N-glycolylated carbohydrates in nature

Abstract

N-glycolylated carbohydrates are amino sugars with an N-glycolyl amide group. These glycans have not been well studied due to their surprising rarity in nature in comparison with N-acetylated carbohydrates. Recently, however, there has been increasing interest in N-glycolylated sugars because the non-human sialic acid N-glycolylneuraminic acid (Neu5Gc), apparently the only source of all N-glycolylated sugars in deuterostomes, appears to be involved in xenosialitis (inflammation associated with consumption of Neu5Gc-rich red meats). Xenosialitis has been implicated in cancers as well as other diseases including atherosclerosis. Furthermore, metabolites of Neu5Gc have been shown to be incorporated into glycosaminoglycans (GAGs), resulting in N-glycolylated GAGs. These N-glycolylated GAGs have important potential applications, such as dating the loss of the Neu5Gc-generating CMAH gene in humans and being explored as a xenosialitis biomarker and/or estimate of the body burden of diet-derived Neu5Gc, to understand the risks associated with the consumption of red meats. This review explores N-glycolylated carbohydrates, how they are metabolized to N-glycolylglucosamine and N-glycolylgalactosamine, and how these metabolites can be incorporated into N-glycolylated GAGs in human tissues. We also discuss other sources of N-glycolylated sugars, such as recombinant production from microorganisms using metabolic engineering as well as chemical synthesis.

Keywords: N-glycolyl; chondroitin; glycobiology; neuraminic acid; xenosialitis.

Fig. 1

Possible pathways for the uptake of Neu5Gc and its incorporation into glycoconjugates in mammalian cells. Free and conjugated sialic acid enter the cell by macropinocytosis. In the lysosome, a sialidase cleaves Neu5Gc from its glycan and exports it out of the lysosome via sialin. Once in the cytoplasm, Neu5Gc can break down into its catabolites or enter the nucleus for activation into CMP-Neu5Gc. In the Golgi, Neu5Gc can be incorporated into newly synthesized glycoconjugates or incorporate its metabolites from the cytoplasm into glycoconjugates. Created with BioRender.com.

Fig. 2

The Neu5Gc degradative pathway. Neu5Gc, when acted on by N-acetylneuraminate lyase (1), forms ManNGc, which in turn gets epimerized to GlcNGc UDP-N-acetylglucosamine 2-epimerase (2). GlcNGc gets phosphorylated at the 6-position by N-acetylglucosamine kinase (3) yielding GlcNGc-6P. Phosphoacetylglucosamine mutase (4) acts on GlcNGc-6P to yield GlcNGc-1P. Alternatively, GlcNGc-6P can be converted to GlcNH₂–6-P and glycolate. Once GlcNGc-1P reacts with UDP-N-acetylglucosamine diphosphorylase (5) to form UDP-GlcNGc, UDP-GlcNGc will also be epimerized by UDP-N-acetylglucosamine 4-epimerase (6) to yield UDP-GalNGc. The final products UDP-GalNGc and UDP-GlcNGc get incorporated into glycosaminoglycans and glycoproteins, but incorporation most commonly occurs into CS for unknown reasons.

Fig. 3

Once incorporated into the human tissue, sources of Neu5Gc, such as red meats, are recognized as xeno-autoantigens. The body responds by producing anti-Neu5Gc antibodies (xeno-autoantibodies). The antibody–antigen interaction leads to a chronic inflammation (xenosialitis) in the tissues, contributing to diseases such as carcinomas, and atherosclerosis. Figure reproduced with permission from (Dhar et al. 2019).

Fig. 4

A timeline of publications with the keyword “N-acetyl” and “N-glycolyl”.

Fig. 5

UDP-GalNGc, when incorporated into CS and DS forms Gc-CS and Gc-DS, respectively, and UDP-GlcNGc when incorporated into HS forms Gc-HS. Representative structure for each GAG is shown.