7: Sugars & Glycobiology

What are the two families of monosaccharides?

Aldoses and Ketoses

Describe the basic structure of a common monosaccharide backbone. Differentiate aldose vs ketose.

Each consecutive connecting carbon within the backbone contains a hydroxyl group. There is also a carbonyl group present as well. An aldose (ald-ehyde) has the carbonyl at the end (top) of the Fischer projection. A ketose (ket-one) has the carbonyl sandwiched between backbone carbons.

Describe key characteristics of a monosaccharide. (solubility, functional groups, number of carbons)

Highly soluble. Polyhydroxyl aldehydes/ketones 3-7 carbons

How do you differentiate a D vs L oriented sugar?

Using a Fischer projection, select the carbon furthest from the carbonyl to be the reference carbon. Then, observe the position of that carbon's hydroxyl relative to the backbone. L: if the -OH of the reference carbon is to the left. D: if the -OH of the reference carbon is to the right.

What are epimers?

Epimers are two sugars that differ only in configuration about one carbon.

Note: they may have differing names, as the difference in configuration may not be on the reference carbon. Therefore, it is not reliable to delineate epimers from D/L distinctions.

How do you differentiate between an α and β oriented sugar?

Relative to the CH2-OH group, α: the C1 -OH group is OPPOSITE SIDE β: the C1 -OH group is SAME SIDE

What are anomers?

Isomeric monosaccharides varying only in configuration about hemiacetal or hemiketal carbon. Makes more sense if you draw a Fischer projection and Haworth projection for a sugar.

Anomers are sugars that vary only in α/β configuration.

What is a pyranose? What is a furanose? Are they planar?

Pyranose: 6-membered sugar ring Furanose: 5-membered sugar ring.

Neither of these are planar.

What are a few classes of hexose derivatives?

Amine-substituted sugars Deoxy-sugars Acidic-sugars Sugar-phosphates

What is a reducing sugar?

A sugar able to have its aldehyde undergo redox with a Cu2+ ion, under basic conditions. The sugar is oxidized into a carboxylic acid. Reduces the Cu2+ to Cu+

What is the key bond within a disaccharide?

An O-glycosidic bond links to monosaccharides into a disaccharide.

Glycosidic bonds are readily hydrolyzed by what? Glycosidic bonds resist cleavage by what?

Readily hydrolyzed by acid. Resist cleavage by base.

Describe interconversion of conformation by sugars.

Sugars usually can easily interconvert between linear and cyclic forms, or between chair conformations. However, within disaccharides and bigger sugars, the O-glycosidic bond prevents this interconversion from happening.

What common disaccharide do plants use to store sugar?

Sucrose

What are 4 ways that polysaccharides differ?

Monosaccharide component sequence Glycosidic linkages Chain length Degree of branching

What are the functions of common polysaccharides in plants and animals and what are their names?

Plants: chitin, cellulose, used for structure; starch used as energy storage Animals: glycogen, used as energy storage

What monosaccharide(s) make up starch? Describe their branching and linkages

Amylose: unbranched glucose polymer with α(1-4) linkages Amylopectin: highly branched glucose polymer, with α(1-6) branch point linkages.

What monosaccharide(s) make up glycogen?

Glucose, α(1-4) linkages, highly branched, more compact than starch.

A glycogen molecule with n number of branches has how many non-reducing ends?

n+1

How to determine if a sugar is reducing or not?

Reducing sugars have hemiacetals. Nonreducing sugars have acetals that are "locked" within the ring and are thereby inactive.

What is cellulose?

A homopolysaccharide that makes up cell walls of plants. Linear and unbranched, its key difference from amylose is that cellulose has β(1-4) linkages.

What is the most stable 3D structure for α(1-4) linkages, such as those present in starch and glycogen?

A helix, stabilized by inter-chain H-bonding

What is the most stable 3D structure for β(1-4) linkages, such as those present in cellulose?

A straight, extended chain, stabilized by intrachain and interchain hydrogen bonding. (They exist in flat planes of many chains) Kinda like b sheets

What is peptidoglycan?

A heteropolysaccharide comprised of β(1-4) linked N-acetylglucosamine and N-acetylmuramic acid.

Makes up bacterial cell walls and creates stability in structure.

What is a glycosaminoglycan? Where is it found, and what are its components? Talk about its charge.

A family of linear polysaccharides found in animals and bacteria, but not plants. It is made up of repeating disaccharide subunits, typically comprised of N-acetyglucosamine and a uronic acid. It has a high density of negative charge.

What is a glycoconjugate?

An information-carrying carbohydrate (sugar) that is covalently linked to either a protein or lipid.

What is a proteoglycan?

A proteoglycan is a glucosaminoglycan chain that is covalently linked to a core protein via a serine residue.

How does the glucosaminoglycan chain attach to the core protein?

It attaches via a tetrasaccharide linker.

Some proteoglycans are membrane proteins. What are the two main types?

Syndecans and glypicans

Describe the general function of the surface membrane syndecans and glypicans.

These proteins can be "shed" from the cell's membrane surface; the cell's surface features plays a role in cell-to-cell communication, recognition, adhesion, proliferation, and differentiation.

Describe proteoglycan aggregates.

Form from many core proteins that attach to a single long strand of hyaluronan. Have multiple chains of sulfates. Contribute strongly to tensile strength and resilience of connective tissue.

What is a glycoprotein? What two types of linkages can occur between the carbohydrate and the protein and which residues are involved?

A glycoprotein is a protein that has multiple oligosaccharides covalently bound. The glycans are branched and very structurally diverse. N-linkages involve asparagine residues covalently linked to an oligosaccharide.

O-linkages involve serine or threonine residues covalently linked to an oligosaccharide.

About what percent of all mammalian proteins are glycosylated?

50%

What are glycolipids?

Glycolipids are components of the plasma membrane that have oligosaccharides as the hydrophobic head component.

What are mucins?

It is a type of glycoprotein containing many o-glycosidic bonds. They are present in most bodily secretions (i.e. mucus)

Give 3 examples of glycoproteins.

Antibodies, certain hormones, mucins.

What is a ganglioside?

Eukaryotic membrane lipids containing a polar head group composed of a complex oligosaccharide with a sialic acid.

What are lipopolysaccharides?

Sugar-lipid membrane molecules that interact with antibodies.

Why are oligosaccharides so information dense?

Compared with amino acids and nucleic acids, there are several orders of magnitude more of different permutations of a given amount of oligosaccharide subunits than of amino acid sequences or nucleic acid sequences of the same amount of subunits.

This is due to the prevalence of branched structures, nonexistent in nucleic or amino acids. Further, oligosaccharides have many more bond types and therefore variations of bound groups.

What is a lectin? What is its function?

Lectins are molecules essential to cell signaling. They are able to separate and detect glycans and glycoproteins with different oligosaccharide groups.

What is a selectin? What is its function?

Selectins are lectins specific to the plasma membrane. They commonly mediate the movement of immune cells to infection sites.

They are the main players of the inflammatory response.

This is done via the interaction of selectins with surface oligosaccharides on the leukocytes; this slows the leukocytes to allow them to bind to the infection site. The second step is where integrin actually stops the leukocyte and mediates its cross over the capillary wall and to the infection site.

This process is known as "lymphocyte homing"

What is E-selectin and L-selectin's functions?

E-selectin: Binds endothelial oligosaccharide L-selectin: Binds leukocyte oligosaccharide

Both during the process of lymphocyte homing.