Azurin is a bacterial blue copper-binding protein. It serves as a redox partner to enzymes such as nitrite reductase or arsenite oxidase. The copper of Azurin is tetrahedrally coordinated by a cysteine, 2 histidines, and a methionine residue. The electron transfer reactions are carried out with the Cu center transitioning between the oxidized Cu(II) form and the reduced Cu(I) form. Azurin can function as tumor suppressor; it forms a complex with p53 that triggers apoptosis in various human cancer cells. Auracyanins A and B are from photosynthetic bacteria. They are very similar blue copper proteins with 38% sequence identity and they are homologous to the bacterial redox protein Azurin. However, auracyanin A is expressed only when C. aurantiacus cells are grown in light, whereas auracyanin B is expressed under dark and in light. Thus, auracyanin A may function as a redox partner in photosynthesis, while auracyanin B may function in aerobic respiration.
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