ARS | Publication request: STARCH DEGRADATION IN GERMINATED BARLEY SEEDS

Submitted to: Brewers Digest
Publication Type: Abstract Only
Publication Acceptance Date: April 14, 1995
Publication Date: N/A

Technical Abstract: Complete degradation of starch results from the concerted action of alpha- amylase, beta-amylase, debranching enzyme and alpha-glucosidase. The least studied of the four enzymes is alpha-glucosidase, which hydrolyzes alpha- glucosyl residues from the nonreducing terminus of the substrate liberat- ing alpha-glucose. We have isolated high pI alpha-glucosidase from germi- nating barley seeds and determined the kinetics of substrate specificity and subsite affinities. The enzyme has only one maltose binding site per molecule and shows high activity on maltooligosaccharides and nigerose. Activity on isomaltose and p-nitrophenyl alpha-glucoside is moderate. Tre- halose is not hydrolyzed at detectable rates. The ratios of maximal veloc- ities for maltose, nigerose, isomaltose, p-nitrophenyl alpha-glucoside and malto-triose, -tetraose, -pentaose, -hexaose, -heptaose are 100:95:21:9: 111:116:119:104:111. The Km values for these substrates are 1.91, 1.29, 5.32, 1.04, 1.11, 2.37, 2.92, 5.44, and 7.89 mM, respectively. Based on the rate parameters for maltooligosaccharides, the subsite affinities in the active site of the enzyme were evaluated. The first three subsites were considered effective for substrate binding to the active site. Dif- ferent arrangement of subsite affinities among alpha-glucosidases, gluco- amylases and amylases was used to explain differences in their substrate specificities. Several amino acids required for alpha-glucosidase to hydrolyze maltodextrins were identified and their subsite location being studied. Kinetics of end product inhibition have been determined. Condi- tions leading to optimally active alpha-glucosidase will be discussed.