Degradation of some sugars and sugar acids by the nonphosphorylated D‐gluconate pathway in Aspergillus ustus

The degradation of fifteen sugars, sugar acids and related substrates were examined using cellfree extracts of Aspergillus ustus growing on D‐glucose, D‐mannose, D‐galactose or D‐gluconate as the only carbon source. D‐gluconate was superior for the induction of the enzymes capable for the degradation of some of these substrates. The addition of 0.5% malt extract with D‐gluconate to the growth medium or the presence of shaking conditions resulted to an increase in the degradation of those substrates, whileas the incorporation of 0.5% malt extract alone to the medium has no effect. Extracts of D‐gluconate‐grown mycelia of A. ustus degraded D‐gluconate > D‐galactonate > 1 : 5 gluconolactone and > L‐arabonate nonphosphorolytically more effectively. Optimum pH and temperature for the degradation of D‐gluconate were found to be 8.0 and 40°C, respectively. Thermal stability studies on the behaviour of D‐gluconate dehydratase showed that this enzyme was stable at 50°C and 60°C for 30 and 5 minutes, respectively. Specific activity of this enzyme was increased three times when cell‐free extracts were incubated at 60°C for 5 minutes. MgCl2 and CoSO4 were good activators, while CaCl2 p‐mercurychlorobenzoate (PMCB), sodium arsenite, ZnSO4, CuSO4, iodoacetic acid, MnCl2 and FeSO4 were potent inhibitors for D‐gluconate dehydratase activity. Km was calculated for D‐gluconate and found to be 2.5 × 10−2 M.