In Silico Three Dimensional Modelling of Mutant Glycogen Synthase Associated with Equine Type 1 Polysaccharide Storage Myopathy

Abstract

IntroductionEquine Type 1 polysaccharide storage myopathy (PSSM1) is associated with a dominant, missense mutation in the equine glycogen synthase gene (GYS1) that increases the enzyme's activity. The amino acid sequence surrounding the PSSM1 mutation is highly conserved amongst species from yeast to horses suggesting that this region plays an important role, but its functional significance has yet to be determined.MethodsComputer modelling can predict three dimensional (3D) protein structures using related proteins as templates. Yeast glycogen synthase (GS) has high sequence homology (73%) to the equine enzyme and since the yeast enzyme's structure has been solved, we hypothesised that in silico modelling based on the yeast protein using MODELLER and SwissPDBViewer software would predict the enzyme's structure and explain the mutant enzyme's increased activity.ResultsHomology‐modelling revealed that, like the yeast enzyme, equine GS likely forms a tetramer surrounding a catalytic cleft and undergoes large structural rearrangements upon G6P binding to highly‐conserved binding sites. The enzyme's glycogen binding sites are located on the extremities of the enzyme and are also conserved in the equine protein. The PSSM1 mutation lay close to the G6P binding site, but did not confer any clear structural rearrangement on the protein's 3D configuration. The mutation was distant from the known phosphorylation sites and the glycogen binding sites.ConclusionsThe close proximity of the mutation to the G6P binding site suggests that it might influence substrate interaction or allosteric regulation. Further studies evaluating G6P ligand binding and enzyme activation are warranted and might best be made following purification of the mutant and wild type proteins.Ethical Animal ResearchNo animals were used in the study. Sources of funding: Petplan Charitable Trust and The Royal Veterinary College. Competing interests: none.