ION-BINDING INDUCES CLOSED CONFORMATION IN PSEUDOMONAS 7A GLUTAMINASE-ASPARAGINASE (PGA) - CRYSTAL-STRUCTURE OF THE PGA-SO42--NH4+ COMPLEX AT 1.7 ANGSTROM RESOLUTION

Pseudomonas 7A glutaminase-asparaginase (PGA) catalyzes the hydrolysis of D- and L-isomers of glutamine and asparagine. X-ray quality type-1 crystals of PGA have been obtained from 2.0 M ammonium sulfate. The s pace group is C222(1) with unit-cell dimensions a = 78.62, b = 135.80, and c = 137.88 Angstrom. The tetrameric molecule is located on a crys tallographic 2-fold axis, and two subunits form the asymmetric portion of the unit cell. The structure was solved by the molecular replaceme nt method and refined at 1.7 Angstrom resolution to an R = 19.9% with a good geometry of the model, G = 0.05. The resultant electron density maps enabled us to resolve individual constituent atoms of most resid ues and introduce minor revisions to the amino acid sequence. The cata lytic loop, Thr20-Gly40, is in the closed conformation with excellent electron density in both subunits. A sulfate ion and an ammonium ion a re bound in the substrate binding site and interact with the loop. Thi s interaction appears to be responsible for the observed closed confor mation. New arguments supporting Thr20 as the catalytic nucleophile in the asparaginase activity are proposed.