Steric and Electrostatic Effects on the Stabilization of the Secondary Quinone in Reaction Centers

Abstract

Plants, cyano- and purple bacteria are able to develop pigment-protein complex which, stabilize light driven electrons on reduced quinones. The role of the quinone acceptor complex is the energetic stabilization of the light separated charges and transporting reducing equivalents to the quinone pool of the membrane in pairs [1]. The function of the acceptor complex depends, basically, on the redox and boundary properties of the quinones. If the quinone is bound to the reaction center (RC), the redox properties are determined by the environment and the chemical identity of the molecule. From the point of view of the RC: the more the environment is polarizable, the more the midpotential of the bound quinone is positive. Therefore the primary quinone, QA has a more negative midpotential compared to the one of the secondary quinone, QB. On the other hand, introducing amino acid changes in the quinone environment the redox potential can be shifted. From the point of view of the quinone: the chemical nature of the molecule is identified by the head group and by the nature and conformation of the substituents. The redox potential is also affected by the resonance equilibrium within the molecule which is dependent not only on the identity of the substituents but their relative orientation to the quinone ring plane as well [2]. If the environment of a substituent is more flexible it allows a larger rotational freedom providing a larger conformational stability. A possible overlap between the lone p-orbitals of the methoxy oxygenes and the delocalized electrons of the quinone ring modifies the partial changes on the carbonyl oxygenes through mesomeric substates [3]. Introducing a larger and more rigid amino acid chain, like methionine instead of the isoleucine at position L229 we put away negative charges, partially from the carbonyl oxygenes giving rise a weaker H-bonds with the protein environment. As a consequence of this the binding of not only the inhibitors, like herbicides, but that of the quinones, specially the semiquinone forms, are destorted in these mutant cells [4]. To test this hipothesis we changed the secondary quinone, QB, which is UQ10 in Rb. sphaeroides to duroquinone, DQ, in the wild type and the L229I1e→Met mutant RCs. This quinone species is substituted by methyl groups at positions 2 and 3 instead of the native methoxy groups.