Conformational response to ligand binding in phosphomannomutase2, insights into inborn glycosylation disorder.

The most common glycosylation disorder is caused by mutations in the gene encoding phosphomannomutase2, producing a disease still without a cure. Phosphomannomutase2, a homodimer where each chain is composed of two domains, require bis-phosphate sugars, mannose or glucose as activators, opening a possible drug-design path for therapeutic purposes. The crystal structure of human phospho-mannomutase2, however, lacks bound substrate and a key active site loop. In order to speed up drug discovery, we present here the first structural model of a bis-phosphate substrate bound to human Phosphomannomutase2. Taking advantage of recent developments in all-atom simulation techniques, in combination with limited and site-directed proteolysis, we demonstrated that alpha-glucose 1,6-bisphosphate can adopt two low energy orientations as required for catalysis. Upon ligand binding, the two domains come close making the protein more compact, in analogy to the enzyme in the crystals from Leismania mexicana. Moreover, proteolysis was also carried out on two common mutants, R141H and F119L. It was an unexpected finding that the mutant most frequently found in patients, R141H, although inactive, does bind alpha-glucose 1,6-bisphosphate and changes conformation.