Structurally characterized enzymes in this subgroup have the C1 or C0 (only small inserts seen at either of the two points of cap insertion) cap type. Characterized functions included in this subgroup include 2-haloacid dehalogenase, beta-phosphoglucomutase, phosphonoacetaldehyde hydrolase, and phosphatases of various specificities.
Lahiri, S.D., et al.
Caught in the act: the structure of phosphorylated beta-phosphoglucomutase from Lactococcus lactis
▸ Abstract
Phosphoglucomutases catalyze the interconversion of D-glucose 1-phosphate and D-glucose 6-phosphate, a reaction central to energy metabolism in all cells and to the synthesis of cell wall polysaccharides in bacterial cells. Two classes of phosphoglucomutases (alpha-PGM and beta-PGM) are distinguished on the basis of their specificity for alpha- and beta-glucose-1-phosphate. beta-PGM is a member of the haloacid dehalogenase (HAD) superfamily, which includes the sarcoplasmic Ca(2+)-ATPase, phosphomannomutase, and phosphoserine phosphatase. beta-PGM is unusual among family members in that the common phosphoenzyme intermediate exists as a stable ground-state complex in this enzyme. Herein we report, for the first time, the three-dimensional structure of a beta-PGM and the first view of the true phosphoenzyme intermediate in the HAD superfamily. The crystal structure of the Mg(II) complex of phosphorylated beta-phosphoglucomutase (beta-PGM) from Lactococcus lactis has been determined to 2.3 A resolution by multiwavelength anomalous diffraction (MAD) phasing on selenomethionine, and refined to an R(cryst) = 0.24 and R(free) = 0.28. The active site of beta-PGM is located between the core and the cap domain and is freely solvent accessible. The residues within a 6 A radius of the phosphorylated Asp8 include Asp10, Thr16, Ser114, Lys145, Glu169, and Asp170. The cofactor Mg(2+) is liganded with octahedral coordination geometry by the carboxylate side chains of Asp8, Glu169, Asp170, and the backbone carbonyl oxygen of Asp10 along with one oxygen from the Asp8-phosphoryl group and one water ligand. The phosphate group of the phosphoaspartyl residue, Asp8, interacts with the side chains of Ser114 and Lys145. The absence of a base residue near the aspartyl phosphate group accounts for the persistence of the phosphorylated enzyme under physiological conditions. Substrate docking shows that glucose-6-P can bind to the active site of phosphorylated beta-PGM in such a way as to position the C(1)OH near the phosphoryl group of the phosphorylated Asp8 and the C(6) phosphoryl group near the carboxylate group of Asp10. This result suggests a novel two-base mechanism for phosphoryl group transfer in a phosphorylated sugar.
