NAD(P) binding domain of methylene-tetrahydrofolate dehydrogenase and methylene-tetrahydrofolate dehydrogenase/cyclohydrolase
NAD(P) binding domains of methylene-tetrahydrofolate dehydrogenase (m-THF DH) and m-THF DH/cyclohydrolase bifunctional enzymes (m-THF DH/cyclohydrolase). M-THF is a versatile carrier of activated one-carbon units. The major one-carbon folate donors are N-5 methyltetrahydrofolate, N5,N10-m-THF, and N10-formayltetrahydrofolate. The oxidation of metabolic intermediate m-THF to m-THF requires the enzyme m-THF DH. In addition, most DHs also have an associated cyclohydrolase activity which catalyzes its hydrolysis to N10-formyltetrahydrofolate. m-THF DH is typically found as part of a multifunctional protein in eukaryotes. NADP-dependent m-THF DH in mammals, birds and yeast are components of a trifunctional enzyme with DH, cyclohydrolase, and synthetase activities. Certain eukaryotic cells also contain homodimeric bifunctional DH/cyclodrolase form. In bacteria, mono-functional DH, as well as bifunctional DH/cyclodrolase are found. In addition, yeast (S. cerevisiae) also express a monofunctional DH. M-THF DH, like other amino acid DH-like NAD(P)-binding domains, is a member of the Rossmann fold superfamily which includes glutamate, leucine, and phenylalanine DHs, m-THF DH, methylene-tetrahydromethanopterin DH, m-THF DH/cyclohydrolase, Shikimate DH-like proteins, malate oxidoreductases, and glutamyl tRNA reductase. Amino acid DHs catalyze the deamination of amino acids to keto acids with NAD(P)+ as a cofactor. The NAD(P)-binding Rossmann fold superfamily includes a wide variety of protein families including NAD(P)- binding domains of alcohol DHs, tyrosine-dependent oxidoreductases, glyceraldehyde-3-phosphate DH, lactate/malate DHs, formate/glycerate DHs, siroheme synthases, 6-phosphogluconate DH, amino acid DHs, repressor rex, NAD-binding potassium channel domain, CoA-binding, and ornithine cyclodeaminase-like domains. These domains have an alpha-beta-alpha configuration. NAD binding involves numerous hydrogen and van der Waals contacts.