BIOL 304 Chapter 11: Carbohydrates and Glycoproteins Part 2

Why is free glucose unable to be stored?

high concentrations of free glucose will disturb the osmotic balance of cells

What are polysaccharides, or glycans?

large polymeric oligosaccharides formed by the linkage of multiple monosaccharides

What are polysaccharides used for?

energy storage and structural support

What is homopolymer?

a polymer where all the monosaccharide units are the same

What is glycogen?

large, unbranched polymer of glucose residue; storage unit of glucose

True or False: glycogen is the most common homopolymer.

True!

How is glycogen structured?

the glucose units are linked by an alpha-1,4-glycosidic linkage and branches are formed through alpha-1,6-glycosidic linkages

Why is it important for a glycogen molecule to have branching?

branching increases the surface area to allow better access for enzymes to rapidly break down glucose

What is starch?

homopolymer that serves as the nutritional reservoir in plants

What are the two forms of starch?

amylose and amylopectin

What is amylose?

unbranched type of starch composed of glucose residues in alpha-1,4 linkage

What is amylopectin?

branched type of starch with ~1 alpha 1,6 linkage per 30 alpha 1,4 linkages

True or False: amylopectin has the same structure as glycogen

True! only difference is amylopectin has a lower degree of branching

What molecule hydrolyzes amylose and amylopectin?

alpha-amylase

What is cellulose?

unbranched polymer of glucose residues joined by beta-1,4 linkages

What role does cellulose play in plants?

helps with structural integrity; forms a rigid, supportive structure

Why is it important that cellulose is structured in the beta configuration?

this configuration allows for cellulose to form long, straight chains that interact with one another through hydrogen bonds

How is cellulose stored in plants?

the alpha linkages of starch and glycogen form compact hollow cylinders suitable for storage

Why are mammals unable to digest cellulose?

mammals lack cellulases which are important for proper digestion

Even though mammals cannot digest insoluble fibers, why are they still important to consume?

these fibers increase the rate at which digestion products pass through the large intestine; soften stool and makes passage easier

What role do soluble fibers play in digestion?

slow down the movement of food in gastrointestinal tract, which facilitates absorption of nutrients

What are two examples of soluble fibers?

pectin and poly galacturonic acid

What is chitin?

a homopolymer of beta-1,4 linked N-acetylglucosamine

Where is chitin commonly found?

inside fungal cell walls and exoskeletons and shells of arthropods

How is chitin structured?

fibers are crosslinked and composited with minerals and proteins to increase rigidity and strength; this is why shellfish have hard shells

What is a glycoprotein?

a carbohydrate group covalently attached to a protein

About how much of the human proteome is made up of glycoproteins?

50%

What is glycosylation?

the processes of adding a carbohydrate to a protein; process that creates glycoproteins

What process increases the complexity of a proteome?

glycosylation

What are glycoforms?

different glycosylated forms that arise due to different types of branching

Why do glycoforms occur?

some proteins have several glycosylation sites (sites where a carbohydrate can be attached)

What are the three classifications of glycoproteins?

glycoproteins, proteoglycans, and mucins (mucoproteins)

How do glycoproteins and proteoglycans differ?

glycoproteins are mostly composed of proteins while proteoglycans are mostly composed of carbohydrates

What are proteoglycans?

predominantly carbohydrates and the protein component is conjugated to a glycosaminoglycan

What is the main function of proteoglycans?

structural components and lubricants

What are mucins (mucoproteins)?

predominantly carbohydrates and the protein components is extensively glycosylated

At what amino acids are mucins usually glycosylated at?

at Ser and Thr residues, usually by N-acetyl galactosamine

What is the main function of mucins?

key component of saliva and functions as a lubricant

What is N-linkage?

links the sugars in glycoproteins to the amide nitrogen atom in the side chain Asn

Which sequence is necessary for linkage of the glycoproteins to nitrogen?

Asn must be a part of an Asn-X-Ser/Thr sequence

What is "X" in the N-linkage sequence?

any amino acid accept for proline

What is O-linkage?

links the sugars in glycoproteins to the oxygen atom in the side chain of Ser or Thr

True or False: there are many different types of sugar groups that can be added through N-linkage

True! there are many that can be added to Asn, however, the type of sugar that is added depends on the type of protein present

True or False: N-linked oligosaccharides have a common core

True! they have a common pentasacchardide core

What is the pentasaccharide in N-linked polysaccharides composed of?

three mannoses and two N-acetylglucosamine residues

What is erythropoietin (EPO)?

a glycoprotein secreted by the kidneys into blood to stimulate red blood cell production

How does glycosylation enhance the stability of erythropoietin (EPO)?

sugar groups help stabilize protein in EPO so that it doesn't break down or get attacked in the bloodstream

How is EPO glycosylated?

N-glycosylated at three Asn residues and O-glycosylated at a Ser residue

What is GlcNAcylation?

the post-transitional, covalent attachment of a single N-acetylglucosamine (GlcNAc) to Ser/Thr residues of proteins

What enzyme catalyzes GlcNAcylation?

O-GlcNAc transferase

When does GlcNAcylation occur?

when nutrients are abundant; sugars are attached to the proteins when nutrients are abundant

How is GlcNAcylation reversible?

when nutrients are low, these sugars are taken off of these proteins and can be used for energy

What reaction often occur at GlcNAcylation sites?

phosphorylation sites, which transfer phosphate to other molecules

How does phosphorylation occur at GlcNAcylation sites?

by the use of O-GlcNAc transferase and protein kinases

What diseases are linked to improper regulation of O-GlcNAc transferase?

insulin resistance, diabetes, cancer, or neuropathological pathologies

What are proteoglycans mostly made up of?

glycosaminoglycan

What is the first function of proteoglycans?

functions as lubricants and structural components in connective tissue; important for building organ muscles and bone structure

What is the second function of proteoglycans?

mediate adhesion of cells to extracellular matrix (ex. the cells that make up linings, cells adhere to proteoglycans so that they stay in place)

What is the third function of proteoglycans?

bind factors that regulate cell proliferation

What are glycosaminoglycans?

composed of repeating units of disaccharides containing a derivative of an amino sugar

Which amino sugar derivatives make up glycosaminoglycans?

glucosamine or galactosamine

What happens if an individual is unable to degrade glycosaminoglycans?

results in diseases marked by skeletal deformities and reduced life expectancies

What is cartilage composed of?

protein collagen protein and proteoglycan aggrecan

What is aggrecan?

large molecule with three globular domains

Why is the aggrecan important?

it is a site of glycosaminoglycan attachment

What glycosaminoglycans bind to the aggrecan?

keratin sulfate and chondroitin sulfate

Where is the site of attachment for glycosaminoglycan on aggrecan?

between the extended region between G2 and G3

The aggrecan has a G1 site, what happens at this site?

G1 noncovalently binds to a central polymer of hyaluronate

Why is water important to aggrecan?

acts like a cushion for compression forces (ex. helps with force that is put on the knee joint)

True or False: water is bound to the glycosaminoglycans of the aggrecan

True! water is squeezed from the glycosaminoglycan under pressure and rebinds when pressure is released

What is osteoarthritis?

form of arthritis that results when water is lost from proteoglycan with aging

What are tandem repeats (VNTR) regions?

regions of a protein backbone of mucins that is rich in O-glycosylated Ser and Thr residues

Why are tandem repeats (VNTR) regions important?

they serve as a place where core carbohydrate structures can bind

What is the first function of mucins?

adhere to epithelial cells and acts as a protective barrier

What is the second function of mucins?

hydrate underlying cells of epithelium

What is the third function of mucins?

plays roles in fertilization, the immune response, and cell adhesion

Where is overexpression of mucins most likely to occur?

in bronchitis, cystic fibrosis, and adenocarcinomas

Where does protein glycosylation mainly take place?

in the endoplasmic reticulum (ER) and Golgi complex, since they play a main role in protein trafficking

Where does N-linked glycosylation take place?

In the ER and in some parts of the Golgi apparatus

Where does O-linked glycosylation take place?

only in the Golgi apparatus, since sugars used for linkage come from smooth plasma reticulum

What is dolichol phosphate?

specialized lipid molecule located in the rough endoplasmic reticulum membrane

Where is dolichol phosphate primarily located?

in regions where large oligosaccharides are destined for attachment to the Asp residues are assembled

What is the site of attachment on dolichol phosphate?

the terminal phosphate

What is the Golgi complex?

a stack of flattened membranous sacs

What happens when proteins leave the Golgi complex?

proceeds to lysosomes, secretory granules, or the plasma membrane

How does a protein known where to go after leaving the Golgi complex?

signals encoded within their amino acid sequences and three-dimensional structures tell them where to go

How does the Golgi complex act as a sorting center?

glycosylation starts in ER, goes to Golgi, travels through stacks of the Golgi until it reaches the trans face and is sent to where it needs to go in the form of granules within a vesicle

What are glycosyltransferases?

catalyze the formation of glycosidic linkage

What are activated sugar nucleotides used for?

they are used as a carbohydrate donor for glycosyltransferases

How are blood groups for RBCs designated?

the presence of three different carbohydrates (A, B, O) attached to glycoproteins and glycolipids on the surface of RBCs

True or False: all blood groups have a core O antigen

True!

What makes A and B antigens different from O antigens?

A, B antigens have one extra monosaccharide through an alpha-1,3 linkage to a galactose moiety of the O antigen, which makes than differentiate from O groups

What is a type A transferase?

adds N-acetylgalactosamin to form the A antigen

What is a type B transferase?

adds galactose to form the B antigen

Do O antigens have type A or B transferase?

No, they lack both these enzymes, which is why type O is produced

What type of blood will a person lacking both type A and type B transferase have?

Type O blood

What type of blood will a person with both type A and type B transferase have?

Type AB blood

What type of blood will a person lacking type A transferase have?

Type B blood

What type of blood will a person lacking type B transferase have?

Type A blood

Why do rejection of blood transfusions occur?

if an antigen not normally present is introduced, the immune system recognizes it as foreign