Purification and properties of potato 1,4-alpha-D-glucan:1,4-alpha-D-glucan 6-alpha-(1,4-alpha-glucano)-transferase. Evidence against a dual catalytic function in amylose-branching enzyme.
Q-Enzyme, the enzyme that synthesizes the 1,6-alpha-glucosidic branch linkages of amylopectin, has been purified from potato to near homogeneity. The molecular weight of the enzyme is 85000. The active enzyme is a monomer, with a molar activity at pH 7.0 and 24 degrees C of 15. The energy of activation is 25 kJ/mol below 15 degrees C, changing sharply to 63 kJ/mol above that temperature. Enzyme activity is not affected by Mg2+ or ATP. There are about 11 readily titratable sulfhydryl groups per molecule. The evidence that the enzyme is a single protein entity, without hydrolytic activity towards amylose, contrasts with an earlier report that Q-enzyme consists of two components, a hydrolase with molecular weight 70000, and a transferase with molecular weight 20000. Q-enzyme acts on native and synthetic amyloses to give products resembling amylopectin in terms of average unit chain length, degress of beta-amylolysis and iodine stain. The profiles of the unit chains of these synthetic products are, however, different from that of native amylopectin. Additional branch linkages are introduced by Q-enzyme into potato amylopectin, but the product bears no resemblance to phytoglycogen.
D015061:1,4-alpha-Glucan Branching Enzyme / Q000302:isolation & purification / Q000378:metabolism*; D000255:Adenosine Triphosphate / Q000494:pharmacology; D000688:Amylose; D001665:Binding Sites; D002151:Calorimetry; D002729:Chloromercuribenzoates / Q000494:pharmacology; D005964:Glucosyltransferases / Q000378:metabolism*; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D008274:Magnesium / Q000494:pharmacology; D008970:Molecular Weight; D010944:Plants / Q000201:enzymology*; D011485:Protein Binding; D011487:Protein Conformation; D013696:Temperature; D013816:Thermodynamics
D002729:Chloromercuribenzoates; D000255:Adenosine Triphosphate; D000688:Amylose; D005964:Glucosyltransferases; D015061:1,4-alpha-Glucan Branching Enzyme; D008274:Magnesium