Sulfite oxidase (SO) family, molybdopterin binding domain. This molybdopterin cofactor (Moco) binding domain is found in a variety of oxidoreductases, main members of this family are nitrate reductase (NR) and sulfite oxidase (SO). SO catalyzes the terminal reaction in the oxidative degradation of the sulfur-containing amino acids cysteine and methionine. Assimilatory NRs catalyze the reduction of nitrate to nitrite which is subsequently converted to NH4+ by nitrite reductase. Common features of all known members of this family are that they contain one single pterin cofactor and part of the coordination of the metal (Mo) is a cysteine ligand of the protein and that they catalyze the transfer of an oxygen to or from a lone pair of electrons on the substrate.
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