• 1,3(4)-β-d-glucanase;
  • 3D protein-ligand structure;
  • glycoside hydrolase family 16;
  • laminarin;
  • lichenin

The 1,3(4)-β-d-glucanases of glycoside hydrolase family 16 provide useful examples of versatile yet specific protein–carbohydrate interactions. In the present study, we report the X-ray structures of the 1,3(4)-β-d-glucanase Phanerochaete chrysosporium Laminarinase 16A in complex with β-glucan products from laminarin (1.6 Å) and lichenin (1.1 Å) hydrolysis. The G6G3G3G glucan, in complex with the enzyme, showed a β-1,6 branch in the acceptor site. The G4G3G ligand–protein complex showed that there was no room for a β-1,6 branch in the −1 or −2 subsites; furthermore, the distorted residue in the −1 subsite and the glucose in the −2 subsite required a β-1,3 bond between them. These are the first X-ray crystal structures of any 1,3(4)-β-d-glucanase in complex with glucan products. They provide details of both substrate and product binding in support of earlier enzymatic evidence.