alternative D-galactose degrading pathway of Aspergillus nidulans proceeds via L-sorbose

• Published in: Archives of microbiology 2004, Vol.181 (1), p.35-44
• Main Authors: Fekete, E, Karaffa, L, Sandor, E, Banyai, I, Seiboth, B, Gyemant, G, Sepsi, A, Szentirmai, A, Kubicek, C.P
• Format: Article
• Language: English
• Subjects: Ammonium; araA1 gene; Aspergillus nidulans; Aspergillus nidulans - enzymology; Aspergillus nidulans - genetics; Aspergillus nidulans - metabolism; biochemical pathways; Biological and medical sciences; Biomass; carbohydrate metabolism; Chromatography, High Pressure Liquid; degradation pathways; Dehydrogenase; Enzymes; Fundamental and applied biological sciences. Psychology; fungus physiology; galactitol; Galactitol - metabolism; galactose; Galactose - metabolism; galE gene; Genes, Fungal; Hexokinase - metabolism; L-Arabitol dehydrogenase; L-sorbose; Magnetic Resonance Spectroscopy; Microbiology; Miscellaneous; Mutants; Mutation; Mycology; Nitrates - metabolism; Oxidation; Quaternary Ammonium Compounds - metabolism; Sorbose - analysis; Sorbose - metabolism; Sugar Alcohol Dehydrogenases - metabolism; Yeasts
• ISSN: 0302-8933; 1432-072X
• DOI: 10.1007/s00203-003-0622-8

The catabolism of D-galactose in yeast depends on the enzymes of the Leloir pathway. In contrast, Aspergillus nidulans mutants in galactokinase (galE) can still grow on D-galactose in the presence of ammonium--but not nitrate--ions as nitrogen source. A. nidulans galE mutants transiently accumulate high (400 mM) intracellular concentrations of galactitol, indicating that the alternative D-galactose degrading pathway may proceed via this intermediate. The enzyme degrading galactitol was identified as L-arabitol dehydrogenase, because an A. nidulans loss-of-function mutant in this enzyme (araA1) did not show NAD+-dependent galactitol dehydrogenase activity, still accumulated galactitol but was unable to catabolize it thereafter, and a double galE/araA1 mutant was unable to grow on D-galactose or galactitol. The product of galactitol oxidation was identified as L-sorbose, which is a substrate for hexokinase, as evidenced by a loss of L-sorbose phosphorylating activity in an A. nidulans hexokinase (frA1) mutant. L-Sorbose catabolism involves a hexokinase step, indicated by the inability of the frA1 mutant to grow on galactitol or L-sorbose, and by the fact that a galE/frA1 double mutant of A. nidulans was unable to grow on D-galactose. The results therefore provide evidence for an alternative pathway of D-galactose catabolism in A. nidulans that involves reduction of the D-galactose to galactitol and NAD+-dependent oxidation of galactitol by L-arabitol dehydrogenase to L-sorbose.