Glycoenzymes: an unusual type of glycoprotein structure for a glucoamylase

Abstract

Glucoamylase, (1→4)(1→6)-α- d-glucan glucohydrolase (EC 3.2.1.3), hydrolyzes starch and glycogen completely to d-glucose and is used industrially in the manufacture of d-glucose from starch. The enzyme is elaborated by many types of fungi and occurs in two isoenzymic forms (glucoamylase I and glucoamylase II) in extracts from certain fungi. The isoenzymes from Aspergillus niger are glycoenzymes containing d-mannose, d-glucose, and d-galactose as integral structural components. New data from experiments on reductive alkaline β-elimination and from methylation analyses show that the carbohydrate chains of glucoamylase I are linked O-glycosidically from d-mannose residues to l-serine or l-threonine residues of the protein moiety. In this enzyme, the carbohydrate residues are present as 20 single d-mannose residues, 11 disaccharides components having the structure 2- O- d-mannopyranosyl- d-mannose, 8 trisaccharides, and 5 tetrasaccharides composed of various combinations of d-mannose, d-glucose, and d-galactose residues joined by (1→3) and (1→6) glycosidic linkages. Such an array of carbohydrate chains in a glycoprotein is unusual, and may account for some of the unique properties exhibited by glucoamylase.