Abstract
The amino acid sequence of soybean β-amylase (isozyme 2) revealed 60 and 30% homology with those of β-amylase from barley and Bacillus polymyxa, respectively. Two SH groups (SH2 and SH4) of soybean β-amylase are located in the highly conserved regions between the sequences of the higher plants and the bacterial enzymes . The chemical modification of the two SH groups resulted in the loss of the enzymatic activity . SH2 and SH4 were identified to be Cys 95 and Cys 343 by the analyses of chymotryptic digest of the enzyme after selective fluorescein-labeling of the two SH groups. As the modification of SH2 decreased the Vmax for amylopectin and the binding ability for glucose depending on the size of the substituents at SH2, it was concluded that SH2 is located near the subsite 1 of the enzyme. The X-ray crystallography of trigonal crystals of soybean β-amylase indicated that the molecule is composed of a large and of a small domain . The high resolution study of soybean β-amylase is now in progress for the hexagonal crystals using the oscillation camera method.
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