Annual Progress Report on Malting Barley Research March, 2007

Introduction
There is general agreement that in germinating barley, the degradation of starch
and production of fermentable sugars results from the concerted action of four
carbohydrases - α-amylase, β-amylase, α-glucosidase and limit dextrinase. Compared
to the extensive body of knowledge about barley α- and β-amylases, there is only
rudimentary knowledge available about α-glucosidase and limit dextrinase. This
research was conducted to increase our fundamental knowledge of α-glucosidases in
germinating barley seeds by examining the genes that encode these enzymes and by
characterizing the functional properties of the α-glucosidases that are expressed in
germinating barley seeds.
α-Glucosidases are a diverse group of enzymes that catalyze several reactions
and are capable of using a wide variety of substrates. One feature that all α-
glucosidases share is the ability to hydrolytically remove the terminal monosaccharide
from the non-reducing end of a carbohydrate. Plant α-glucosidases have been
reported to hydrolyze di, - oligo- and polysaccharides, soluble starch, starch granules
with intact crystalline structure, and the carbohydrate structures attached to
glycoproteins. They can hydrolyze α-D-glycosidic bonds with –1,1-, -1,2-, -1,3-, 1,4-,
and -1,6- linkages. Plant α-glucosidases can hydrolyze glycosidic bonds between
glucose residues, between glucose and fructose residues, between glucose and
mannose residues, and between xylose residues. Additionally, they can catalyze
hydrolytic, condensation and transglycosylation reactions.
All plant α-glucosidases sequenced as of this time are categorized as members
of the glycosyl hydrolase family 31. In most cases this categorization is based upon
sequence similarity alone as enzyme function has not yet been determined. All
members of the glycosyl hydrolase family 31 share the same active site sequence and
have at least one additional highly conserved region. The first α-glucosidase cloned
from a plant was in 1996 when Dr. Ron Skadsen’s lab cloned Agl1 from barley (Tibbot
and Skadsen, 1996). Since that time, other genes encoding α-glucosidases have been
identified in plants although, in almost all cases, more is known about the gene than the
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