What is the Sugar Code?

Abstract

A code is defined by the nature of the symbols, which are used to generate information-storing combinations (e. g. oligo- and polymers). Like nucleic acids and proteins, oligo- and polysaccharides are ubiquitous, and they are a biochemical platform for establishing molecular messages. Of note, the letters of the sugar code system (third alphabet of life) excel in coding capacity by making an unsurpassed versatility for isomer (code word) formation possible by variability in anomery and linkage position of the glycosidic bond, ring size and branching. The enzymatic machinery for glycan biosynthesis (writers) realizes this enormous potential for building a large vocabulary. It includes possibilities for dynamic editing/erasing as known from nucleic acids and proteins. Matching the glycome diversity, a large panel of sugar receptors (lectins) has developed based on more than a dozen folds. Lectins 'read' the glycan-encoded information. Hydrogen/coordination bonding and ionic pairing together with stacking and C-H/π-interactions as well as modes of spatial glycan presentation underlie the selectivity and specificity of glycan-lectin recognition. Modular design of lectins together with glycan display and the nature of the cognate glycoconjugate account for the large number of post-binding events. They give an entry to the glycan vocabulary its functional, often context-dependent meaning(s), hereby building the dictionary of the sugar code.

Keywords: adhesion; glycoproteins; glycosylation; lectins; proliferation.

Figure 1

Illustration of the four routes to transfer the Fuc moiety from its GDP‐Fuc donor (GDP given as X) to the 2, 3, 4, or 6 position of glycan acceptors by mammalian fucosyltransferases (top panel), examples of resulting oligosaccharides in N‐ and O‐glycans that define the R’ position are presented in the bottom panel. This part shows glycans with α1,2‐fucosylation (histo‐blood group ABH(0) epitopes), with α1,3‐fucosylation ((sialyl) Le^x), with α1,4‐fucosylation (Le^a) and with α1,2/4‐fucosylations (Le^y) as well as the N‐glycan stem with α1,6‐fucosylation termed core fucosylation (examples for lectins that bind the respective structure are named.

Figure 2

Overview on DiLacNAc synthesis (for details, please see Supporting Information, Scheme S1) and the calorimetric titration profiles of its interaction with human galectin‐3 in H₂O (bottom, left) and D₂O (bottom, right). For details, please see Ref. [63e].

Figure 3

Illustrations of the syntheses of a triiodobenzene‐based trivalent glycocluster (top) and of a 16mer starburst glycodendrimer (bottom). For further information on the syntheses, please see Supporting Information, Scheme S2 and the original reports with details on results of lectin assays.

Figure 4

Lectin histochemical localization of glycans in sections through retina and anterior segments of fixed adult chicken eyes. Detection of the mucin‐type O‐glycan core 1 disaccharide (TF antigen) in retina's photoreceptor layer (inset: inhibition control with cognate sugar) (A), of LacNAc oligomers in connective tissue and epithelial cells of the ciliary body (B), of α2,3 (C)‐ or α2,6 (D)‐sialylated N‐glycans in immune cell aggregates of Haderian gland, of β1,6‐branched N‐glycans between lens fibers (E) and of β‐galactosides (bound by the labeled chicken galectin CG‐1B) in corneal epithelium (inset: inhibition control with cognate sugar) (F). Scale bars: 20 μm (Reproduced with permissions from Ref. [97b] Copyright 2017 John Wiley and Sons and from Ref. [97c] Copyright 2018 Elsevier; for technical details and information on the lectins used as tools, please see Ref. [97]).

Figure 5

Illustrations of galectin binders from the class of natural β‐galactosides and naming of examples of mammalian galectins with preference (galectins in parentheses bind with lesser affinity) for a glycan, for example galectin‐8 (Gal‐8) for 3’‐sulfated LacNAc and the hexasaccharide of ganglioside GD1a or galectins‐1, ‐2, ‐3 and ‐7 (Gal‐1, ‐2, ‐3 and ‐7) for the pentasaccharide of ganglioside GM1 (please see Figure 11 for examples of bioactivity of GM1 binding by these adhesion/growth‐regulatory galectins).

Figure 6

Illustrations of the contact pattern of 3’‐sulfated Lac with the N‐terminal CRD of human Gal‐8 (PDB 3AP6) or Gal‐4 (PDB 5DUW) (A, B), of the synthesis of a bioactive derivative of the sulfatide headgroup (for details, please see Supporting Information, Scheme 3) (C) and crystal/modeled structures of its binding profile with the two CRDs, the water‐mediated contact to sphingosine's hydroxyl group highlighted by arrows (D, E). For details, please see Ref. [60].

Figure 7

Overview of the synthesis of the trifluorinated N‐glycan core trimannoside (for further information, please see Supporting Information, Scheme S4) (A), the crystallographic information on trimannoside binding in two modes (PDB 1RIN) (B) and NMR‐spectroscopical information on binding of the trifluoro‐trimannoside (2F‐Man3) by Pisum sativum (pea) agglutinin (C); from left to right: 1D ¹H of Man3, 1D ¹H 2F‐Man3; 2D ¹H,¹⁹F TOCSYreF correlation spectrum; 2D ¹H,¹⁹F STD TOCSYreF spectra (strips) of 2F‐Man3 in the bound state revealing the 2 : 1 ratio of its two modes of docking via a terminal residue, i. e. the α1,3‐ or the α1,6‐linked Man moiety, respectively (for details, please see Ref. [67d]).

Figure 8

Illustration of routes within mucin‐type core 1/2 O‐glycan biosynthesis. The functional meaning of these words of the glycan vocabulary is indicated by naming of examples for mammalian lectins that bind respective glycans.

Figure 9

Illustration of examples of lectin design starting with a single CRD that can have a short or long tail (the latter for self‐association). In clockwise manner, lectins with modules for covalent subunit association (via disulfide bonds), for non‐covalent and linker‐mediated modes of CRD associations and for building a puzzle‐like architecture with intracellular domains for outside‐in signaling are displayed. Abbreviations are given to define distinct lectins for each type of shown architecture (Reproduced with permission from Ref. [98c] Copyright 2015 Elsevier; for further information, please see Refs. [77b,g–k, 82f, 83c,e, 86i, 98]).

Figure 10

Illustration of the route of galectin‐driven osteoarthritis pathogenesis by upregulation of pro‐degradative/‐inflammatory effectors such as interleukins (IL) and matrix metalloproteinases (MMPs) that starts with dysregulated galectin expression. Their secretion, cell surface binding and the triggered outside‐in signaling to reprogram IL/MMP gene expression via a downstream effector, i. e. the transcription factor NK‐κB, lead to matrix degradation in vitro and in vivo (for details, please see Ref. [99]).

Figure 11

Effect of wild‐type and of engineered human galectins on neuroblastoma cell (SK−N−MC) growth. Galectin architecture, microphotographs of representative cultures and a bar graph of cell numbers are shown. Galectins are tested at 100 μg/mL (* 10 μg/mL), wild‐type Gal‐3 and its Gal‐3NT/1 variant are used in 10fold excess in the mixtures with Gal‐1 (for details on proteins, impact of architecture on lattice formation by testing synthetic glycoclusters and assay conditions, please see Refs. [95c,d, 100]).