MODELS FOR THE ACTION OF BARLEY ALPHA-AMYLASE ISOZYMES ON LINEAR SUBSTRATES

The formation of maltodextrins, G(1) to G(12), during the hydrolysis of amylose by alpha-amylases 1 and 2 from barley malt was followed by HPLC. Similar, but not identical, distributions of products were obtained with-the two alpha-amylase components. Maltose, G(6), and G(7) were major products, but G(7) was degraded as hydrolysis proceeded. alpha-Amylase 1 produced more G(1) and G(3) than did alpha-amylase 2 at all stages of hydrolysis. Products formed during the hydrolysis of G(9), G(10), G(11), and G(12) by the two alpha-amylases were also determined. A different spectrum of products was observed with each substrate and small differences were observed in the action pattern of the two alpha-amylases, e.g., G(3) and G(7) were the major products formed during the hydrolysis of G(10) by alpha-amylase 1, whereas G(2) and G(8) were the major products formed by alpha-amylase 2 on the same substrate. These results were used to develop a model of the active site of barley malt alpha-amylases. This site contains ten contiguous subsites with the catalytic site situated between subsites 7 and 8. The model can be used to predict hydrolysis patterns of amylose and maltodextrins by cereal alpha-amylases.