X-ray crystallographic studies of the nucleotide binding sites on adenylosuccinate synthetase mutants from Escherichia coli

Abstract

Crystal structures of mutant adenylosuccinate synthetase, in space group P3221, have been refined to R factors of 0.167 (K16Q), 0.150 (R143L) and 0.194 (R303L) against data to 2.5 Å resolution. All the crystal structures presented in this work contain 6-phosphoryl-IMP, GDP, and Mg⁺². The K 16Q and R 143 L mutants have hadacidin bound, whereas R3 03 L has only water molecules in the putative aspartate binding site. As a consequence the 300's loop is disordered in the R3 03 L structure. In the K 16Q structure, NE2 of Gln 16 is in approximately the same position as NZ of Lys 16 of the wild-type enzyme. The void created by the elimination of the guanidinium group in the R143L mutant is filled by water molecules. In all three structures OD1 of Asp 13 coordinates Mg⁺², as well as hydrogen bonds with Ni of 6-phosphoryl-IMP. In addition, distances between Asp 13 and 6-phosphoryl-IMP decrease as distances between His 41 and 6-phosphoryl-IMP increase; the relative separation of Asp 13 and His 41 from 6-phosphoryl-IMP is correlated with k꜀ₐₜ