Superposition of the active sites of D-sorbitol dehydrogenase (SDH), xylitol dehydrogenase (XDH) and L-arabitol dehydrogenase (LAD). Crystal structure of D-sorbitol dehydrogenase (1PL6)  is depicted in green. The substrate analogue which was co-crystalised is shown as grey sticks. Oxygen, nitrogen and sulphur residues are shown in red, blue and yellow, respectively. Active site residues are shown as sticks and are labelled. Residues that are different in LAD are in magenta and are labelled with the one letter code in magenta. All residues shown are identical in SDH and XDH. Numbers in the figure are from the SDH sequence: F59 corresponds to F62 and M70 in A. niger XdhA and LadA, respectively; F297 corresponds to F302 and Y318 in A. niger XdhA and LadA, respectively.