COMPARATIVE ANALYSIS OF QUINONE HYDROGEN BINDING SCHEMES IN TYPE I VERSUS TYPE II REACTION CENTERS

摘要

In oxygenic photosynthesis, two reaction center (RC) complexes, Photosystem I (PS I) and Photosystem II (PS II), function in tandem. For both types of RCs X-ray structures are available (Ermler et al 1994, Jordan et al 2001, Ferreira et al 2004). In the course of photo-induced charge separation an electron is transferred from the primary electron donor P, located near the lumen side of the membrane, to the first quinone acceptor Q on the stroma side of the membrane. In both types of RCs the first quinone acceptor functions as a one electron gate but delivers electrons to different sets of terminal acceptors. In PS I the quinone cofactor (phylloquinone) operates at one of the lowest redox potentials (from — 760 mV (Semenov et al 2000) to -820 mV (Vos & van Gorkom 1988)) of any quinone known to function in biological systems. The native quinone (plastoquinone, PQ-9) of PS II has been shown to operate in PS I as well, i.e., with a redox potential lowered by at least 700 mV as compared to PS II (Johnson et al 2000, Zybailov et al 2000). It has therefore been concluded that the redox potential is mainly controlled by the specific protein environment. Hydrogen bonding between quinones in photosynthetic reaction centers (RC) has been realized as an important protein-cofactor interaction for both binding specificity and function. This work presents a comparative analysis of the H-bonding schemes of quinone cofactors in type I versus type II RC based on various electron paramagnetic resonance (EPR) techniques.