Sorbitol dehydrogenase of Aspergillus niger , SdhA, is part of the oxido‐reductive d ‐galactose pathway and essential for d ‐sorbitol catabolism

In filamentous fungi d ‐galactose can be catabolised through the oxido‐reductive and/or the Leloir pathway. In the oxido‐reductive pathway d ‐galactose is converted to d ‐fructose in a series of steps where the last step is the oxidation of d ‐sorbitol by an NAD‐dependent dehydrogenase. We identified a sorbitol dehydrogenase gene, sdhA (JGI53356), in Aspergillus niger encoding a medium chain dehydrogenase which is involved in d ‐galactose and d ‐sorbitol catabolism. The gene is upregulated in the presence of d ‐galactose, galactitol and d ‐sorbitol. An sdhA deletion strain showed reduced growth on galactitol and growth on d ‐sorbitol was completely abolished. The purified enzyme converted d ‐sorbitol to d ‐fructose with K m of 50 ± 5 mM and v max of 80 ± 10 U/mg.