A structural and functional analysis of α-glucan recognition by family 25 and 26 carbohydrate-binding modules reveals a conserved mode of starch recognition

Abstract

Starch-hydrolyzing enzymes lacking α-glucan-specific carbohydrate-binding modules (CBMs) typically have lowered activity on granular starch relative to their counterparts with CBMs. Thus, consideration of starch recognition by CBMs is a key factor in understanding granular starch hydrolysis. To this end, we have dissected the modular structure of the maltohexaose-forming amylase from Bacillus halodurans (C-125). This five-module protein comprises an N-terminal family 13 catalytic module followed in order by two modules of unknown function, a family 26 CBM (BhCBM26), and a family 25CBM (BhCBM25). Here we present a comprehensive structure-function analysis of starch and α-glucooligosaccharide recognition by BhCBM25 and BhCBM26 using UV methods, isothermal titration calorimetry, and x-ray crystallography. The results reveal that the two CBMs bind α-glucooligosaccharides, particularly those containing α-1,6 linkages, with different affinities but have similar abilities to bind granular starch. Notably, these CBMsappear to recognize the same binding sites in granular starch. The enhanced affinity of the tandem CBMs for granular starch is suggested to be the main biological advantage for this enzyme to contain two CBMs. Structural studies of the native and ligand-bound forms of BhCBM25 and BhCBM26 show a structurally conserved mode of ligand recognition but through non-sequence-conserved residues. Comparison of these CBM structures with other starch-specific CBM structures reveals a generally conserved mode of starch recognition. © 2006 by The American Society for Biochemistry and Molecular Biology, Inc.