To investigate the enantioselectivity of Pseudomonas cepacia lipase, inhibition studies were performed with S C -and R C -(R P ,S P )-1,2-dialkylcarbamoylglycero-3-O-p-nitrophenyl alkylphosphonates of different alkyl chain lengths. P. cepacia lipase was most rapidly inactivated by R C -(R P ,S P )-1,2-dioctylcarbamoylglycero-3-O-p-nitrophenyl octylphosphonate (R C-trioctyl) with an inactivation half-time of 75 min, while that for the S C -(R P ,S P )-1,2-dioctylcarbamoylglycero-3-O-p-nitrophenyl octyl-phosphonate (S C -trioctyl) compound was 530 min. X-ray structures were obtained of P. cepacia lipase after reaction with R Ctrioctyl to 0.29-nm resolution at pH 4 and covalently modified with R C-(RP ,SP)-1,2-dibutylcarbamoylglycero-3-O-p-nitrophenyl butyl-phosphonate (R C -tributyl) to 0.175-nm resolution at pH 8.5. The three-dimensional structures reveal that both triacylglycerol analogues had reacted with the active-site Ser87, forming a covalent complex. The bound phosphorus atom shows the same chirality (S P) in both complexes despite the use of a racemic (R P ,S P ) mixture at the phosphorus atom of the triacylglycerol analogues. In the structure of R C -tributyl-complexed P. cepacia lipase, the diacylglycerol moiety has been lost due to an aging reaction, and only the butyl phosphonate remains visible in the electron density. In the R C-trioctyl complex the complete inhibitor is clearly defined; it adopts a bent tuning fork conformation. Unambiguously, four binding pockets for the triacylglycerol could be detected : an oxyanion hole and three pockets which accommodate the sn-1, sn-2, and sn-3 fatty acid chains. Van der Waals' interactions are the main forces that keep the radyl groups of the triacylglycerol analogue in position and, in addition, a hydrogen bond to the carbonyl oxygen of the sn-2 chain contributes to fixing the position of the inhibitor.Keywords : crystal structure; transition-state analog; enantioselectivity; lipase; stereospecificity.Lipases are lipolytic enzymes, which hydrolyze ester bonds fold, a structural motif common to a wide variety of hydrolases of triacylglycerols. However, their substrate specificity is not [3]. Their active sites contain a catalytic triad, Ser-His-Asp/Glu, limited to triacylglycerols. They may also hydrolyze ester bonds similar to those of serine proteases [4Ϫ6]. Studies with lipases of other compounds such as acetyl-arylpropionic acid esters, covalently complexed with organosulfates [7], organophosphates which are precursors for the nonsteroidal anti-inflammatory [8, 9], or organophosphonates [10Ϫ12] demonstrated that, in the agents naproxen and ibuprofen [1]. Because of this broad sub-presence of lipid-like compounds or organic solvents, their strate specificity, and because of their distinct stereopreferences, active-site regions may undergo drastic conformational changes, lipases have found widespread application in the enantioselec-exposing the catalytic residues and the surrounding hydrophobic tive synthesis of organic compounds, and in the resolution of ...