The four major forms of barley β-amylase. Purification, characterization and structural relationship

Abstract

Four major forms of barley β-amylase have been purified in the presence of 0.1 m-thiol from extracts of flour fractionated by ammonium sulfate precipitation and chromatography on DEAE-cellulose and DEAE-Fractogel. The β-amylases were NH2-terminally blocked, single polypeptide chains of approx. Mr 59,700, 58,000, 56,000 and 54,000, with corresponding pI 5.2, 5.3, 5.5 and 5.7. All forms displayed optimal activity on soluble starch between pH 4.5 and 7.5; all Km and Vmax values were 2.5 mg·ml-1 and 17 μmol maltose·min-1·(nmol protein)-1, respectively. The β-amylase 4 consisted of different components terminating between Gly493 and Gln497 in the amino acid sequence deduced from the nucleotide sequence of a full length cDNA (Kreis et al., Eur. J. Biochem., in press (1987)). β-Amylase 4 is the main product from limited proteolysis of the large form, β-amylase 1, catalysed by malt protease(s), papain or subtilisin. Intermediate forms of β-amylase with pI 5.3 and 5.5 were generated with crude malt enzymes, α-chymotrypsin or thermolysin. The conversion effected by malt enzymes was prevented by leupeptin and EDTA in combination. © 1987 Carlsberg Laboratory.