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Factors affecting D‐galactonate degradation by extracts of Aspergillus niger

Biochemical studies on the degradation of D‐galactonate by cell‐free extracts of Aspergillus niger indicated that the pH value and temperature optima were 8.0 and 7.5 and 40 °C and 50 °C for the two enzymes responsible for this degradation namely D‐galactonate dehydratase and 2‐keto‐3‐deoxy‐D‐galactonate (KDGal) aldolase respectively. The effects of the nature of the buffer substance, buffer molarity and enzyme concentration were also studied. Thermal stability behaviour studies show that d‐galactonate dehydratase was stable at 40 °C for 60 minutes and about 41%, 80% and 90% of enzyme activity were lost by exposing the extracts to 50 °C, 60 °C and 70 °C, respectively, for the same period. However, exposing the extracts to 70 °C after 60 minutes caused a complete inhibition for KDGal aldolase activity and a gradual decrease in activity was noticed by incubation the extracts at 60 °C. The results of freezing and thawing treatment indicated that KDGal aldolase was more stable than d‐galactonate dehydratase in this respect, as only 27% of enzyme activity was lost after 5 days of storage at –5 °C. Dialyzing the extracts significantly affects KDGal formation from d‐galactonate. Results obtained also indicated the non‐requirement of metal ions for activation of KDGal aldolase. On the other, hand d‐galactonate dehydratase has a requirement for Mg++ and Mn++, however ZnSO4 and HgCl2 caused a complete inhibition of the enzymatic activity of this enzyme.