This entry represents aldo-keto reductase family 1 member D1 (AKR1D1). It catalyses the stereospecific NADPH-dependent reduction of the C4-C5 double bond of bile acid intermediates and steroid hormones carrying a delta-4-3-one structure to yield an A/B cis-ring junction. This cis-configuration is crucial for bile acid biosynthesis and plays important roles in steroid metabolism []. It is capable of reducing a broad range of delta-4-3-ketosteroids from C18 (such as, 17beta-hydroxyestr-4-en-3-one) to C27 (such as, 7alpha-hydroxycholest-4-en-3-one) [].In general, the aldo-keto reductase (AKR) protein superfamily members reduce carbonyl substrates such as: sugar aldehydes, keto-steroids, keto-prostaglandins, retinals, quinones, and lipid peroxidation by-products [, ]. However, there are some exceptions, such as the reduction of steroid double bonds catalysed by AKR1D enzymes (5beta-reductases); and the oxidation of proximate carcinogen trans-dihydrodiol polycyclic aromatic hydrocarbons; while the beta-subunits of potassium gated ion channels (AKR6 family) control Kv channel opening [].Structurally, they contain an (alpha/beta)8-barrel motif, display large loops at the back of the barrel which govern substrate specificity, and have a conserved cofactor binding domain. The binding site is located in a large, deep, elliptical pocket in the C-terminal end of the beta sheet, the substrate being bound in an extended conformation. The hydrophobic nature of the pocket favours aromatic and apolar substrates over highly polar ones []. They catalyse an ordered bi bi kinetic mechanism in which NAD(P)H cofactor binds first and leaves last []. Binding of the NADPH coenzyme causes a massive conformational change, reorienting a loop, effectively locking the coenzyme in place. This binding is more similar to FAD- than to NAD(P)-binding oxidoreductases [].
