Chemoenzymatic Approach for the Preparation of Asymmetric Bi-, Tri-, and Tetra-Antennary N-Glycans from a Common Precursor

Abstract

Progress in glycoscience is hampered by a lack of well-defined complex oligosaccharide standards that are needed to fabricate the next generation of microarrays, to develop analytical protocols to determine exact structures of isolated glycans, and to elucidate pathways of glycan biosynthesis. We describe here a chemoenzymatic methodology that makes it possible, for the first time, to prepare any bi-, tri-, and tetra-antennary asymmetric N-glycan from a single precursor. It is based on the chemical synthesis of a tetra-antennary glycan that has N-acetylglucosamine (GlcNAc), N-acetyllactosamine (LacNAc), and unnatural Galα(1,4)-GlcNAc and Manβ(1,4)-GlcNAc appendages. Mammalian glycosyltransferases recognize only the terminal LacNAc moiety as a substrate, and thus this structure can be uniquely extended. Next, the β-GlcNAc terminating antenna can be converted into LacNAc by galactosylation and can then be enzymatically modified into a complex structure. The unnatural α-Gal and β-Man terminating antennae can sequentially be decaged by an appropriate glycosidase to liberate a terminal β-GlcNAc moiety, which can be converted into LacNAc and then elaborated by a panel of glycosyltransferases. Asymmetric bi- and triantennary glycans could be obtained by removal of a terminal β-GlcNAc moiety by treatment with β-N-acetylglucosaminidase and selective extension of the other arms. The power of the methodology is demonstrated by the preparation of an asymmetric tetra-antennary N-glycan found in human breast carcinoma tissue, which represents the most complex N-glycan ever synthesized. Multistage mass spectrometry of the two isomeric triantennary glycans uncovered unique fragment ions that will facilitate identification of exact structures of glycans in biological samples.

Figure 1

Asymmetric bi-, tri, and tetra-antennary N-glycans from common intermediate 1.

Figure 2

(A) Multistage mass spectrometry can distinguish the isomer glycans 8 and 9. CID MS of the triply charged precursor ion at m/z 1082.39. Fragment ions indicative for distinguishing the two isomers are color-coded. (B) CID-MS. The MS fragment ion at m/z 1112.39 was subjected to MS to reveal the structural differences between these two isobaric fragment ions. Fragment ions that can distinguish the two isomers are color-coded.

Scheme 1

Chemical Assembly of Common Intermediate 1

Scheme 2

Synthesis of Asymmetrical Tetra-Antennary Glycan 7 from Precursor 1

Scheme 3

Preparation of Isomeric Tri-Antennary Glycans 8 and 9 from Common Precursor 22

Scheme 4

One-Pot Multiple-Enzyme Approach for the Preparation of Biantennary Glycans from Common Precursor 1