Opposite effect of polymorphic mutations on the electrostatic aggregation of human alanine:glyoxylate aminotransferase: implications for the pathogenesis of Primary Hyperoxaluria Type I

Protein aggregates formation is the basis of several misfolding diseases, including those displaying loss-of-function pathogenesis. Although aggregation is often attributed to the population of intermediates exposing hydrophobic surfaces, the contribution of electrostatic forces has recently gained attention. Here we combined computational and in vitro studies to investigate the aggregation process of human peroxisomal alanine:glyoxylate aminotransferase (AGT), a pyridoxal 5'-phosphate (PLP)-dependent enzyme involved in glyoxylate detoxification. We demonstrated that AGT is susceptible to electrostatic aggregation due to its peculiar surface charge anisotropy, and that PLP binding counteracts the self-association process. The two polymorphic mutations P11L and I340M exert opposite effects. The P11L substitution enhances the aggregation tendency, by probably increasing surface charge anisotropy, while the I340M plays a stabilizing role. In light of these results, we examined the effects of the most common missense mutations leading to Primary Hyperoxaluria Type I (PH1), a rare genetic disorder associated with abnormal calcium oxalate precipitation in the urinary tract. All of them endow AGT with a strong electrostatic aggregation propensity. Moreover, we predicted that pathogenic mutations of surface residues could alter charge distribution, thus inducing aggregation under physiological conditions. A global model describing the AGT aggregation process is provided. Overall, the results indicate that the contribution of electrostatic interactions in determining the fate of proteins as well as the effect of amino acid substitutions should not be underestimated, and provide the basis for the development of new therapeutic strategies for PH1 aimed at increasing AGT stability. This article is protected by copyright. All rights reserved.