Metalloproteins in the evolution of photosynthesis

Certain metalloproteins are common to all photosynthetic electron transfer chains. These include soluble proteins such as ferredoxins and cytochromes of the c₂ type, and membrane-bound components such as cytochrome b, c₁ and the Rieske iron-sulphur protein. The sequence of electron transfer Quinone → (cyt b, FeS, cyt c₁) → cyt c₂ indicates a common precursor to these systems and to the mitochondrial respiratory chain. In cyanobacteria the cytochrome c₁, can be interchanged with the copper protein plastocyanin, and furthermore in chloroplasts of higher plants the latter is used exclusively. The ferredoxins in anaerobic photosynthetic bacteria are mostly of the 4Fe4S] type, probably derived from those of the fermentative bacteria. These could readily be formed in the earliest cells from iron, sulphide and a very simple peptide. In the oxygen-evolving cyanobacteria and the aerobic halobacteria the 2Fe2S] ferredoxins predominate. The electron transfer chains of the cyanobacteria have been incorporated almost unchanged into the chloroplasts of plants. The electron transfer chains of purple photosynthetic bacteria were probably the precursors of the mitochondrial respiratory chain, as shown by similarities of cytochromes c₂ and succinate dehydrogenase. However a different origin of the eukaryotic cytoplasm is indicated by the presence of the copper/zinc superoxide dismutase.