The alternative D-galactose degrading pathway of Aspergillus nidulans proceeds via L-sorbose.
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.
Fekete
Erzsébet
E
Department of Microbiology and Biotechnology, University of Debrecen, Faculty of Sciences, P.O.Box 63, 4010 Debrecen, Hungary.
Karaffa
Levente
L
Sándor
Erzsébet
E
Bányai
István
I
Seiboth
Bernhard
B
Gyémánt
Gyöngyi
G
Sepsi
Adél
A
Szentirmai
Attila
A
Kubicek
Christian P
CP
D001233:Aspergillus nidulans / Q000201:enzymology* / Q000235:genetics / Q000378:metabolism*; D018533:Biomass; D002851:Chromatography, High Pressure Liquid; D004376:Galactitol / Q000378:metabolism; D005690:Galactose / Q000378:metabolism*; D005800:Genes, Fungal; D006593:Hexokinase / Q000378:metabolism; D009682:Magnetic Resonance Spectroscopy; D009154:Mutation; D009566:Nitrates / Q000378:metabolism; D000644:Quaternary Ammonium Compounds / Q000378:metabolism; D013013:Sorbose / Q000032:analysis / Q000378:metabolism*; D013401:Sugar Alcohol Dehydrogenases / Q000378:metabolism
D009566:Nitrates; D000644:Quaternary Ammonium Compounds; D004376:Galactitol; D013401:Sugar Alcohol Dehydrogenases; D006593:Hexokinase; D013013:Sorbose; D005690:Galactose