Improved activity and modulated action pattern obtained by random mutagenesis at the fourth beta-alpha loop involved in substrate binding to the catalytic (beta/alpha)(8)-barrel domain of barley alpha-amylase 1

The functionality of the sequence Arg(183)-Gly(184)-Tyr(185) of the substrate binding fourth beta-alpha loop in the (beta/alpha)(8)-barrel of barley alpha-amylase isozyme 1 (AMY1) was studied by random mutagenesis. A motif of polar Gly(184) hydrophobic residues was present in active mutants, selected by starch plate screening of yeast transformants. Gly(184) was important, probably due to the carbonyl group binding to Ca2+ and the spatial proximity of Phe(181). Mutation of both flanking residues as in Ser(183)-Gly(184)-Met(185) (SGM-) and TGL-AMY1 decreased the Ca2+ affinity. SGM-AMY1 has 2-fold increased activity for amylose but reduced activity on maltooligosaccharides, whereas KGY-AMY1 has up to 3-fold elevated activity toward the oligosaccharides. TGL-AMY1 has modest activity on all substrates. Shifted action pattern on maltooligosaccharides for NGY-, SGM-, and TGL-AMY1 support that Arg(183) in wild type is located at subsites +1 and +2, accommodating two sugar rings toward the reducing end from the site of cleavage. In the crystal structure of barley alpha-amylase 2 (AMY2), Lys(182) (equivalent to AMY1 Arg(183)) is hydrogen-bonded with sugar OH-3 in subsite +2. Higher K-i app for acarbose inhibition of KGY-AMY1 and parent AMY1 compared with the other mutants suggests favorable substrate interactions for Arg/Lys(183). KGY-AMY1 was not inhibited by the AMY2-specific proteinaceous barley alpha-amylase/subtilisin inhibitor, although Lys(182) Of AMY2 is salt-linked to the inhibitor.