Week 5 (Carbohydrate Polymers) Flashcards
What are carbohydrates composed of?
Carbon, hydrogen and oxygen
What is a monosaccharide?
- One sugar residue
- Any of the class of sugars (e.g. glucose) that cannot be hydrolysed to give a simpler sugar
What is a disaccharide?
Two monosaccharide units
What is an oligosaccharide?
More than two monosaccharides (from three to six simple monosaccharide units)
What is a polysaccharide?
Lots of monosaccharide molecules bonded together
How are carbohydrates stored in plant cells?
As the complex carbohydrate starch
What is the function of carbohydrates in plant cells?
- act as structural elements
- supply energy (in the form of simple sugars)
How are carbohydrates stored in animal cells?
Stored as glycogen
however they also occur as both simple and complex sugars
What are the function of carbohydrates in animal cells?
- Storage of glucose, in the form of polymers like glycogen, makes it slightly less accessible for metabolism; however, this prevents it from leaking out of the cell or creating a high osmotic pressure that could cause excessive water uptake by the cell
- Carbohydrates form a part of genetic material like DNA and RNA in the form of deoxyribose and ribose sugars
- Act as recognition elements e.g, blood groups, cell markers
How many arrangements of a simple disaccharide comprising two glucose residues be joined together?
Eleven
Approximately how many different monosaccharides exist?
What does this give rise to?
- 200
- This gives rise to millions of different polymers
What extends the range of polysaccharide possibilities?
Functional group inter conversion
Explain the structure, function and properties of glycogen
- used by animals to store glucose
- a large branched polymer of glucose
- alpha-1,4- glycosidic binds
- branches formed by alpha-1-6- glycosidic bonds
What is the advantage of branching in glycogen?
- Branching increases solubility and makes the sugar more accessible
- Furthermore, branching creates a large number of terminal residues, the sites of action of glycogen phosphorylase and synthase. Thus, branching increases the rate of glycogen synthesis and degradation.
(Glycogen branching requires a single transferase activity. Glycogen debranching requires two enzyme activities: a transferase and an α-1,6 glucosidase)
What is the structure of amylose?
- unbranched glucose chains with alpha-1,4- glycosidic bonds
- left handed helix with 6 glucose units per turn
What is the structure of amylopectin?
- branched
- one alpha-1-6 linkage per 30 alpha 1-4 linkages
What is the purpose of the alpha 1-4 linkages in glycogen and starch?
The alpha 1-4 linkages of glycogen and starch produce molecules with hollow helices
- well suited to forming accessible surface stores
What is the main differences in the structure of amylopectin and glycogen?
- Glycogen has more branching and shorter branches
- Amylopectin has less frequent branching but longer side chains
- Amylopectin is an insoluble form whereas glycogen is a soluble form
What is the function of dextran?
- A storage polysaccharide in yeasts and bacteria
What is the structure of dextran?
- Nearly all linkages in the main chain are alpha 1-6
- Depending upon the species occasional branches formed by: Alpha 1-2 linkages, Alpha 1-3 linkages (most common) and Alpha 1-4 linkages
What is the function of cellulose?
- major polysaccharide of plants
- has a structural rather than nutritional role
What is the structure of cellulose?
- an unbranched polymer of glucose residues joined by beta 1-4 linkages
- this particular configuration allows cellulose to form very long straight chains
- each glucose molecule is rotated 180 degrees relative to its neighbours (This means that the repeating unit is cellobiose, not glucose)
- Intrachain hydrogen bonds form between the ring oxygen of one residue and the hydrogen of the C3 hydroxyl of a neighboring residue. (Interchain hydrogen bonds occur between hydroxyl and oxygen atoms on adjacent chains)
- H bonds are formed between different strands (Several cellulose chains come together to form a crystalline or paracrysatalline lattice, which is stabilized by both intrachain (intramolecular) and interchain (intermolecular) hydrogen bonds.)
- fibrils are formed by parallel chains
- the linear structure is ideal for forming fibres having a high tensile strength (several dozen such chains lie against each other and are bonded together to form what is known as a cellulose fibril)
What are glycoasaminoglycans?
- disaccharide repeating units containing a derivative of an amino sugar:
Glucosamine Or Galactosamine
- at least one of the sugars in the repeating unit has a negatively charged carboxylate or surface group
What are the major glycosaminoglycans?
- chondroitin 6-sulphate
- keratan sulphate
- heparin
What is heparin synthesised as?
- synthesised as a non-sulfated proteoglycan
- this is then deacelated and sulfated
- incomplete modification leads to a mixture of variously sulfated consequences
Describe the chemical structure of heparin
- Disaccharide repeating unit
- Carboxylate group (COO-)
- OH which has be esterified with surfuric acid (OSO3-) →Sulfonamide link (acidic)
- https://en.wikipedia.org/wiki/Sulfonamide
What are proteoglycans?
- A diverse class of proteins contains one or more covalently linked glycosaminoglycan chains
E.g. the proteoglycan in the extracellular matrix of cartilage
Explain the composition of cartilage
How does this help with its funcition?
- A very long filament of hyaluronate in the middle
keratin sulfate and chondrotin sulfate chains are covalently attached (through serine chains) to a polypeptide backbone called core protein
- about 140 of these proteins are noncovalently bound at intervals of 30 nm to the long filament of hyaluronate
- this interaction is promoted by a small link protein
- cartilage can cushion compressive forces because these highly hydrated polyanions spring back together after being deformed
(Lots of negative charges close together which can repel each other- when there is no pressure on the cartilage they are far apart (expanded state) when you walk the cartilage is compressed and the negative charges are pushed close together)
Describe the physical structure of cartilage
- Hyaluronate is the main thread that runs down the middle
- Core proteins in the chains that come out at the sides
- Chondroitin sulfate and keratan sulfate attached to the core protein
- Link protein hold the various structures together
- Lots of negative charges close together which can repel each other- when there is no pressure on the cartilage they are far apart (expanded state) when you walk the cartilage is compressed and the negative charges are pushed close together
What two ways can oligosaccharides and proteins be linked to form glycoproteins?
- N- linked glucoses (saccharide chains) via asparagine amino groups
- O linked glycine via threonine or serine hydroxyls
How are N- linked glycans usually attached to protein?
- Through N- actetylglucosamine
- Sometimes through N-acetylgalacosamine to side chain amino group in an asparagine residue
How are O linked glycans usually attached to protein?
- by an O-glycosidic bond between N-acetylgalactosamine and the hydroxyl group of threonine or serine residue
- in a few cases, collagen, hydroxylysine or hydroyproline is employed
What are blood group substances?
- a set of antigenic oligosaccharides attached to the surface of red cells
- on some cells the antigens are attached as O- linked glycans to membrane proteins
- alternatively, the oligosaccharide may be linked to a lipid molecule to form a glycolipid.
- The lipid portion of the molecule helps anchor the antigen in the outside surface of erythrocyte membranes
- these oligosaccharides determine the blood group types in humans.
- Their presence in a blood sample is detected by blood typing
- determining wether antibodies to a particular antigen cause red cells of that blood sample to agglutinate
Explain the oligosaccharides corresponding to the blood types A, B and O
(The R group is the anchoring point to the surface of the red blood cell)
- almost all humans can produce the type O saccaride. There are 4 carbohydrate building blocks joined together
- addition of either galactose (to make type B) or N-acetylgalactosamine (to make type A) requires special enzymes
- some individuals posses one or other of these enzymes
- a few heterozygous can produce both.
- The heterozygous individuals have type AB blood with both A and B oligosaccharides present on cell surfaces
How can oligosaccharides act as cell markers?
- the surface of many cells are nearly covered with saccharides attached to either lipids or proteins in the cell membrane
- these act as recognition sites by binding to particular proteins e.g. lectins (carbohydrate-binding proteins that are highly specific for sugar groups of other molecules)
What is the difference between heparin and heparin sulfate?
Heparan sulfate is like heparin but has few N- and O- sulfate groups and more N- acetylcholine groups
Why can cartilage cushion compressive forces?
These highly hydrated polyanions spring back after being deformed
Why are oligosaccharides used as cell markers?
They can offer up a massive variety of structures in relatively short chains -Monomers - linkages - branching patterns
How many different possibilities of tri saccharides are there?
In a tri-saccharide several thousand permutations are possible
What is the difference between heparan sulfate and heparin?
Heparan sulfate is like heparin but has fewer N- and O-sulfate groups and more N-acetyl groups
Draw the structure of Dermatan Sulfate
Draw the structure of Hyaluronate
Name some sugar residues commonly found in glycoproteins
Why are oligosaccharides so often used as celluar markers?
They can offer up a massive variety of structures in relatively short chains
Describe the structure and properties of chondroitin 6-sulphate
- Disaccharide repeating unit
- Glucose is usually CH2OH however this structure has gone up 2 oxidation levels to a caboxylic acid in one component
- Under physiological conditions the carboxylic acid is depronated to COO-
- In the other componet what was the CH2OH is effectively now the ester of sulfuric acid which would also be deprotonated in physiological pH: CH2OSO3-
- At the bottom of this component what was the OH group would have become an amino group and then an amide group
Name some carbohydrates commonly attached to protein
