The X-ray Crystal Structures of Human α-Phosphomannomutase 1 Reveal the Structural Basis of Congenital Disorder of Glycosylation Type 1a*

Congential disorder of glycosylation type 1a (CDG-1a) is a congenital disease characterized by severe defects in nervous system development. It is caused by mutations in α-phosphomannomutase (of which there are two isozymes, α-PMM1 and α-PPM2). Here we report the x-ray crystal structures of human α-PMM1 in the open conformation, with and without the bound substrate, α-d-mannose 1-phosphate. α-PMM1, like most haloalkanoic acid dehalogenase superfamily (HADSF) members, consists of two domains, the cap and core, which open to bind substrate and then close to provide a solvent-exclusive environment for catalysis. The substrate phosphate group is observed at a positively charged site of the cap domain, rather than at the core domain phosphoryl-transfer site defined by the Asp19 nucleophile and Mg2+ cofactor. This suggests that substrate binds first to the cap and then is swept into the active site upon cap closure. The orientation of the acid/base residue Asp21 suggests that α-phosphomannomutase (α-PMM) uses a different method of protecting the aspartylphosphate from hydrolysis than the HADSF member β-phosphoglucomutase. It is hypothesized that the electrostatic repulsion of positive charges at the interface of the cap and core domains stabilizes α-PMM1 in the open conformation and that the negatively charged substrate binds to the cap, thereby facilitating its closure over the core domain. The two isozymes, α-PMM1 and α-PMM2, are shown to have a conserved active-site structure and to display similar kinetic properties. Analysis of the known mutation sites in the context of the structures reveals the genotype-phenotype relationship underlying CDG-1a.