Role of amino‐acid sidechains in electron‐transfer reactions in reaction center of rhodopseudomonas viridis as revealed by extended hückel molecular orbital calculations

Abstract

AbstractThe 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 (Clustering System for Molecules/Sequences), and conserved and character‐conserved amino acids were extracted. Extended Hückel molecular orbital (MO) calculations were made, based on the three‐dimensional structure of the Rhodopseudomonas 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 Å from both of each pair of electron donor and acceptor were included. Calculations of the electronic coupling between each pair of donor and acceptor as well as evaluations of mixing of the lowest unoccupied molecular orbital (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 bacteriopheophytin in the M‐branch (HM), accessory bacteriochlorophyll in the M‐branch (BM), one of the special‐pair bacteriochlorophylls in the M‐branch (PM), one of the special‐pair bacteriochlorophylls in the L‐branch (PL), accessory bacteriochlorophyll in the L‐branch (BL), and bacteriopheophytin in the L‐branch (HL), and can affect the special‐pair bacteriochlorophylls (P) → HL electron transfer; (2) M250‐Trp plays a crucial role in the HL → primary quinone (QA) electron transfer, but L216‐Phe does not facilitate the HM → secondary quinone (QB) electron transfer; (3) the Fe2+ ion and the ligating histidines facilitate the QA → QB electron transfer; (4) L162‐Tyr plays a key role in the third heme in the cytochrome subunit (Heme) → P (PM plus PL) electron transfer. © 1995 John Wiley & Sons, Inc.