The axial ligands of the acceptor chlorophylls, A0A and A 0B, 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 P 700 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 A 1 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.
All Science Journal Classification (ASJC) codes
- Cell Biology