Carbohydrates Flashcards
(24 cards)
What is the difference between monosaccharides, oligosaccharides and polysaccharides?
monosaccharides - simple sugars
oligosaccharides - short chains of monosaccharides, less than 20
polysaccharides - consist of hundreds or thousands of units
Describe the features of monosaccharides
- colourless, crystalline solids
- water soluble
- most are sweet tasting
- exist in open chain and ring structures
- two families –> aldoses (aldehydes) and ketoses (ketone)
List 3 examples of trioses
- D-glyceraldehyde
- L- Glyceraldehyde
- Dihydroxyacetone
What is the equation for stereoisomers?
A molecules with n chiral centers can have 2^n stereoisomers
Describe the features of aldoses
- 6 carbon sugar - hexose
- carbonyl group at end of chain
- reducing sugars
- asymmetric (chiral) centers at C2, C3, C4 and C5
- L and D configuration based on spatial orientation of C5 groups
- 16 possible isomers
What are epimers?
Epimers are a type of diastereomers that differ in the configuration of only one chiral center. Diastereomers are stereoisomers that are not mirror images of each other.
Describe the features of ketoses
- 6 carbon sugar - hexose
- carbonyl group at C2 position
- reducing sugars
- asymmetric (chiral) centers C3, C4 and C5
- L and D configuration based on spatial orientation of C5 groups
- 8 possible isomers
Describe the ring formation of monosaccharides
- in aqueous solution, aldotetroses and all monosaccharides >5 carbons exist as cyclic structures
- covalent bond formation between carbonyl group and an oxygen atom of a hydroxyl group
- produces another chiral centre
- anomers - cyclic monosaccharides that are epimers
Describe the conformation of pyranose rings
- tetrahedral geometry of saturated carbons produce non-planar structures
- axial substituents sterically hinder each other
- chair form of B-D glucopyranose predominates as all axial positions are occupied by H atoms
Describe the conformation of furanose rings
- are not planar
- conformation is known as envelope form
- in ribose either the C2 or C3 is out of the plane
Describe glycosidic bond formation
- O-Glycosidic bonds are formed when a hydroxyl group reacts with the anomeric carbon
- determines structure of oligo- and polysaccharides
- non-reducing sugar
- N-glycosidic bonds are formed when the anomeric carbon reacts with a nitrogen atom
What are oligosaccharides and list some
- a few disaccharides occur naturally and are a source of energy
- they consist of two monosaccharides linked covalently by an O-glycosidic bond
- lactose (GalB1-4Glu) - found in milk
- trehalose (Glua1-1aGlu) - found in insects
- maltose (Glua1-4Glu) - hydrolysis of starch
- sucrose (Glua1-2BFru) - found in plants
Describe the roles of sugars
- Energy source - mono-, disaccharides, starch, glycogen and ATP
- Structural - Cell walls, exoskeletons and connective tissue
- information - recognition (glycoproteins and glycolipids) and nucleic acids
Why are most sugars stored as polysaccharides?
- compact granule
- bonding means less osmotic pull
- little water associated with it
Describe the structure of starch
- unbranched - amylose
- branched - amylopectin –> a1-6 linkage every 30 residues
Describe the structure of glycogen in storage
- branched - a1-6 linkage every 10 residues
Describe the structural roles of sugars
- homopolymers - cellulose (B1-4 linked glucose) and chitin (B1-4 linked N-acetyl glucosamine)
- heteropolymers - bacterial cell walls (B1-4 linked NAG and NAM)
- glycoconjugates - connective tissue
Describe the structure of cellulose
- one of the most abundant organic compounds found on earth
- unbranched polymer of glucose residues joined by B1-4 linkages
- the B configuration allows formation of very long straight chains
- parallel chains can interact by hydrogen bonding forming fibrils
- fibrils have high tensile strength
Describe the structure of chitin
- linear homopolymer composed of N-acetlyglucosamine linked by B1-4 glycosidic bonds
- differs from cellulose by replacement of the C2 hydroxyl group with an acetylated amino group
- parallel chains interact by hydrogen bonding leading to high tensile strength
- second most abundant polysaccharide on earth
What are proteoglycans?
- glycans with some protein associated
- polysaccharide chains –> glycosaminoglycan (GAG)
- GAG are anionic polysaccharide chains made up of repeating disaccharide units
- monosaccharide + amino sugar = N-acetylglucosamine or N-acetylgalactosamine
- can attach to long filament of hyaluronan to form enormous assemblies
- can interact with fibrous matrix protein
- form cross-linked meshwork giving strength and resilience
What can sugar residues from our fingerprints tell us and why not use protein?
- sugars allow more variety of structure than protein
- important for tissue typing
- define blood groupings
- mount immune response
How is oligosaccharide assembly used for blood groups?
- oligosaccharides synthesised by specific enzymes
- specific to sugars being linked
- carbohydrates attached to protein and lipids on cell surface
- common foundation known as the O antigen
What are lectins and what is their function?
- proteins that recognise and bind specific carbohydrate structures
- can be divided on the basis of their amino acid sequences and biochemical properties
- serve wide variety of cell-cell recognition and adhesion processes
- recognise old erythrocytes and remove them
- microbial/ viral pathogens utilise lectins for adhesion or toxin entry
Describe the action of the influenza virus
- hemagglutinin (lectin) recognises sialic acid residues on host surface glycoproteins
- after penetrating the membrane neuramidase (sialidase) breaks the glycosidic bond to release the virus
- without neuramidase the virus is non invasive –> tamiflu and relenza inhibit the enzyme
