An Evolutionary Proposal

It is now widely accepted that the evolution of earth's atmosphere is linked tightly to the evolution of its biota, with microorganisms determining its basic composition since the origin of life. This must be attributed to fundamental connections between genome evolution and the geochemical environment. Such a strong interplay of geochemical change and biological evolution raises questions about how new biochemical capabilities arise and spread in response to environmental changes in biogeochemical cycling [7]. Oxygenic photosynthesis, in particular, had to be initiated only once by a primitive cyanobacterium for the process to evolve. Microorganisms thus created the breathable, O2-rich air that we enjoy today; but they also caused the formation of dioxygen in an anaerobic earth [8]. Consequently, cyanobacteria altered the solubility of metals-in particular, iron and copper - on a global scale, as a by-product of dioxygen-evolving photosynthesis.

In a reducing environment, the predominance of relatively soluble ferrous iron readily permits cellular acquisition; during early evolution this will have encour aged widespread recruitment of iron as a cofactor in biological redox chemistry. By contrast, under oxidizing conditions ferric iron is poorly soluble [9]. Today, all multi-cellular and, essentially, all single -cell organisms require iron for growth, despite the biological availability of iron being extremely limited by the insolubility of iron hydroxide. This is the reason why microbes synthesize low-molecular weight chelating agents, called siderophores, to bind and solubilize iron. Such ferric siderophore complexes are then transported into the bacteria by specific receptor proteins. In fact, competition for iron between a host and a bacterium is an important factor in determining the course of a bacterial infection. Because of that, different organisms utilize structurally varied siderophores to also competitively bind iron and gain selective growth advantages. Such competition even occurs in mammals where dietary iron is absorbed and bound to transferrin, the iron transport protein, and is then stored by the protein ferritin in cells.

Copper, on the other hand, first existed as cuprous sulfide-which is very insoluble in aqueous media-in the early and middle Precambrian period when the stationary oxygen pressure in the atmosphere was quite low; thus, copper might have been unavailable to organisms. Copper became Cu(II) upon the rise of atmospheric oxygen pressure. This probably occurred in the middle of the Proteozoic Era, when the first eukaryotic organisms appeared on earth. The change in the oxidation state of copper, combined with the new environment, made copper more accessible to more organisms. Copper may thus be considered an indicator element for the atmospheric evolutionary switch from anoxygenic to oxygenic, and for the evolution of higher organisms [10].

Iron and copper play an important role in the living world. From a brief consideration of their chemistry and biochemistry, one can conclude that the early chemistry of life used water soluble ferrous iron but had only limited access to copper as the latter was present in the form of water-insoluble Cu(I). The rise of atmospheric O2 enabled the oxidation of iron, which led to a loss of bioavailability as insoluble Fe(III). Conversely, the oxidation of insoluble Cu(I) led to soluble Cu(II) [11] .

Based on these findings, it has been proposed that Cyt c6 was first incorporated in nature at a time when the reducing character of earth's atmosphere made iron more available than copper -8] . As the atmospheric molecular oxygen concentration was rising because of photosynthetic activity (see Chapters 4 and 5 for structure ofPhotosystem II and the oxygen-evolving complex), the relative bioavailabilities of iron and copper were going down and up, respectively, and Cyt c6 was replaced with Pc [2]. In plants, copper is usually not a limiting element so Cyt c6 would have disappeared, whereas Pc would have become a constitutively synthesized protein.

In fact, the absence of Cyt c6 in plants was a widely accepted paradigm for many years, until a number of plant genomes became sequenced. In 2002, a modified Cyt c6-the so-called Cyt cP [2], or Cyt c6A [12]-was discovered in some plants [13], and it was proposed that such a cytochrome could replace Pc in Arabidopsis [14]. However, such a conclusion was challenged from two different approaches. On the one hand, the structural and functional analysis of the Arabidopsis Cyt cP, compared with plant Pc and algal Cyt c6, demonstrated that the plant heme protein is not an effective donor to its own PSI. Actually, the physicochemical parameters and surface electrostatic potential of Cyt cP and Pc are so different in plants that the former cannot replace the latter [ 15]. On the other hand, inactivation of the two Pc genes of Arabidopsis by stable frame -shift mutations resulted in plants unable to grow photoautotrophically, even when the Cyt c6-like protein was over-expressed at the same time [16].

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