Primary electron acceptor complex of photosystem I in spinach chloroplasts

Abstract

PHOTOSYSTEM I (PSI) is the photochemical reaction complex involved in the generation of a low potential reductant in oxygen-evolving photosynthetic organisms. The primary photochemical reaction in PSI has been identified as the photo-oxidation of a reaction centre chlorophyll complex P700 (ref. 1). This photo-oxidation occurs both at room temperature and in the frozen state at temperatures as low as 4.2 K. At cryogenic temperatures this photo-oxidation has generally been found to be irreversible². Malkin and Bearden³ showed that the electron acceptor for this photo-oxidation reaction is a bound ferredoxin. This ferredoxin has two iron–sulphur centres with very low redox potentials (Em₁₀ = −550 and −590 mV)^4–6. It has been proposed that these centres are the primary electron acceptor of PSI^3,4. More recent evidence suggests it may not be^7–11, and we have shown¹¹ that when the bound ferredoxin is chemically reduced, illumination at low temperature results in the photo-oxidation of P700 without any change in the bound ferredoxin, this oxidation is reversible. In photosynthetic bacteria a component with an EPR signal at g = 1.82 has been identified as the primary electron acceptor of the photochemical system^12,13 and it is possible that a similar component might be involved in chloroplast photochemical reactions. McIntosh et al.¹⁴ obtained kinetic evidence for the presence in PSI particles of a component with an EPR signal at g = 2.06 and g = 1.76 which showed a reversible redox change on illumination of the particles at low temperature parallel to that of P700. The changes observed were small and it was not possible to obtain a spectrum of the component. Using a more highly purified PSI preparation we have now obtained strong evidence that this component is the primary electron acceptor of PSI.