Computer-aided modelling of stereoselective triglyceride hydrolysis catalyzed by Rhizopus oryzae lipase

Lipase from Rhizopus oryzae catalyzes the stereoselective hydrolysis of triglycerides and analogues. Stereopreference and degree of enantiomeric excess of the product varies with the structure of the substrate: trioctanoylglycerol ('ester') and the sn-2 analogues 2-X-1,3-dioctanoylpropandiol, where 2-X = 2-O-octyl ('ether') and 2-hexyl('alkane'), are preferentially hydrolyzed at sn-1, substitution by 2-phenyl ('phenyl') reverses the stereopreference to sn-3. We have modelled the stereoselectivity of Rhizopus oryzae lipase by docking the tetrahedral intermediates of these substrates in two orientations, appropriate to hydrolysis at sn-1 or sn-3, respectively. The initial complexes were further relaxed by molecular dynamics simulations. The favoured orientation of a substrate is characterized by three factors: (1) The substrate fits well into the binding site. The glycerol backbone is relaxed and the sn-2 chain points to a well defined hydrophobic binding site. (2) The oxyanion is stabilized by an extra hydrogen bond from the side chain of Thr 83. (3) The substrate lacks repulsive interactions with protein side chains, especially of Leu 258. Our model is consistent with experimental data and explains qualitatively the ranking of four different substrates with regard to stereoselectivity. It can be used to design lipase mutants with modified stereoselectivity.