Identification of tryptophanyl residues involved in binding of carbohydrate ligands to barley α-amylase 2

Abstract

In barley α-amylase 2, two and three tryptophans are protected against reaction with dimethyl(2-hydroxy-5-nitrobenzyl)sulphonium bromide by β-cyclodextrin and the pseudooligosaccharide inhibitor aplanin, respectively. Fragments were generated from the enzyme derivatives by digestion with Armillaria mellea protease and trypsin, and isolated by RP-HPLC. The substituted tryptophans were identified by amino acid and sequence analyses of modified peptides. Aplanin and β-cyclodextrin both reduced the accessibility of Trp276 and -277. In addition, aplanin hindered modification of Trp206, and only this derivative retained activity. Trp206 probably belongs to the active site region, whereas Trp276 and -277 are located in a different binding site. This suggestion is supported by a comparison with the 3-D structure of Taka-amylase A guided by sequence homology between it and barley α-amylase. © 1987 Carlsberg Laboratory.