In vitro biosynthesis of homogalacturonan by a membrane-bound galacturonosyltransferase from epicotyls of azuki bean.
A membrane preparation of 7-d-old seedlings from azuki bean (Vigna angularis) contained galacturonosyltransferase (GalAT) capable of transferring galacturonic acid (GalA) from UDP-GalA into polygalacturonic acid (PGA) as an exogenous acceptor. The enzyme was maximally active at pH 6.8-7.8 and 25-35 degrees C in the presence of 5 mM Mn2+ and 0.5% (w/v) Triton X-100. Acid-soluble low-Mr (average Mr 10,000) PGA was a more efficient acceptor substrate than acid-insoluble polymer (Mr 70,000). The apparent Michaelis constants for UDP-GalA and low-Mr PGA were 0.14 mM and 0.02 mg/ml, respectively. Various pectins with different degrees of methyl-esterification (DE) were poor acceptors, and the enzyme activity tended to decrease with decreasing DE of the pectins. The transfer products from incubation of the enzyme with UDP-14C-GalA and the low-Mr PGA yielded 14C-GalA2 as the major product upon digestion with an endopolygalacturonase (EPGase), confirming the incorporation of GalA into PGA through contiguous alpha-1,4-linkages.
Takeuchi
Y
Y
Department of Biochemistry and Molecular Biology, Faculty of Science, Saitama University, Japan.
Tsumuraya
Y
Y
D002236:Carbohydrate Conformation; D007887:Fabaceae / Q000201:enzymology*; D014453:Glucuronosyltransferase; D016695:Glycosyltransferases / Q000737:chemistry / Q000302:isolation & purification / Q000378:metabolism*; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D008566:Membranes / Q000201:enzymology; D008861:Microsomes / Q000201:enzymology; D008970:Molecular Weight; D010368:Pectins / Q000096:biosynthesis* / Q000737:chemistry; D010940:Plant Proteins; D010944:Plants / Q000201:enzymology
D010940:Plant Proteins; D010368:Pectins; D016695:Glycosyltransferases; D014453:Glucuronosyltransferase; C438373:alpha-1,4-galacturonosyltransferase; C003181:polygalacturonic acid