Active site of Cellulomonas uda cellobiose phosphorylase in complex with cellobiose. The crystal structure of C. uda CBP is shown in complex with cellobiose [PDB:3S4A]. CBP is a homodimer whose active sites comprise an (α/α)6-barrel domain of one subunit (blue) and the helical extension from the N-terminal domain of the adjacent subunit (green). Cellobiose is bound in the active site with its reducing end pointing toward the N-terminal extension from the adjacent subunit (green). Arg166 and Gln165 on the adjacent subunit (green) might be in contact with cellobiose bound at the active site of the blue subunit. The 6-methoxy group that is present in cellobiose but absent in the GX molecule is circled. Xylose is expected to bind at the reducing end site, resulting in a possible interaction with the N-terminal extension of the adjacent subunit.