Structure and Chemical Reaction Mechanism of LigU, an Enzyme That Catalyzes an Allylic Isomerization in the Bacterial Degradation of Lignin.

LigU from Novosphingobium sp. strain KA1 catalyzes the isomerization of (4E)-oxalomesaconate (OMA) to (3Z)-2-keto-4-carboxy-3-hexenedioate (KCH) as part of the protocatechuate (PCA) 4,5-cleavage pathway during the degradation of lignin. The three-dimensional structure of the apo form of the wild-type enzyme was determined by X-ray crystallography, and the structure of the K66M mutant enzyme was determined in the presence of the substrate OMA. LigU is a homodimer requiring no cofactors or metal ions with a diaminopimelate epimerase structural fold, consisting of two domains with similar topologies. Each domain has a central α-helix surrounded by a β-barrel composed of antiparallel β-strands. The active site is at the cleft of the two domains. 1H nuclear magnetic resonance spectroscopy demonstrated that the enzyme catalyzes the exchange of the pro-S hydrogen at C5 of KCH with D2O during the isomerization reaction. Solvent-deuterium exchange experiments demonstrated that mutation of Lys-66 eliminated the isotope exchange at C5 and that mutation of C100 abolished exchange at C3. The positioning of these two residues in the active site of LigU is consistent with a reaction mechanism that is initiated by the abstraction of the pro-S hydrogen at C3 of OMA by the thiolate anion of Cys-100 and the donation of a proton at C5 of the proposed enolate anion intermediate by the side chain of Lys-66 to form the product KCH. The 1,3-proton transfer is suprafacial.