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1H and 15N assignments and secondary structure of the starch-binding domain of glucoamylase from Aspergillus niger.

1H and 15N NMR resonance assignments of the granular starch-binding domain (SBD) of glucoamylase from Aspergillus niger have been made by multi-dimensional homonuclear and heteronuclear NMR techniques. Secondary structure analysis based on chemical shifts, 1H-1H NOEs, coupling constants and backbone amide exchange data indicates the presence of a well-defined beta-sheet structure. This consists of one parallel and five antiparallel pairs of beta-strands forming two beta-sheets. Cis-trans isomerisation of proline residues and O-glycosylation of threonine residues are observed and compared between the proteolytically derived SBD fragment and the recombinant protein. Structural features of the SBD in solution were compared to the X-ray crystal structure of a homologous domain of cyclodextrin glycosyltransferase from Bacillus circulans. There are some differences in the locations of the start and end of beta-strands but overall the two structures are very similar. This study will form the basis for the structure determination of the granular SBD and of its complexes.