Endohydrolysis of (1->4)-alpha-D-glucosidic linkages in polysaccharides containing three or more (1->4)-alpha-linked D-glucose units. Binds 3 Ca(2+) ions per subunit. Monomer. Production of alpha-amylase is hormonally regulated. Germinating embryos produce the hormone gibberellic acid, which within 10 hours stimulates the aleurone cells covering the endosperm of the seed to produce alpha-amylase. The enzyme then degrades the starch within the endosperm for use by the developing plant embryo. Type B isozyme mRNA is undetectable in unstimulated cells and increases a hundred-fold after stimulation with gibberellic acid. There are at least 4 types of alpha-amylase in barley. Binds starch not only at the active site, but also via accessory binding sites on the protein surface that are important for efficient binding to starch granules and thereby increase enzyme activity. Belongs to the glycosyl hydrolase 13 family.