Carbohydrates Flashcards
What are carbohydrates ?
- most abundant biomolecules on Earth
- complex organic molecules formed from carbon dioxide and water which are used as a source of chemical energy by living organisms
What is the empirical formula of carbohydrates ?
(CH2O)n where n is greater than or equal to 3
What are monosaccharides ?
Simple sugars
What are oligosaccharides ?
Short chains of monosaccharides which are less than 20 residues
What are polysaccharides ?
Consists of hundreds or thousands of monosaccharide units
What are the properties of monosaccharides ?
- colourless and crystalline solids
- water soluble
- most are sweet tasting
- open chain and ring structures
- two families : aldehydes and ketones
Why are most monosaccharides sweet tasting ?
They interact with the protein receptors on the tongue and form hydrogen bonds with them
Summarise the properties of glucose
- 6 carbon sugar (hexose)
- aldose
- reducing sugar - reduces copper 2 into copper 1 and forms a red brick ppt in Benedict’s reagent
- asymmetrical with chiral centres at C2, C3, C4 and C5
- the L and D configuration is based on spatial orientation of C5 groups
- 16 possible isomers
What is an epimer ?
A molecule which has 2 different stereoisomers because only 1 carbon attachment is different
What are the epimers of glucose ?
- D mannose (C2)
- D galactose (C4)
Are mannose and galactose epimers of each other ?
No because 2 carbons are different and not one
Summarise the properties of fructose
- hexose sugar
- ketone sugar (carbonyl group at C2 position)
- reducing sugar because the hydroxyl group can be oxidised to another carbonyl which is then reduced
- asymmetrical - chiral centres at C3, C4 and C5
- L and D configurations based on spatial orientation of C5
- 8 possible isomers
What are anomers ?
What type of rings are pyranose rings ?
Hexagonal rings e.g. glucose
What type of rings are furanose rings ?
Pentagonal rings e.g. fructose
What is derivatisation ?
Hydroxyl groups can be derivatised which means they can be replaced by substituents
What can hydroxyl groups be replaced by ?
- addition of a phosphate group e.g. ATP is the energy currency of cells
- addition of sulphate groups e.g. heparin reduces blood clotting
- addition of amine groups e.g. glucosamine is found in connective tissue
- addition of hydrogen or alkyl groups e.g. deoxyribose found in DNA
How do O-glycosidic bonds form ?
- form when a hydroxyl group reacts with the anomeric carbon
- non reducing sugar because the anomeric carbon is trapped by the glycosidic bond and so it can’t be reduced
How do N-glycosidic bonds form ?
They form when the anomeric carbon reacts with a nitrogen atom e.g. nucleotides and glycoproteins
What are disaccharides ?
2 monosaccharides linked covalently by an O-glycosidic bond
Give some examples of disaccharides
- Lactose
- Trehalose
- Maltose
- Sucrose
Briefly describe lactose
- disaccharide
- made of ß D galactose and glucose
- found in milk
- reducing sugar because there is at least 1 free anomeric carbon
Briefly describe trehalose
- non reducing sugar
- found in insects
- 2 alpha glucose with 1-1 linkage
Briefly describe maltose
- important in hydrolysis of starch
- glucose 1-4 linkage
- reducing sugar
Briefly describe sucrose
- non reducing sugar
- found in plants
- glucose and fructose (1-2)
How do polysaccharides vary ?
- the monosaccharide units involved
- chain length
- type of bonds linking the units
- degree of branching
What are homopolymers ?
Identical monosaccharides
What are heteropolymers ?
Variable monosaccharides
What are the roles of sugars ?
- energy source
- structural function
- important in information
Which carbohydrates can be used an energy source ?
- monosaccharides
- disaccharides
- starch
- glycogen
- ATP
What are the structural functions of sugars ?
- cell walls
- exoskeletons
- connective tissue
Why do sugars have so many roles ?
- diverse structural forms
- 3 hexose sugars can form 1056 different trisaccharides because each hydroxyl group can react with an anomeric carbon
Why are polysaccharides mostly used for storage ?
- compact granules
- bonding means less osmotic pull and so more can be stored in a small space
- little water associated with it
Give some examples of storage polysaccharides
- starch
- glycogen
Describe the structure of starch
- homopolymer of alpha D glucose
- the alpha D glucose molecules are linked together by 1-4 linkages
- unbranched = amylose
- branched = amylopectin > 1-6 linkage every 30 residues
Describe the structure of glycogen
- more compact than amylopectin
- branched > 1-6 linkage every 10 residues
What is the importance of alpha linkages ?
They provide the coiled structure which is good for storage carbohydrates
What do all structural sugars have in common ?
They all have ß linkages which provides a linear structure allowing hydrogen bonding which adds to the tensile strength
What are the structural homopolymers ?
- cellulose (ß1-4 linked glucose)
- chitin (ß1-4 linked N-acetylglucosamine)
What are the structural heteropolymers ?
Bacterial cell walls (ß1-4 linked peptidoglycan)
Describe the structure of cellulose
- unbranched polymer of glucose joined by ß1-4 linkages
- the ß configuration allows the formation of very long straight chains
- parallel chains can interact by hydrogen bonds forming fibrils
- fibres have high tensile strength
What is the most abundant biological molecule on Earth ?
Cellulose
What is chitin used for ?
Insect exoskeleton
Describe the structure of chitin
- very similar to cellulose
- differs from cellulose by the replacement of C2 hydroxyl group with an acetylated amino group
- parallel chains interact by hydrogen bonding leading to high tensile strength
Describe the structure of bacterial cell walls
ß1-4 linked
What are glycosaminoglycans ?
- family of heteropolysaccharides composed of repeating disaccharide units
- anioinic
- one of the monosaccharides is always either N-acetylgalactosamine or N-acetylglucosamine
- the other sugar is mostly an uronic acid (C6 carboxylic acid)
- it is a component of the extracellular matrix
Where is connective tissue found ?
- bone
- teeth
- cartilage
- skin
- blood vessels
Describe the structure of connective tissue
- made of collagen which is fibrous so provides strength
What are proteoglycans ?
- glycans with a bit of protein associated with it (95% carbohydrates and 5% protein)
What are glycoproteins and glycolipids ?
- oligosaccharides associated with proteins and lipids
- play important roles in recognition and communication
Describe the importance of sugar in fingerprints
- sugar residues of our cell surfaces which are unique to us (recognise self from non self)
- sugars allow more variety than proteins
- important for tissue typing
- define blood groupings
- mounts an immune response
How do the blood groups differ ?
- glycosyltransferases add specific sugars/carbohydrates to the base antigen which causes differentiation
- common foundation is known as the O antigen
- O phenotype is a result of a mutation resulting in no active glycosyltransferase
How do the A and B blood groups differ ?
- A blood group has the addition of N-acetylgalactose
- B blood group has the addition of galactose
- there is a type A and type B glycosyltransferase
Which blood group is the universal recipient ?
AB
Which blood group is the universal donor ?
O
What are mucins ?
- proteins found in the mouth to help with digestion
- they contain lots of carbohydrates
- they increase the viscosity of the fluid in which they are dissolved
- aid the delivery of the food to the gullet and ease its passage
What are lectins ?
- proteins which recognise and bind specific carbohydrate structures
- serve a wide variety of cell to cell recognition and adhesion processes
Give some functions of lectins
- lectins recognise old red blood cells and remove them
- viral pathogens utilise lectins for adhesion or toxic entry into host cells
What are selectins ?
They are a family of lectins involved in cell to cell recognition and adhesion e.g. the movement of T lymphocytes to a site of infection or inflammation
Summarise the roles of carbohydrates in the influenza virus
- hemagglutinin (lectin) recognises sialic acid residues on the host surface glycoproteins
- after penetrating the membrane, neuramidase (sialidase) breaks the glycosidic bond to release the virus
- without neuramidase, the virus is non invasive
