Studies on Structure, Function, and Protein Engineering of Starch-Degrading Enzymes.

Abstract

During the past few years great progress has been made in the understanding of the function of starch-degrading enzymes. Combining mutational analysis, multiple sequence alignment, and three dimensional structures of enzyme substrate analogue complexes formed the basis for developing amylolytic and related enzymes through protein engineering. In the case of barley a-amylase this has involved rational site-directed mutagenesis, regional random mutagenesis and isozyme sequence exchange as well as generation of isozyme chimeras. The approach has enabled modulation of enzyme specificity and identification of the functional roles of distinct residues and regions. The mutational analysis included the complex between barley α-amylase 2 and a proteinaceous inhibitor from barley seeds, BASI (barley a-amylase/subtilisin inhibitor). The interplay between the catalytic and the starch binding domains in glucoamylase from Aspergillus niger was investigated. The two domains are connected by a highly O-glycosylated linker and bind with 1:1 stoichiometry double-headed heterobifunctional synthetic analogues targeted to each domain. Glucoamylase is an inverting hydrolase and replacement of the catalytic base, a glutamic acid, by a cysteinesulfinic acid, gave rise to activity surpassing that of the wild-type enzyme, indicating a most surprising flexibility for the catalytic base.