Study on the electron and energy transfer mechanisms in photoreaction center proteins

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The amino-acid sequences of the L and M subunits of the reaction centers (RC) from four purple photosynthetic bacteria were compared by the use of CLUSMOL/S,and conserved and character-conserved amino acids were extracted. Extended Huckel molecular orbital calculations were made, based on the three-dimensional structure of the R.viridis RC,for eleven composites of pigments, in which the sidechains (except for aliphatic sidechains) of the above amino acids within a distance of 6 A from both of each pair of electron donor and acceptor were included. Calculations of the electronic coupling H_<DA> between each pair of donor and acceptor as well as evaluations of mixing of the LUMO of one chromophore with the orbitals of another chromophore and amino-acid sidechains elucidated the roles of the sidechains in the electron transfer reactions : (1) L181-Phe and M208-Tyr function as a pair of bridges in the electronic coupling among pigments H_M, B_M, P_M, P_L, B_L and H_L, and can affect the P * H_L electron transfer. (2) M250-Trp plays a crucial role in the H_L * Q_A electron transfer, but L216-Phe does not facilitate the H_M * Q_B electron transfer. (3) The Fe^<2+> ion and the ligating histidines facilitate the Q_A * Q_B electron transfer. (4) L162-Tyr plays a key role in the Heme * P (P_M plus P_L) electron transfer.