Sulfate: a neglected (but potentially highly relevant) anion

Sulfation pathways in times of change

Sulfation pathways are an essential part of overall sulfur metabolism. Sulfation pathways are mainly about sulfate activation, and the making and breaking of biological sulfate esters. This special issue features some extended reflection on what was presented at the SUPA 2023 meeting on Sulfation Pathways. Novel insights into the synthesis and analytics of sulfate, of sulfated conjugates, and of protein persulfides are presented. Oxysterol sulfates are presented as emerging sulfo-metabolites. Sulfation pathways enzymes are discussed in various disease settings. This special issue also presents insights into polysaccharide sulfotransferases and their functional characterization. Finally, cytoplasmic sulfotransferases are highlighted with regards to their impact on DNA-modification, and in the context of endocrine disruptors. All in all, thought-provoking findings, with the potential to guide and stimulate future research in the field of sulfation pathways and beyond.

Interactions of proteins with heparan sulfate

Heparan sulfate (HS) is a glycosaminoglycan, polysaccharides that are considered to have arisen in the last common unicellular ancestor of multicellular animals. In this light, the large interactome of HS and its myriad functions in relation to the regulation of cell communication are not surprising. The binding of proteins to HS determines their localisation and diffusion, essential for embryonic development and homeostasis. Following the biosynthesis of the initial heparosan polymer, the subsequent modifications comprise an established canonical pathway and a minor pathway. The more frequent former starts with N-deacetylation and N-sulfation of GlcNAc residues, the latter with C-5 epimerisation of a GlcA residue adjacent to a GlcNAc. The binding of proteins to HS is driven by ionic interactions. The multivalent effect arising from the many individual ionic bonds between a single protein and a polysaccharide chain results in a far stronger interaction than would be expected from an ion-exchange process. In many instances, upon binding, both parties undergo substantial conformational change, the resulting hydrogen and van der Waal bonds contributing significant free energy to the binding reaction. Nevertheless, ionic bonds dominate the protein-polysaccharide interaction kinetically. Together with the multivalent effect, this provides an explanation for the observed trapping of HS-binding proteins in extracellular matrix. Importantly, individual ionic bonds have been observed to be dynamic; breaking and reforming, while the protein remains bound to the polysaccharide. These considerations lead to a model for 1D diffusion of proteins in extracellular matrix on HS, involving mechanisms such as sliding, chain switching and rolling.