TRANSITION-STATE ANALOG L-LEUCINEPHOSPHONIC ACID BOUND TO BOVINE LENSLEUCINE AMINOPEPTIDASE - X-RAY STRUCTURE AT 1.65 ANGSTROM RESOLUTION IN A NEW CRYSTAL FORM

The three-dimensional structure of bovine lens leucine aminopeptidase (blLAP) complexed with L-Leucinephosphonic acid (LeuP) has been determ ined by molecular replacement using the structure of native blLAP as a starting model. Cocrystallization of the enzyme with the inhibitor yi elded a new crystal form of space group P321 which has cell dimensions a = 130.4 Angstrom and c = 125.4 Angstrom. Refinement of the model ag ainst data from 7.0 to 1.65 Angstrom resolution resulted in a final st ructure with a crystallographic residual of 0.160 (R(free) = 0.191). T he N-tenminal amino group of LeuP is coordinated to Zn-489, one phosph oryl oxygen atom bridges both metal ions, and another phosphoryl oxyge n atom is coordinated to Zn-488. The side chain of Arg-336 interacts w ith the inhibitor via three water molecules. LeuP resembles the presum ed tetrahedral gem-diolate transition state after direct attack of a w ater or hydroxide ion nucleophile on the scissile peptide bond. On the basis of the LeuP binding mode and the previous structural and bioche mical data, three plausible reaction pathways are evaluated. The two-m etal ion mechanisms discussed herein share as common features a metal- bound hydroxide ion nucleophile and polarization of the carbonyl group by the zinc ions. Possible catalytic roles of Arg-336 and Lys-262 in the direct or indirect (through H2O) protonation of the leaving group, in the stabilization of a zinc-bound OH- nucleophile and in the stabi lization of the negatively charged intermediate, are discussed. A site 3 metal ion approximately 12 Angstrom away from the active site 2 zin c ion probably serves a structural role.