The role of trace metals in photosynthetic electron transport in O-2-evolving organisms

Iron is the quantitatively most important trace metal involved in thylakoid reactions of all oxygenic organisms since linear (= non-cyclic) electron f low from H2O to NADP(+) involves PS II (2-3 Fe), cytochrome b(6)-f (5 Fe), PS I (12 Fe), and ferredoxin (2 Fe); (replaceable by metal-free flavodoxin in certain cyanobacteria and algae under iron deficiency). Cytochrome c(6) (1 Fe) is the only redox catalyst linking the cytochrome b(6)-f complex to PS I in most algae; in many cyanobacteria and Chlorophyta cytochrome c(6) a nd the copper-containing plastocyanin are alternatives, with the availabili ty of iron and copper regulating their relative expression, while higher pl ants only have plastocyanin. Iron, copper and zinc occur in enzymes that re move active oxygen species and that are in part bound to the thylakoid memb rane. These enzymes are ascorbate peroxidase (Fe) and iron-(cyanobacteria, and most algae) and copper-zinc- (some algae; higher plants) superoxide dis mutase. Iron-containing NAD(P)H-PQ oxidoreductase in thylakoids of cyanobac teria and many eukaryotes may be involved in cyclic electron transport arou nd PS I and in chlororespiration. Manganese is second to iron in its quanti tative role in the thylakoids, with four Mn (and 1 Ca) per PS II involved i n O-2 evolution. The roles of the transition metals in redox catalysts can in broad terms be related to their redox chemistry and to their availabilit y to organisms at the time when the pathways evolved. The quantitative role s of these trace metals varies genotypically (e.g. the greater need for iro n in thylakoid reactions of cyanobacteria and rhodophytes than in other O-2 -evolvers as a result of their lower PS II:PS I ratio) and phenotypically ( e.g. as a result of variations in PS II:PS I ratio with the spectral qualit y of incident radiation).