Excited-state redox potentials and the Z scheme of photosynthesis

Abstract

The Z scheme for electron flow in oxygen-evolving photosynthetic organisms was first proposed 25 years ago by R. Hill and F. BendalP. Their original diagram is shown in Fig. 1. The basic idea, that two photochemical systems are arranged in series so that two photons act sequentially to drive electrons from the very poor electron donor 1-I20 to the weak electron acceptor NADP, is now embraced by essentially every worker in the field. This scheme rightly stands as the central integrating construct for all work on electron transport in oxygenevolving photosynthetic organisms. Introductory biochemistry texts invariably contain a version of the scheme, usually with an explanation of some of the classic experiments (such as the red drop and enhancement effects) that led to the acceptance of the idea of a sequence of two photochemical steps, each with a somewhat different wavelength optimum. The Z scheme is really a synthesis of thermodynamic and kinetic data. The midpoint redox potentials of the various electron carriers are now usually plotted vertically so that more reducing compounds are above more oxidizing ones. This ensures that thermodynamically favorable reactions are 'downhill' on the diagram. The horizontal axis corresponds to a reaction sequence and the kinetically important reactions are connected by arrows. Unfortunately, many authors are extremely careless about constructing their Z schemes, so much so that the understanding of what is in reality a simple concept is greatly impeded. Several Z schemes in the literature have P~0, the reaction centerchlorophyll of photosystem II, at an E m (oxidation-reduction midpoint potential) of +450 mV, and show electrons flowing uphill to it from 1420 against a