Recycle:
-Little known currently about Mn recycling
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Mn
Manganese
54.938
Essential:
almost all organisms
Manganese
Major functions in cells: (1)
-Oxygenic photosynthesis
-Resistance to damage by reactive oxygen species
-Cofactor for mitochondrial SOD (Sce)
Environmental and health impacts:
-Manganese limitation can limit growth of photosynthetic microbes and plants
-Manganese deficiency has also been observed in animals
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(1) Manganese: Widely Used in Biology
Mn is widely used in biology and is often critical for growth in many types of microbes, animals, and plants. The single most critical role for Mn, considered globally, is for its role in the water-splitting complex of PS II in oxygenic photosynthesis. Thus, phototrophic growth has a high Mn requirement in both bacteria and eukaryotic organisms.
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Mn is also required for the growth of many nonphotosynthetic bacteria, although a universal requirement is by no means established. The highly radioresistant bacterium Deinococcus radiodurans accumulates high intracellular levels of Mn, which in this and other organisms is correlated with greater resistance to damage by reactive oxygen species (Daly, 2009).
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Mn also plays a critical role in S. cerevisiae, where it serves as cofactor for a mitochondrial SOD (Sod2). Since Sod2 also can bind Fe, which leads to an inactive enzyme, mechanisms that ensure proper metallation are thought to be present in cells but are, as yet, poorly understood (Aguirre and Culotta, 2012).
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Although the Mn-deprivation responses have been characterized in many different organisms, and the corresponding regulatory proteins have often been defined, there is surprisingly little evidence for a Mn-sparing response.
(2) Remodeling Photosynthetic Complexes
The cyanobacterium Synechocystis has a high Mn quota because of the critical role on Mn in the water-splitting complex of PS-II in oxygenic photosynthesis. Synechocystis cells grow well at levels of Mn as low as 100 nM; at lower levels, they remodel their photosynthetic complexes to prevent photooxidative damage (Salomon and Keren, 2011).
(3) Prioritization of Mn Usage
Phototrophic growth has a high Mn requirement in the eukaryote, C. reinhardtii. The first response to declining Mn availability is the expression of acquisition pathways (NRAMP1) and induction of two PHO84 family transporters thought to import phosphate:Mn complexes. Further reduction in Mn availability leads to a loss of MnSOD activity which, in general, precedes the eventual loss of photosytem II activity. This suggests a prioritization of Mn usage, although the corresponding mechanisms are not well understood (Allen et al., 2007).
(4) Lack of Sparing Response
Although the Mn-deprivation responses have been characterized in many different organisms, and the corresponding regulatory proteins have often been defined, there is surprisingly little evidence for a Mn-sparing response.
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The cyanobacterium Synechocystis, which has a high Mn quota, concentrates Mn from the environment and stores it in association with the cell envelope (Keren et al., 2002). However, there is little evidence for an obvious Mn-sparing response, perhaps because no other single component of the cell, other than PSII, has such a high demand for Mn.
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The B. subtilis MntR regulator senses Mn directly and represses acquisition systems, but little else (Que and Helmann, 2000).
Similarly, Staphylococcus aureus upregulates Mn import and this helps resist the inhibitory effects of the divalent metal sequestration protein calprotectin (Hammer and Skaar, 2012). Despite the fact that cells fail to grow in the absence of Mn, the essential processes requiring this metal have not been identified. One possibility is suggested by the presence of a single, Mn-dependent RNR in B. subtilis and related Firmicutes (Zhang and Stubbe, 2011).
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