[X-ray structural study of leucine aminopeptidase with a resolution of 4 Angstroms]

An X-ray crystallographic structure determination has been carried out on bovine lens leucine aminopeptidase at 4.0 A resolution by using a combination of isomorphous replacement and solvent flattening. The two heavy atom derivatives used were obtained by soaking crystals in ethyl mercury chloride, which bound at four sites, and phenyl mercury acetate, which bound at one site in the monomer. The electron density map reveals that the enzyme hexameric oligomer, arranged in 32 symmetry, has a triangular barrel appearance and dimensions, of height 88 A and maximal width 118 A in barrel equatorial plane. Each subunit in an elongated ellipsoid of approximate length 92 A. Subunits contacts have been described. From an analysis of the map each subunit appears to contain some 36% alpha-helix and is organized into two distinct globular domains. Direct location of zinc cluster and competitive inhibitor binding site are presented.