Fungal glucoamylases.

Glucoamylase (alpha-1,4-glucan glucohydrolase, EC 3.2.1.3) from fungal sources is one of the microbial glycoproteins that has received considerable attention particularly because it is used in the commercial production of dextrose. Several investigators have isolated glucoamylase from various fungal sources. In many instances the presence of more than one form of enzyme is common. The enzymes from most sources have pH optima between 4 and 5 and exhibit maximum activity between 40 and 60 degrees C. The enzyme does not require any cofactors for activity or for stability. The enzyme has an Mr between 48,000 and 80,000 and usually has no subunit structure. The amino acid composition of multiple forms of glucoamylases differ in general, but all of them are glycoproteins. The carbohydrate content of the enzyme ranges from 3 to 30% containing mainly mannose, but glucose, galactose, and in some instances glucosamine and xylose are also present. In the enzyme from Aspergillus the carbohydrate structures are present as mono-, di-, tri-, and tetrasaccharide units linked O-glycosidically through mannose to the hydroxyl groups of serine and threonine. In the enzyme from Rhizopus part of the carbohydrate is present as disaccharide (Man-Man-) units linked O-glycosidically and the remainder is present as large heterosaccharide structures attached by N-glycosidic linkages involving aspargine and glucosamine. Carbohydrate moieties seem to have no influence on the enzyme activity or antigenicity but appear to stabilize the enzyme by preserving the three-dimensional structure.

Methionine-repressible homoserine dehydrogenase of Serratia marcescens: purification and properties.

Serratia marcescens Sa-3 possesses two homoserine dehydrogenases and neither has any aspartokinase activity unlike the case of Escherichia coli enzymes. The two enzymes have been separated. One of them is active with either NAD+ or NADP+ and has been purified about 180-fold to homogeneity. This enzyme is completely repressed by the presence of 1 mM methionine or homoserine in the growth medium, but its activity is unaffected by any amino acid of the aspartate family either singly or together. In many of its properties (such as pH optimum, Km for substrate and cofactors), it resembles its counterpart in E. coli K12. Potassium ions stabilize the enzyme but are not essential for activity. Its molecular weight is around 155,000 as determined by gel filtration and approximately 76,000 by SDS-polyacrylamide gel electrophoresis. This suggests that the enzyme has two subunits (polypeptide chains) in the molecule: 8 M urea has no effect on enzyme activity. This enzyme represents approximately 30% of the total homoserine dehydrogenase activity of S. marcescens unlike in Salmonella typhimurium and E. coli K12 where it is a minor or a negligible component.