ARS | Publication request: STARCH DEGRADATION IN GERMINATING BARLEY SEEDS: THE CHARACTERISTICS AND ROLES OF ALPHA-GLUCOSIDASE

Submitted to: Preharvest Sprouting of Cereals Proceedings
Publication Type: Abstract Only
Publication Acceptance Date: May 15, 1995
Publication Date: N/A

Technical Abstract: The complete degradation of starch results from the concerted action of alpha-amylase, beta-amylase, debranching enzyme and alpha-glucosidase. A mathematical study of the interactions between these four enzymes as they work together to hydrolyze starch revealed that alpha-glucosidase is a key enzyme in hydrolysis of both native and gelatinized starches. This study also revealed that alpha-glucosidase's primary contribution to native starch granule hydrolysis was by interacting with alpha-amylase whereas its contribution to hydrolysis of gelatinized starches was nearly half noninteractive and half via interactions with alpha- and beta-amylases. Alpha-glucosidase is the least studied of the four starch degrading enzymes in germinating barley seeds. We have isolated a high pI isoform and determined its substrate specificity and subsite affinities. The enzyme has only one maltose binding site per molecule and shows high activity on alpha-1,4-maltodextrins and nigerose. Activity on isomaltose and p-nitrophenyl alpha-glucoside is moderate. Trehalose is not hydrolyzed. Kinetic rate parameters for maltodextrin hydrolysis were determined. Km's were dependent upon the substrate chain length. Based on experimentally determined rate parameters, the subsite affinities within the active site were calculated. The first three subsites effectively bind substrate to the active site. Arrangements of the subsite within the active site of alpha-glucosidase is different from those of glucoamylases, alpha-amylase and beta-amylase. These differences account for differences in these enzymes substrate specificities. End product inhibition kinetics were studied. Optimal conditions for alpha-glucosidase activity and its probable roles in starch degradation will be discussed.