Protonation States of Proton-Sensing Glutamate Residues in Sialin Transport

Sialic acids are a diverse class of widely distributed monosaccharides that are engaged in a wide range of biological processes. Sialin, a sialic acid/proton symporter, transports sialic acid across membranes between the lysosomal lumen and cytosol, playing a critical role in sialin metabolism. Taking advantage of recently published experimental structures of sialin, we report here the first computational study that probes the molecular mechanism of ligand transport through sialin, which is yet to be fully understood. In particular, we carry out steered molecular dynamics simulations of the transport of N-acetylneuraminic acid, the most widely spread natural derivative of sialic acids, through sialin with two key glutamate residues (E171 and E175) in various protonation states. The previously proposed model is refined with enriched atomistic details from this study for the cotransport of sialic acid and proton. With additional quantum calculations, our data suggest a possible explanation for why mutation R168A retains most of the transport activities, but R168K does not.