Three-dimensional structure of homodimeric cholesterol esterase-ligand complex at 1.4 Å resolution

The three-dimensional structure of a Candida cylindracea cholesterol esterase (ChE) homodimer (534 × 2 amino acids) in complex with a ligand of proposed formula C₂₃H₄₈O₂ has been determined at 1.4 Å resolution in space group P1 using synchrotron low-temperature data. The structure refined to R = 0.136 and R_free = 0.169 and has revealed new stereochemical details in addition to those detected for the apo- and holo-forms at 1.9 and 2.0 Å resolution, respectively [Ghosh et al. (1995), Structure, 3, 279-288]. The cholesterol esterase structure is a dimer with four spatially separated interfacial contact areas and two symmetry-related pairs of openings to an internal intradimer cavity. Hydrophobic active-site gorges in each subunit face each other across a central interfacial cavity. The ChE subunits have carbohydrate chains attached to their Asn314 and Asn351 residues, with two ordered N-acetyl-D-glucosoamine moieties visible at each site. The side chains of 14 residues have two alternative conformations with occupancy values of 0.5 ± 0.2. For each subunit the electron density in the enzyme active-site gorge is well modeled by a C₂₃-chain fatty acid.