Whereas zinc may be the most common transition metal in enzymes, manganese is perhaps the least common. Part of the reason is that complexes of manganese with proteins tend to be weakly stable and dissociate readily. Notable manganese metal-loenzymes include pyruvate carboxylase and manganese superoxide dismutase in the mitochondria and arginase in the urea cycle. Manganese can also function as a metal-activating cofactor for many enzymes that require magnesium.
Reactivity Although manganese is not considered a redox metal based on reactivity, it nonetheless can exist in six oxidation states (Mn2+ to Mn7+), three of which (Mn5+ to Mn7+) are not seen in biological systems. The most common form of manganese is Mn2+. The highest number of multiple valences of manganese occurs in the water splitting enzyme that is found in chloroplasts of plants as part of photosystem II.
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