NADP-mannitol dehydrogenase catalyzes the conversion of fructose to mannitol, an acyclic 6-carbon sugar. MDH is a tetrameric member of the SDR family. This subgroup also includes various other tetrameric SDRs, including Pichia stipitis D-arabinitol dehydrogenase (aka polyol dehydrogenase), Candida albicans Sou1p, a sorbose reductase, and Candida parapsilosis (S)-specific carbonyl reductase (SCR, aka S-specific alcohol dehydrogenase) which catalyzes the enantioselective reduction of 2-hydroxyacetophenone into (S)-1-phenyl-1,2-ethanediol. SDRs are a functionally diverse family of oxidoreductases that have a single domain with a structurally conserved Rossmann fold (alpha/beta folding pattern with a central beta-sheet), an NAD(P)(H)-binding region, and a structurally diverse C-terminal region. Classical SDRs are typically about 250 residues long, while extended SDRS are approximately 350 residues. Sequence identity between different SDR enzymes are typically in the 15-30% range, but the enzymes share the Rossmann fold NAD-binding motif and characteristic NAD-binding and catalytic sequence patterns. These enzymes have a 3-glycine N-terminal NAD(P)(H)-binding pattern (typically, TGxxxGxG in classical SDRs and TGxxGxxG in extended SDRs), while substrate binding is in the C-terminal region. A critical catalytic Tyr residue (Tyr-151, human 15-hydroxyprostaglandin dehydrogenase (15-PGDH) numbering), is often found in a conserved YXXXK pattern. In addition to the Tyr and Lys, there is often an upstream Ser (Ser-138, 15-PGDH numbering) and/or an Asn (Asn-107, 15-PGDH numbering) or additional Ser, contributing to the active site. Substrates for these enzymes include sugars, steroids, alcohols, and aromatic compounds. The standard reaction mechanism is a proton relay involving the conserved Tyr and Lys, as well as Asn (or Ser).
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