A previous study suggested the presence of a single L-arabitol dehydrogenase encoding gene involved in the L-arabinose catabolism [6], as a UV mutant of this gene was devoid of L-arabitol dehydrogenase activity. It is therefore likely that LadB and LadC have different biological functions f LadA. Modelling of the structure from A. niger LadA Eltanexor chemical structure and XdhA on human D-sorbitol dehydrogenase revealed a large number of amino acids that are conserved in all three types of dehydrogenases, including the residues involved in Zinc binding (H80, E81 and E166, numbers from LadA sequence) [13].

None of the residues that were conserved in L-arabitol and D-sorbitol dehydrogenases, but different in xylitol dehydrogenases were in close proximity of the substrate cleft. However, two of the residues (F62 and F302 from XdhA) that were conserved in xylitol and D-sorbitol dehydrogenases, but different in L-arabitol dehydrogenases (corresponding to M70 and Y318 from LadA) were located very close to the substrate, suggesting that they may be important for substrate specificity. As both XdhA and D-sorbitol dehydrogenase are active on D-sorbitol, whereas LadA has very little

activity on this substrate [5] this could indicate that these residues are important for activity on D-sorbitol. The M70F mutation of LadA of A. niger resulted in almost complete inactivation of the AZD7762 cost enzyme on a variety of substrates. The reason for this is not clear at this point, but a possible

explanation could be that M70 in this particular enzyme influences the 3-dimensional structure; thus Bioactive Compound Library concentration promoting enzyme activity. As the aim of this study was to identify residues important in substrate specificity, we did not further investigate this mutation. The Y318F mutation of LadA resulted in increased affinity of the enzyme for D-sorbitol, while the Vmax and Kcat increased for L-arabitol and xylitol. Projection of the catalytic site of LAD, SDH and XDH predicts that the tyrosine residue in LAD and the phenylalanine in SDH Glutamate dehydrogenase and XDH are in exactly the same position (Fig. 3). This suggests that the OH group on the Y318 is the only structural difference between LadA and the Y318F mutant protein. This demonstrates that the presence of a phenylalanine at this position contributes significantly to D-sorbitol dehydrogenase activity. This OH-group probably affects positioning of D-sorbitol by hydrogen-bond formation in the substrate binding site, which prevents efficient catalysis in native A. niger LadA. The tyrosine residue does not affect affinity of LadA for L-arabitol and xylitol. However, the increased activity in the mutant suggests that the presence of the OH-group delays release of the products (L-xylulose and D-xylulose). D-sorbitol and xylitol differ structurally from L-arabitol with respect to positioning of the OH-group on C2 and C4, while D-sorbitol has an additional OH group at C5 compared to xylitol (Fig. 4).