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. There are at least 4 types of alpha-amylase in barley. Type B isozyme mRNA is undetectable in unstimulated cells and increases a hundred-fold after stimulation with gibberellic acid. Belongs to the glycosyl hydrolase 13 family.