text: Glycogen branching enzyme belongs to the α-amylase family of enzymes, which include α-amylases, pullulanas/isoamylase, cyclodextrin glucanotransferase (CGT), and branching enzyme.  Shown by x-ray crystallography, glycogen branching enzyme has four marginally asymmetric units each that are organized into three domains: an amino-terminal domain, involved in determining the length of the chain transfer, a carboxyl-terminal domain, involved in substrate preference and catalytic capacity, and a central (α/β) barrel catalytic domain. The amino-terminal domain consists of 128 residues arranged in seven β-strands, the carboxyl-terminal domain with 116 residues also organized in seven β-strands, and the (α/β) barrel domain with 372 residues. While the central (α/β) barrel domain is common in members of the α-amylase family, numerous variations exist between the various barrel domains. Additionally, there are striking differences between the loops connecting elements of the secondary structure among these various α-amylase members, especially around the active site. In comparison to the other family members, glycogen binding enzyme has shorter loops, which result in a more open cavity, favorable to the binding of a bulkier substrate such as branched sugar. Through primary structure analysis and the x-ray crystallographic structures of the members of the α-amylase family, seven residue were conserved, Asp335, His340, Arg403, Asp 405, Glu458, His525, and Asp526 (E coli. numbering). These residues are implicated in catalysis and substrate binding.
  Glycogen binding enzymes in other organisms have also been crystallized and structurally determined, demonstrating both similarity and variation to GBE found in Escherichia coli.