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 degrees C and 50 degrees 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 degrees C for 60 minutes and about 41%, 80% and 90% of enzyme activity were lost by exposing the extracts to 50 degrees C, 60 degrees C and 70 degrees C, respectively, for the same period. However, exposing the extracts to 70 degrees 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 degrees 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 degrees 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.
Elshafei
A M
AM
Department of Microbial Chemistry, National Research Centre, Dokki, Cairo, Egypt. alielshafei@yahoo.com
Abdel-Fatah
O M
OM
D000446:Aldehyde-Lyases / Q000378:metabolism*; D001234:Aspergillus niger / Q000201:enzymology*; D001673:Biodegradation, Environmental; D006836:Hydro-Lyases / Q000378:metabolism*; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D008274:Magnesium / Q000378:metabolism / Q000494:pharmacology; D008670:Metals / Q000378:metabolism / Q000494:pharmacology; D013400:Sugar Acids / Q000378:metabolism*; D013696:Temperature
D008670:Metals; D013400:Sugar Acids; C012991:galactonic acid; C093801:2-keto-3-deoxy-D-galactonate aldolase; D000446:Aldehyde-Lyases; D006836:Hydro-Lyases; C019607:galactonate dehydratase; D008274:Magnesium