Transient EPR studies of Photosystem I containing non-native quinones

Abstract

The quinone acceptors in both type I and type II reaction centres play a pivotal role in light induced photosynthetic electron transfer. Although, the quinones found in the various reaction centres are structurally similar, the rates of electron transfer and nature of the subsequent acceptors vary greatly. The relationship between the properties of the quinone binding-site, the structure of the quinone and the rates of electron transfer are still not well understood. Transient EPR data for the forward and back reaction kinetics will be presented for Photosystem I, in which the native phylloquinone has been replaced by a series of foreign quinones. Our results suggest that the ability of quinones to function in the electron transport chain is governed primarily by their midpoint potentials. We find that the less negative the midpoint potential of a given quinone is the slower the forward electron transfer rate. The back reaction from the iron sulphur centres is also influenced by the presence of a foreign quinone in the A1 site clearly showing that the back reaction from FA/FB proceeds via A1. In contrast to the forward rate, the back reaction rate increases as the midpoint potential of the quinone becomes less negative. The EPR data also suggest that naphthoquinone derivatives bind more strongly to the A1 site than benzoquinone derivatives do. This observation is can be understood as a result of differences in the proposed p-stacking arrangement between PsaA-W697 (PsaB-W677) and phylloquinone.