Carbohydrate Structure

What are the different classes of carbohydrates in nature?

● Carbohydrates, also called glycans, can be divided into three major groups: (1) simple sugars consist of monosaccharides, disaccharides, and oligosaccharides; (2) polysaccharides consist of glucose homopolymers or disaccharide heteropolymers in which one of the two sugars is a hexosamine; and (3) glycoconjugates of proteins or lipids with covalently attached glycans.

● The most abundant carbohydrate on Earth is cellulose, a homopolymeric molecule that accounts for the majority of plant cell walls. It consists of up to 1000 repeating glucose disaccharides called cellobiose, which are linked by β(1–>4) glycosidic bonds. Most organisms cannot metabolize cellobiose because they lack the enzyme cellulase, a β-1,4 glycosidase.

● Chitin is a linear hexosamine polysaccharide consisting of repeating GlcNAc units linked by β(1–>4) glycosidic bonds. Chitin provides many types of insects and crustaceans with an excellent biomaterial for building a strong exoskeleton.

● Glycosaminoglycans are another type of linear hexosamine polysaccharide. Unlike chitin, however, glycosaminoglycans are covalently attached to proteins to form a class of glycoconjugates called proteoglycans.

● Starch and glycogen are glucose homopolymers used as quick sources of metabolic energy in plants and animals, respectively. Starch is generated during daylight hours by plants, using energy from photosynthesis, whereas glycogen stores in animals are synthesized from carbohydrates and proteins in the diet.

● The presence of α(1–>6) glycosidic bonds in amylopectin and glycogen creates branch points that greatly increase the number of nonreducing ends. Because glucose units can only be metabolized from the nonreducing ends of polysaccharides, a higher number of α(1–>6) glycosidic bonds results in more efficient glucose retrieval. Glycogen contains about three times as many α(1–>6) glycosidic bonds per 100 glucose residues than amylopectin.

What are the structures and functions of glycoconjugates?

● Glycan modification of proteins takes place within the lumen of the endoplasmic reticulum compartment of the cell, whereas glycolipids are primarily generated in the Golgi apparatus.

● Proteoglycans are protein glycoconjugates that consist mostly of carbohydrate with only a small protein component. Peptidoglycans are proteoglycans that are found in bacterial cell walls and contain peptide linkers of D and L amino acids between adjacent polysaccharide strands.

● Lectins are glycan binding proteins that mediate two classes of glycoconjugate binding interactions: (1) intrinsic glycoconjugate binding between glycans and lectins on human cells, and (2) extrinsic glycoconjugate binding between glycans and lectins on human cells and pathogen cells.

● Glycan attachments to glycoproteins occur through either the amide nitrogen atom of asparagine, leading to the generation of N-linked oligosaccharides, or the oxygen atom of serine or threonine residues, resulting in O-linked oligosaccharides.

● The most common N-glycosidic bond in glycoproteins is between asparagine and GlcNAc, whereas the most common monosaccharide used to create the O-glycosidic bond is GalNAc.

● Glycosyltransferases use nucleotide sugars as the carbohydrate donor and sequentially add sugar residues to extend the glycan group. A large number of glycosyltransferases are required to build the variety of glycan structures found in any one organism.

● Bacterial cell walls are made of peptidoglycans, which are proteoglycans consisting of multiple strands of hexosamine polysaccharide chains. The chains consist of repeating units of a β(1–>4)-linked MurNAc-GlcNAc disaccharide, which are tethered together by linkages between short oligopeptides.

What are some of the biochemical methods used in glycobiology?

● High-performance liquid chromatography (HPLC), in combination with glycosylase cleavage, can provide information about the arrangement of sugars in a highly purified glycan fraction using standard HPLC equipment. In contrast, mass spectrometry, which requires mass analyzers, identifies common glycans present in a mixed glycan sample on the basis of predicted mass-to-charge ratios.

● The fluorescent dye 2-aminobenzamide (2-AB) is covalently attached to glycan groups prior to analysis of glycan structures by HPLC. The glycans labeled with 2-AB are subjected to stepwise treatment with sugar-specific glycosylases to yield related glycan structures that can be separated and identified by HPLC.

● Mass spectrometry is used either as the primary analytical method for glycan characterization or as a complementary technique to augment elution data derived from HPLC. By comparing the observed masses obtained from mass spectrometry to predicted masses of common N-linked glycan groups, specific peaks in the spectrum can be assigned to glycan groups.

● Lectin and glycan arrays provide platforms to screen large numbers of samples for specific binding interactions using fluorescently labeled molecules. Two basic types of arrays have been developed for screening purposes: (1) protein arrays, which contain covalently attached lectin proteins or antibodies; and (2) glycan arrays, which contain covalently attached glycoproteins with intact glycan groups or chemically synthesized glycan groups.