Evolution of photosynthesis and oxygen evolution: Implications from the structural comparison of Photosystems I and II

Two large membrane protein complexes, Photosystem I and II (PS I and PS II), perform the first step in the conversion of the light energy from the sun into chemical energy: the light-induced transmembrane charge separation. They function in series; PS II provides the strong positive redox potential for water oxidation, while PS I generates a strong negative redox potential, which makes it able to reduce ferredoxin and deliver the electrons for the reduction of hydrogen in the form of NADPH. The structural comparison of PS I and PS II sheds light on the evolution of oxygenic photosynthesis. Both Photosystems show similarities in their core structure, indicating that they have been derived from a common ancestor. Striking differences in the arrangement and coordination of cofactors and in their protein environment, however, may contain the secret to the functional differences between the Photosystems. In this article, we address how the oxygen-evolving complex may have developed, and the main similarities and differences in the electron carriers and the organization of the antenna systems of these two complexes.