Chapter7: Polysaccharides Structure and Analysis

What are the two kinds of polysaccharides?

• Homopolysaccharides: one kind of sugar

• Heteropolysaccharides: multiple kinds of sugar

What is glycogen’s function in animals?

Energy (carbohydrate) storage

Describe the structure of glycogen

• Glc(alpha1→4)Glc chains

• Alpha1→6 branches (about every 8-12 units of glucose)

• Reducing end connected to a protein in the middle (glycogenin)

Where do we find glycogen stores?

Muscles and liver cells

Why is glycogen being insoluble a good thing?

Prevents excess intracellular glucose solutes from drawing in water and causing high osmotic pressure

What is the function of starch?

Energy (carbohydrate) storage in plants

Describe the structure of starch

• Still all glucose

• Amylose and amylopectin strands

• Amylose: only alpha 1→4, LINEAR, but can cause helices to form

• Amylopectin: Alpha 1→4 chain and alpha 1→6 branches

• Secondary structure can make H-bonded double helix

What kind of linkage does cellulose have?

Glc(Beta 1→4) Glc

Is cellulose branched?

No! All linear

What chemical properties allow cellulose to be used as structural support in plant cell walls, for example?

Since cellulose is unbranched and flat, they can form sheets

What’s the sugar making up chitin?

N-acetylglucosamine (glucose molecule modified with amide group)

What is the function of chitin?

Composes insect exoskeletons

Chitin linkages

GlcNAc (Beta 1→4)GlcNAc

Enzymes recognize the glycosidic bonds they are made to cleave by…

Recognizing the specific monomers AND the linkagethey form

Give two examples of glycosidic bond cleaving enzymes

The enzyme alpha amylase is present where in our body? What polymer does it hydrolyze?

Saliva, starch alpha1→4 linkages

Endo versus exo enzymes

• Endo: can cut anywhere in the polysaccharide with the right substituents, like in between monomers

• Exo: can only cut at terminal, non-reducing end, chops one at a time

Is glycogen phosphorylase an endo or exo enzyme?

Exo! Cuts terminal non-reducing glucose in glycogen(alpha 1→4) and adds a phosphate group

Alpha amylase can cleave both…

Glycogen and starch because they both have alpha 1→4 glucose linkages

What cleaves the alpha 1→6 glucose linkages in glycogen and starch?

Isomaltase

What is cellulose cut by?

Cellulase

Why can cows digest cellulose?

Their gut microbiome harbors a microbe (rumen) that produces cellulase

What are our 4 major classes of heteropolysaccharides?

1. Glycosaminoglycans- repeating disaccharides (DOESN’T contain proteins!!!)

2. Peptidoglycan- bacterial cell wall

3. Proteoglycan- protein + glycosaminoglycans

4. Glycoproteins- proteins with polysaccharides attached

Is peptidoglycan found in mammals?

No!

Describe glycosaminoglycans

• Repeating disaccharides: ABABABA…

  • A= GlcNAc or GalNAc

  • B= a uronic acid

• Linear heteropolysaccharide

What is GlcNAc? What is GalNAc? What is a uronic acid?

• GlcNAc= N-acetylglucoseamine

• GalNAc= N-acetylgalactoseamine

• Uronic acid= a monosaccharide with a carboxylic acid functional group instead of -OH

Why are glycosaminoglycans usually very negatively charged?

1. Carboxylic acid group in uronic acid can be deprotonated

2. The amino sugar can sometimes be sulfated (-OSO₃^- )

What is the longest glycosaminoglycan?

Hyaluronic acid

What monomers comprise hyaluronic acid?

D-glucuronic acid and N-acetylglucosamine

Describe hyaluronic acid

• Strongly negative at pH=7

• Long, linear chains of repeating D-glucuronic acid and N-acetylglucosamine

• High viscosity because hydrophilic

• Found in our eyeballs as a lubricant, also eye structure and protection. Can also be found in cartilage and tendons

How might other glycosaminoglycans differ from hyaluronic acid?

1. Can be shorter

2. Can be linked to specific proteins

3. One or both monosaccharides might be different than hyaluronic acid

Name Three other pertinent glycosaminoglycans and their functions

• Chondroitin sulfate: tensile strength of cartilege, tendoms, ligaments, etc, highly negatively charged bc the GlcNAc is sulfate at C4

  • N-acetylglucosamine-4-sulfate

• Keratan sulfate: nails, hair, hoofs, no uronic acid present, GlcNAc is also sulfated so highly negatively charged

  • N-acetylglucosamine-6-sulfate

• Heparan sulfate: Inhibit coagulation of blood through its activation of anti-thrombin, made of a uronic acid and amino sugar

What is peptidoglycan made of?

• N-acetylglucosamine and N-acetylmuramic acid(B1→4)

Peptidoglycan is connected to…

Proteins that have alternating chirality amino acids.

Connected by pentaglycine cross-links

What enzyme degrades peptidoglycan to help prevent infections?

Lysozyme

What are three examples of glycoconjugates?

1. Glycoproteins

2. Glycolipids

3. Proteoglycans

Glycoconjugates are usually found where in the cell?

Surface

Proteoglycan def

• Macromolecules at the cell surface in which one or more sulfated glycosaminoglycan chains are joined covalently to membrane protein or secreted protein

Glycoproteins def

Protein that has one or several oligosaccharides of varying complexity joined covalently to a protein

Glycolipid def

Plasma membrane components in which the hydrophilic head groups are oligosaccharides

Glycoproteins can have very a very __ array of sugars

Complicated

What is the backbone of proteoglycans

Hyaluronate →NOT a protein

Many smaller glycosaminoglycans are bundled up in…

Large proteoglycans

Where are proteoglycans mostly found?

Extracellular matrix (ECM)

What is the ECM?

Gel-like region around the cell that holds cells together and provides a porous pathway for the diffusion of nutrients and oxygen to other cells

Describe the structure of proteoglycans

• Hyaluronate backbone

• Link protein attached to backbone, which attach to Aggrecan core proteins

• Many smaller glycosaminoglycans conjugate with the aggrecan, creating a feather-like structure. Glycosaminoglycans conjugate with Serine residues!!

Where are glycoproteins primarily found?

Cell membrane and membrane proteins, and on secreted proteins, important in cell-signaling

What is the role of carbohydrate moieties in glycoproteins

• Can target proteins for transport to specific organelles

• Protect proteins from proteolysis (like COVID spike protein)

• Aiding folding and stability

• Provide point of recognition

• Restrict access to substrate, etc

There are a lot of glycoproteins in what organ?

Intestines!

Pointed on the outside where digested food is passing through. Protects intestinal cells and helps with absorption a bit

Oligosaccharides on glycoproteins can form two kinds of linkages, what are they?

• O linkages on Ser and thr

• N linkages on Asn

Where is the glycoprotein glycophorin found?

On the surfaces of RBCs

What special sugar is on the surface of RBCs and what is its function?

• Sialic acid: 9 C sugar

• Highly negatively charged to prevent RBCs from clumping together

Once a protein is made by a ribosome, they are __ so they can be sent to a specific location

Tagged

Oligosaccharides are tagged onto proteins in what cellular organelle?

Endoplasmic reticulum

Where are glycoproteins further processed?

Golgi apparatus

Oligosaccharides can dictate the __ of __ of a protein

Function or location

What is the role of Mannose 6-Phosphate?

Targets enzymes to lysosomes

What is I-Cell disease?

• Autosomal recessive

• Cant put a phosphate group of the mannose sugar that targets proteins for the lysosome

• Leads to 8 enzymes missing from the lysosomes → instead they get secreted into the blood

• Severe effects