NAD binding domain of methylene-tetrahydrofolate dehydrogenase
The NAD-binding domain of methylene-tetrahydrofolate dehydrogenase (m-THF DH). 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. M-THF DH is a component of an unusual monofunctional enzyme; in eukaryotes, m-THF DH is typically found as part of a multifunctional protein. NADP-dependent m-THF DHs 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, monofunctional DH, as well as bifunctional DH/cyclodrolase are found. In addition, yeast (S. cerevisiae) also express an monofunctional DH. This family contains only the monofunctional DHs from S. cerevisiae and certain bacteria. 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.