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Crystal Structure of the Complex of Pseudomonas Cepacia Lipase with Transition State Analog

Lipases have found widespread applications in the enantioselective synthesis of organic compounds because of their substrate specificity and disinct stereopreferences. Study of kinetic resolutions of racemic sec alcohols in n-hexane with a series of twenty commercialy available microbial lipases revealed that Pseudomonas cepacia lipase (PCL) is the most efficient in resolving racemic 1-phenoxy-2-butanol (1). In order to understand better enantioselectivity of PCL, (RP, SP)-methyl-O-((2'R)-1-phenoxybut-2'-yl)phosphonic acid chloride was synthetised for complexation with protein. The lipase-transition state analog complex was prepared by adding freshly prepared (RP, SP)-methyl-O-((2'R)-1-phenoxybut-2'-yl)phosphonic acid chloride dissolved in acetonitrile to the protein solution (20 mg/ml in 6.3 mM Na phsophate, mM piperazine, pH=6) in a molar excess of 60:1. Crystals grew within a few days at 50C using hanging drop vapour-diffusion technique. The crystal grown from 42% 2-methyl-2, 4-pentanediol, 0.1 M sodium citrate, 0.1 M Hepes, pH=7.4 was used for data collection. X-ray diffraction data to 2.3 Å were collected using MAR Research 345 mm diameter imaging plate detector system mounted on a Rigaku rotating-anode X-ray generator operated at 50 kV and 100 mA at 100 K. The crystal structure of the complex is isomorphous with the structure of the native protein ; space group C2, cell dimensions a = 89.06, b = 46.63, c = 84.30 Å , β = 120.86° and one molecule in the asymmetric unit. The structure was refined by restrained refinement methods to the R-factor of 17 %. The three-dimensional structure revealed that analog had reacted with the active-site Ser87, forming a covalent bond. The bound phosphorus atom shows S chirality. The arrangement of the catalytic residues Ser87, His286 and Asp264 is similar to that of native PC lipase. One of the phosphonyl oxygen atoms occupies the oxyanion hole, forming hydrogen bonds to the main-chain nitrogen atoms of Gln88 and Leu17.

Pseudomonas cepacia lipase; racemic sec alcohols; transition state (TS) analogue; crystal structure

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