A highly efficient tree structure for the biosynthesis of heparan sulfate accounts for the commonly observed disaccharides and suggests a mechanism for domain synthesis

Abstract

The form of the biosynthetic pathway of the biologically and medically important polysaccharides heparan sulfate (HS) and the closely related heparin remain obscure despite significant progress characterising the biosynthetic machinery. Considering possible biosynthetic schemes using a graph approach and applying known constraints of enzyme order and specificity, a previously unreported system with a highly efficient tree structure emerged with two features: (1) All commonly occurring HS disaccharides could be synthesised through a common route, the major branch. (2) The least common disaccharides also occurred on a separate common branch, termed here the minor branch. This suggested that the relative abundance of these two sets of structures were the result of the specificity of a single enzyme (HS epimerase) acting at an early point in the scheme, to convert GlcA-GlcNS to IdoA-GlcNS in preference to converting GlcA-GlcNAc to IdoA-GlcNAc. A third key finding was that the common substrates for 3-O-sulfation all lie on the same (major) branch. The proposed scheme is consistent with a wide body of experiments comprising both biochemical data and results from HS biosynthetic enzyme knockout experiments in the literature. The major branch also contains a bifurcation, providing a choice of two distinct backbone geometries with the same charge. Further development of this novel biosynthetic scheme, in which frame shifts in the site of action of the enzymes were permitted to occur, while maintaining their order of action, suggested a mechanism by which domains could be generated, or further modification blocked. The relationship between the proposed pathway and the geometric and charge possibilities it allows were also explored.

Heparan Sulfate Proteoglycans Containing a Glypican 5 Core and 2-O-Sulfo-iduronic Acid Function as Sonic Hedgehog Co-receptors to Promote Proliferation

“…Alternatively, cleavage of the glycan chain by a heparanase could give rise to a specialized Shh-binding epitope. It is notable that the nonreducing end of glycans, which appears to be the sites for Shh binding, are remarkably regulated during proteoglycan synthesis (54,55). Although treatment with IDS alters binding and biological responses to wild type Shh, IDS does not alter binding and biological responses of a mutant isoform Shh Ala , providing validation that basic amino acids in the Shh Cardin-Weintraub motif are important for binding to proteoglycan co-receptors (12,23,24).…”

“…Thus, proteoglycan receptors for Shh contain 2-O-sulfated iduronic acid at the nonreducing end of the glycosaminoglycan chains. Although sulfo-iduronic acid residues are unusual at the nonreducing end of uncleaved heparan sulfate chains (53), frame shifts in heparan sulfate biosynthetic enzymes may allow formation of such epitopes (54). Alternatively, cleavage of the glycan chain by a heparanase could give rise to a specialized Shh-binding epitope.…”

Enhanced Tumorigenic Potential of Colorectal Cancer Cells by Extracellular Sulfatases

“…Recently, a scheme for the biosynthesis at the level of disaccharides has been proposed (44), in which two branches, VECTOR SULF1 VECTOR SULF1 VECTOR SULF1 VECTOR SULF1 VECTOR SULF1 VECTOR SULF1 VECTOR SULF2 VECTOR SULF2 VECTOR SULF2 VECTOR SULF2 VECTOR SULF2 VECTOR one comprising disaccharides containing N-acetylated glucosamine and another, containing N-sulfated glucosamine, are suggested. The branch point is the action of the NDST enzyme, and the subsequent epimerase activity is much more efficient in the conversion of GlcA-GlcNS than GlcA-GlcNAc (45), resulting in a branch containing the less common, IdoA/GlucA(AE2S)-GlcNAc (AE6S) disaccharides.…”

Heparan sulfate and heparin interactions with proteins

“…The biosynthetic process and its machinery have been studied extensively [20] and a sequential hierarchical order of events, in which selective enzymatic modifications are required to allow further editing of the polymer, are widely accepted. While this model does provide for some of the possible substitution patterns observed in both heparin and HS, several disaccharides and their respective substitution patterns cannot be explained within the scheme, for example IdoAGlcNAc,6S, if the proposed order of events and their known specificities are respected rigidly [21]. Furthermore, unlike nucleic acids and proteins, HS and heparin biosynthesis is not template driven and the conventional experimental approaches of gene knockout and single enzyme inhibition do not provide conclusive answers, because compensatory mechanisms can be activated [22,23].…”