Manganese photosynthesis process

Copper is an essential element. It is of a great importance in the metabolic processes of plants, its content in plant tissues being very low, much lower than those of zinc and manganese. Copper ions form complexes with proteins and with the other biopolymers in plant tissues. These complexes are more stable than those of the other metals. Binding to proteins enhances the catalytic activity of copper. Copper participates in many oxidation-reduction enzymatic systems, and it is thus of importance particularly in respiration and photosynthesis processes. In plants it is also essential in the nitrogen metabolism. Its concentration in plants ranges... 

Figure 39 Schematic representation of the S-state cycle operating in the water oxidation process of photosynthesis together with the oxidation states of the manganese atoms of the Mn4 cluster in the different S-states...

By far the most important role of manganese in nature is its direct involvement in the photocatalytic, four-electron oxidation of water to dioxygen in green plant photosynthesis, an essential process for the maintenance of life. Pirson, in 1937, first discovered the requirement of manganese in photosynthesis by showing that plants grown in a Mn-deficient medium lost their water oxidation capacity (184). During the next four decades, several researchers showed that two photosystems, photosystem I (PSI) and photosystem II (PSII), were involved in photosynthesis and that 02 evolution and Mn were localized at PSII (for a review, see Ref. 185). 

M Kusunoki (1984) A probable model of catalytic binuclear manganese complex for photosynthetic water splitting and oxygen evolving process. In C Sybesma (ed) Advances in Photosynthesis Research, Vol 1, pp 275-278. Martinus Nijhoff/Dr W Junk. The Plague... 

Electron-transfer (ET) reactions play a central role in all biological systems ranging from energy conversion processes (e.g., photosynthesis and respiration) to the wide diversity of chemical transformations catalyzed by different enzymes (1). In the former, cascades of electron transport take place in the cells where multicentered macromolecules are found, often residing in membranes. The active centers of these proteins often contain transition metal ions [e.g., iron, molybdenum, manganese, and copper ions] or cofactors as nicotinamide adenine dinucleotide (NAD) and flavins. The question of evolutionary selection of specific structural elements in proteins performing ET processes is still a topic of considerable interest and discussion. Moreover, one key question is whether such stmctural elements are simply of physical nature (e.g., separation distance between redox partners) or of chemical nature (i.e., providing ET pathways that may enhance or reduce reaction rates). 

In plant photosynthesis molecular oxygen is produced from water. In this process tyrosine molecules carry electron holes from oxidized chlorophyll molecules to manganese-protein complexes. Short-lived tyrosine radicals keep the photoexcited chlorophyll molecules apart from reactive oxygen intermediates, such as peroxides and oxygen atoms (Calvin, 1978 Tommos et al, 1995, 1998)... 

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