electron paramagnetic resonance
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