Abstract
Aspergillus niger produces two forms (isoenzymes) of glucoamylase that are separable by electrophoresis or by chromatography on DEAE-cellulose and that are designated glucoamylase I and II. The molecular weight of glucoamylase I is 99,000, and that of glucoamylase II is 112,000. Both forms of the glucoamylase contain covalently linked carbohydrate (containing d-mannose, d-glucose, and d-galactose residues) and are therefore glycoenzymes. The carbohydrate-protein linkage in the glycoenzyme is primarily glycosidic to the hydroxyl group of l-serine and l-threonine residues, but glycosylamine linkages to l-asparagine and l-glutamine may also be present. Glucoamylase I and II possess identical amino acid compositions and, presumably, identical amino acid sequences. However, the two glycoenzymes differ in carbohydrate content, glucoamylase II containing nearly twice as many carbohydrate residues per molecule as glucoamylase I. Accordingly, it is suggested that the two forms of glucoamylase are isoglycoenzymes. The difference in electrophoretic and chromatographic properties of the isoglycoenzymes is probably due to a difference in the number of amide groups or glycosylaminically linked carbohydrate units in the polypeptide chains.
Published Version
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