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.