Synthesis of N-Glycolylneuraminic Acid (Neu5Gc) and Its Glycosides

Abstract

Sialic acids constitute a family of negatively charged structurally diverse monosaccharides that are commonly presented on the termini of glycans in higher animals and some microorganisms. In addition to N-acetylneuraminic acid (Neu5Ac), N-glycolyl neuraminic acid (Neu5Gc) is among the most common sialic acid forms in nature. Nevertheless, unlike most animals, human cells loss the ability to synthesize Neu5Gc although Neu5Gc-containing glycoconjugates have been found on human cancer cells and in various human tissues due to dietary incorporation of Neu5Gc. Some pathogenic bacteria also produce Neu5Ac and the corresponding glycoconjugates but Neu5Gc-producing bacteria have yet to be found. In addition to Neu5Gc, more than 20 Neu5Gc derivatives have been found in non-human vertebrates. To explore the biological roles of Neu5Gc and its naturally occurring derivatives as well as the corresponding glycans and glycoconjugates, various chemical and enzymatic synthetic methods have been developed to obtain a vast array of glycosides containing Neu5Gc and/or its derivatives. Here we provide an overview on various synthetic methods that have been developed. Among these, the application of highly efficient one-pot multienzyme (OPME) sialylation systems in synthesizing compounds containing Neu5Gc and derivatives has been proven as a powerful strategy.

Keywords: Neu5Gc; chemical synthesis; chemoenzymatic synthesis; sialic acid; sialoside.

Graphical Abstract

Chemical and chemoenzymatic synthetic methods for Neu5Gc and Neu5Gc-sialosides are reviewed. One-pot multienzyme (OPME) chemoenzymatic strategy has advantages in accessing a large number of Neu5Gc-sialosides and their derivatives.

Figure 1

(A) Three basic forms of sialic acids including N-acetylneuraminic acid (Neu5Ac), N-glycolylneuraminic acid (Neu5Gc), and 3-deoxy-D-glycero-D-galacto-2-nonulosonic acid (Kdn); (B) Biosynthesis of Neu5Gc and its sialosides in eukaryotic cells. Enzymes and abbreviations: GNE, UDP-GlcNAc 2-epimerase/ManNAc-6-kinase; NANS, Neu5Ac-9-P synthetase; NANP, Neu5Ac-9-P phosphatase; CSS, CMP-sialic acid synthetase; CMAH, cytidine 5′-monophosphate-N-acetylneuraminic acid hydroxylase.

Figure 2

Naturally occurring Neu5Gc derivatives that have been identified.

Figure 3

(A) A general chemoenzymatic synthetic strategy of sialic acid aldolase (EcNanA or PmNanA)-catalyzed synthesis of Neu5Gc and derivatives containing modifications at C5, C7, and/or C9 from ManNGc and derivatives, (B) PmNanA-catalyzed synthesis of Neu5Gc8Me, (C) EcNanA-catalyzed synthesis of 3-fluoro-Neu5Gc, and (D) EcNanA-catalyzed synthesis of disaccharides containing Neu5Gc at the reducing end.

Figure 4

Synthesis of Neu5Gc-glycosides by de-N-acetylation of Neu5Ac-glycoside derivatives without (A) or with (B) a microwave-assisted process and (C) by glycosylation of protected Neu5Gc formed enzymatically from ManNGc and pyruvate.

Figure 5

Efficient synthesis of Neu5Gc-glycosides using phosphite donors in good yields and α-selectivity.

Figure 6

An example of one-pot chemical synthesis of Neu5Gc-containing trisaccharides using 5-N-phthaloyl group protected N-phenyltrifluoroacetimidate sialyl donor.

Figure 7

Chemoenzymatic synthesis of Neu5Gcα2–3Galβ1–3GalNAc.

Figure 8

Synthesis of sialosides containing Neu5Gc or derivative using one-pot multienzyme (OPME) sialylation systems containing (A) three enzymes including sialic acid aldolase (NanA), CMP-sialic acid synthetase (CSS), and sialyltransferase (SiaT) or (B) two enzymes including CSS and SiaT.

Figure 9

Synthesis of biotinylated human serum albumin-sialoglycoside conjugates containing Neu5Gc or Neu5Gc9Ac including (A) sTn epitopes, (B) sialyl lactoside, and (C) chemoenzymatic synthesis of dabsyl fluorophore-tagged glycopeptides including sTn, T, and ST-antigens containing Neu5Gc. Adopted and modified from Yu et al. (148) and Malekan et al. (162) with permission.