3WKG: Crystal Structure Of Cellobiose 2-epimerase In Complex With Glucosylmannose

Cellobiose 2-epimerase (CE) reversibly converts d-glucose residues into d-mannose residues at the reducing end of unmodified beta1,4-linked oligosaccharides, including beta-1,4-mannobiose, cellobiose, and lactose. CE is responsible for conversion of beta1,4-mannobiose to 4-O-beta-d-mannosyl-d-glucose in mannan metabolism. However, the detailed catalytic mechanism of CE is unclear due to the lack of structural data in complex with ligands. We determined the crystal structures of halothermophile Rhodothermus marinus CE (RmCE) in complex with substrates/products or intermediate analogs, and its apo form. The structures in complex with the substrates/products indicated that the residues in the beta5-beta6 loop as well as those in the inner six helices form the catalytic site. Trp-322 and Trp-385 interact with reducing and non-reducing end parts of these ligands, respectively, by stacking interactions. The architecture of the catalytic site also provided insights into the mechanism of reversible epimerization. His-259 abstracts the H2 proton of the d-mannose residue at the reducing end, and consistently forms the cis-enediol intermediate by facilitated depolarization of the 2-OH group mediated by hydrogen bonding interaction with His-200. His-390 subsequently donates the proton to the C2 atom of the intermediate to form a d-glucose residue. The reverse reaction is mediated by these three histidines with the inverse roles of acid/base catalysts. The conformation of cellobiitol demonstrated that the deprotonation/reprotonation step is coupled with rotation of the C2-C3 bond of the open form of the ligand. Moreover, it is postulated that His-390 is closely related to ring opening/closure by transferring a proton between the O5 and O1 atoms of the ligand.
PDB ID: 3WKGDownload
MMDB ID: 116180
PDB Deposition Date: 2013/10/21
Updated in MMDB: 2013/12
Experimental Method:
x-ray diffraction
Resolution: 1.47  Å
Source Organism:
Similar Structures:
Biological Unit for 3WKG: monomeric; determined by author and by software (PISA)
Molecular Components in 3WKG
Label Count Molecule
Protein (1 molecule)
Cellobiose 2-epimerase
Molecule annotation
Chemicals (4 molecules)
* Click molecule labels to explore molecular sequence information.

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