Crystal structure of the starch-binding domain of glucoamylase from<i>Aspergillus niger</i>

<jats:p>Glucoamylases are widely used commercially to produce glucose syrup from starch. The starch-binding domain (SBD) of glucoamylase from<jats:italic>Aspergillus niger</jats:italic>is a small globular protein containing a disulfide bond. The structure of<jats:italic>A. niger</jats:italic>SBD has been determined by NMR, but the conformation surrounding the disulfide bond was unclear. Therefore, X-ray crystal structural analysis was used to attempt to clarify the conformation of this region. The SBD was purified from an<jats:italic>Escherichia coli</jats:italic>-based expression system and crystallized at 293 K. The initial phase was determined by the molecular-replacement method, and the asymmetric unit of the crystal contained four protomers, two of which were related by a noncrystallographic twofold axis. Finally, the structure was solved at 2.0 Å resolution. The SBD consisted of seven β-strands and eight loops, and the conformation surrounding the disulfide bond was determined from a clear electron-density map. Comparison of X-ray- and NMR-determined structures of the free SBD showed no significant difference in the conformation of each β-strand, but the conformations of the loops containing the disulfide bond and the L5 loop were different. In particular, the difference in the position of the C<jats:sup>α</jats:sup>atom of Cys509 between the X-ray- and NMR-determined structures was 13.3 Å. In addition, the<jats:italic>B</jats:italic>factors of the amino-acid residues surrounding the disulfide bond are higher than those of other residues. Therefore, the conformation surrounding the disulfide bond is suggested to be highly flexible.</jats:p>