Molecular decoding of a key step in the maturation process of heparan sulfate

Heparan sulfate belongs to the family of glycosaminoglycans, a group of negatively charged polysaccharides, present in large quantities on cell surfaces and in interstitial tissues. They exert their activities by interacting with a large number of proteins, controlling their mechanism of action and thus intervening in most of the major biological functions (morphogenesis, division, signalling and cell migration, inflammation and immune responses, angiogenesis and tissue repair,… etc.) as well as in their pathological dysfunctions. These polysaccharides comprise various glycan domains, constituting the recognition zones for heparan binding proteins and are therefore essential for "coding" the various biological functions of the molecule. The molecular mechanisms associated with the biogenesis of these domains remain poorly documented.
A collaborative project involving the laboratory Architecture et Fonction des Macromolécules Biologiques, the Institut de Biologie Structurale and the Institut de Chimie Moléculaire et des Matériaux d’Orsay, made it possible to describe the mode of action of a key enzyme in the biogenesis of heparan sulfates, the C5-epimerase, which converts glucuronic acids (GlcA) into iduronic acids (IdoA). This function is essential to the maturation process of heparan sulfate since iduronic acids are systematically present at polysaccharide interaction sites. By combining glycan engineering and chemistry, protein biochemistry and structural biology (X-ray crystallography) approaches, the residues forming the catalytic site were identified as well as the binding modes of the substrate and the product. The mechanism of action of the enzyme involves conformational changes of the polysaccharide associated with selective distortions of the glucuronic entity to be epimerized.
These results provide the molecular and mechanistic basis for new strategies to modify glucuronic/iduronic acid residues at the polymer level and to generate, by chemo-enzymatic synthesis, heparan sulfate analogues for biotechnological or therapeutic applications.

Substrate binding mode and catalytic mechanism of human heparan sulfate D-glucuronyl C5 epimerase. Debarnot C, Monneau Y R, Roig-Zamboni V, Delauzun V, Le Narvor C, Richard E, Hénault J, Goulet A, Fadel F, Vivès R R, Priem B, Bonnaffé D, Lortat-Jacob H, Bourne Y. Proc Natl Acad Sci USA published ahead of print March 14, 2019