X-RAY STRUCTURE OF LEUCINE AMINOPEPTIDASE AT A RESOLUTION OF 4 ANGSTROM

X-ray structural analysis was carried out using crystals of bovine len s leucine aminopeptidase by isomorphic replacement and solvent flatten ing. Two heavy atom derivatives were obtained by soaking crystals of t he enzyme in ethylmercuric chloride solutions, for which the enzyme ha s four binding sites, and in phenylmercuric acetate solutions, for whi ch the enzyme monomer has only one binding site. The electron density map showed that the protein hexamer, with 32-fold symmetry, has the sh ape of a triangular barrel 88 angstrom high and with a maximum width o f 118 angstrom in the equatorial plane. Each subunit has an extended e llipsoid shape with a length of 92 angstrom. The contacts between subu nits were identified. Analysis of the electron density map showed that 36% of the residues are in the alpha-helical conformation, and each s ubunit consists of two distinct globular domains. The positions of zin c atoms were identified, as was the active center binding site of a co mpetitive inhibitor of the enzyme.