Redox enzymes manganese role

Mn " " also activates several oxidases. The role of manganese in oxidation-reduction processes of plants is its most important function, and is related to the valency change between Mn " " and Mn (Kabata-Pendias and Pendias 2001). Because of its high redox potential, manganese is the main metallic activator of enzymes in the citrate cycle and controls oxidation, reduction, and carboxylation reactions in carbohy-... 

Enzymes containing amino acid radicals are generally associated with transition metal ions—typically of iron, manganese, cobalt, or copper. In some instances, the metal is absent it is apparently replaced by redox-active organic cofactors such as S -adenosylmethionine or flavins. Functionally, their role is analogous to that of the metal ion in metalloproteins. 

In the case of Mn, there is evidence for a role of ectoenzymes in both oxidation and reduction reactions (Fig. 6). Manganese cycling in the photic zone of natural waters involves a redox transition between dissolved Mn(II) and particulate Mn(IV). In freshwater, Mn(II) oxidation by algal photosynthetic activity has been reported (Richardson et al., 1988) and may indirectly involve the enzyme carbonic anhydrase, which catalyzes the conversion of bicarbonate to carbon dioxide extracellularly. For bicarbonate utilizing species of algae, a more rapid... 

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). 

The biological applications of manganese are numerous and quite varied. Although most of these biological roles utilize manganese in structural or hydrolytic systems, some very striking redox systems are also known. The hydrolytic roles are, of course, not to be overlooked with respect to the widespread and critical arginase enzyme, which exhibits a strict requirement for two Mn" ions. The ribo-nuclease hydrolases of retroviral reverse transcriptase are another key nonredox application—for example, the ribonuclease hydrolase from HIV-1. 

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