Structure and synthesis of polyisoprenoids used in N-glycosylation across the three domains of life

Abstract

N-linked protein glycosylation was originally thought to be specific to eukaryotes, but evidence of this post-translational modification has now been discovered across all domains of life: Eucarya, Bacteria, and Archaea. In all cases, the glycans are first assembled in a step-wise manner on a polyisoprenoid carrier lipid. At some stage of lipid-linked oligosaccharide synthesis, the glycan is flipped across a membrane. Subsequently, the completed glycan is transferred to specific asparagine residues on the protein of interest. Interestingly, though the N-glycosylation pathway seems to be conserved, the biosynthetic pathways of the polyisoprenoid carriers, the specific structures of the carriers, and the glycan residues added to the carriers vary widely. In this review we will elucidate how organisms in each basic domain of life synthesize the polyisoprenoids that they utilize for N-linked glycosylation and briefly discuss the subsequent modifications of the lipid to generate a lipid-linked oligosaccharide.

Fig 1

Schematic overview polyisoprenoid alcohol formation and utilization in N-glycosylation. The MVA and DOXP pathways provide the isoprenoid precursors DMAPP and IPP, which are used to synthesize dolichol and polyprenol. Dolichol and polyprenol and modified by sugar additions into a completed LLO, then the oligosaccharide is transferred off of the carrier lipid and onto asparagine residues on the protein.

Fig 2

The MVA biosynthetic pathway for IPP and DMAPP. Acetyl-CoA is used to synthesize IPP and DMAPP with MVA as an intermediate. In the modified archaeal pathway, MVA-5-phosphate is first decarboxylated then phosphorylated to form IPP, the opposite of what occurs in the traditional MVA pathway. *Enzyme is theoretical and has yet to be discovered. For a review of the MVA pathway with chemical structures see [108].

Fig 3

The DOXP biosynthetic pathway for IPP and DMAPP. Glyceraldehyde and pyruvate are used to synthesize IPP and DMAPP with DOXP as an intermediate. For a review of the DOXP pathway with chemical structures see [108].

Fig 4

Polyprenol is reduced via polyprenol reductase to form dolichol. The structure of polyprenol and dolichol differ based on the saturation of the α-isoprene unit. The three isoprene units originating from farnesyl pyrophosphate (FPP) at the ω end of the molecule are in the trans configuration, while the remaining isoprene units are in the cis configuration.

Fig 5

The biosynthetis of Poly-PP from IPP and DMAPP through the intermediates GPP and FPP. n = number of isoprene units.

Fig 6

The sugar structures of the final lipid-linked oligosaccharide in eukaryotes (A.) and bacteria (B.)

Fig 7

Dolichol synthesis in Coluria geoides hairy root. Cytosolic IPP from the MVA pathway can enter the plastid. Dolichol synthesis begins in the plastid where a Poly-PP with less than 14 isoprene units is formed using IPP from both the MVA and DOXP pathways. The Poly-PP is then transported to the cytosol, where synthesis continues using only IPP from the MVA pathway.