The electron transport between the bf complex and Photosystem I is mediated by the soluble electron transfer proteins cytochrome c6 and plastocyanin. The structure, function, and evolution of these electron transfer proteins are discussed in Chapter 8 . Plastocyanin is the exclusive luminal electron carrier in plants, while most cyanobacteria contain genes for both cytochrome c6 and plastocyanin, which are expressed as a function of the iron availability in the environment. Despite their functional similarity, the proteins show no structural homology. Plastocyanin consists mainly of beta-sheets and contains Cu as the redox active cofactor, while cytochrome c6 is a typical c-type cytochrome which consists mainly of alpha helices and contains a c-type heme as the redox active cofactor.
As discussed in Chapter 8 . there is strong evidence that cytochrome c6 is the more ancient electron carrier and that plastocyanin was developed later in response to the limitation of iron and the increase of Mg availability in the ocean.
Structures of both plastocyanin and cytochrome have been determined from a wide variety of photosynthetic organisms ranging from thermophilic cyanobacteria to several higher plants (see [20-22, 48]) and references in Chapter 8).
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