Cytochrome b5 reductase catalyzes the reduction of 2 molecules of cytochrome b5 using NADH as an electron donor. Like ferredoxin reductases, these proteins have an N-terminal FAD binding subdomain and a C-terminal NADH binding subdomain, separated by a cleft, which accepts FAD. The NADH-binding moiety interacts with part of the FAD and resembles a Rossmann fold. However, NAD is bound differently than in canonical Rossmann fold proteins. Nitrate reductases, flavoproteins similar to pyridine nucleotide cytochrome reductases, catalyze the reduction of nitrate to nitrite. The enzyme can be divided into three functional fragments that bind the cofactors molybdopterin, heme-iron, and FAD/NADH.
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