Lecture 4 - Carbohydrates Flashcards
What are saccharides
- Make up nearly 50% dry weight of cells
- Often described with the prefix Glyco
- And suffix -ose
- Originally compounds of the formula CnH2nOn
What are the functions of saccharides
- Metabolic energy source
- Structural component in cell wall and extracellular matrix
- Recognition sites on cell surfaces- for adhesion and signalling
What are glycoconjugates
- General classification for carbohydrates covalently linked with other chemical species
- Glycoconjugates are formed via glycosylation
- Modify solubility, stability and life span
What are 4 examples of glycoconjugates
- Glycoproteins
- Proteoglycans
- Glycolipids
- Oligosaccharides
What are the 4 classes of saccharide
- Monosaccharide
- Oligosaccharide
- Polysaccharide
- Monosaccharide derivatives
What is a monosaccharide
- Sugars which cannot be hydrolysed to give a simple sugar
- e.g. Fructose
What is an oligosaccharide
- A carbohydrate which is composed of a small number of monosaccharide units (2-3)
What is a polysaccharide
- A carbohydrate whose molecules consist of a number of sugar molecules bonded together
- They range in structure from linear to highly branched
- E.g starch, cellulose, glycogen
What are examples of monosaccharide derivatives
- sugar acids
- Sugar alcohols
- Alditols
- Amino sugars
- Deoxysugars
- L-ascorbic acid
- Sugar phosphates
What is an aldose
- A monosaccharide which contains an aldehyde
What is a ketose
- A monosaccharide which contains a ketone
- Can also use the suffix -ulose instead of -ose
What is the simplest unit of a sugar
- Glyceraldehyde
- Not sugar itself- what they are built up from
- Contains chiral carbon
4.
How do you draw Fischer projections
- Line longest chain down sheet of paper
- All molecules drawn in plain of paper
- But actually Side chains are coming out towards us.
- D means first hydroxyl up on chain is on right
- If L every side chain is swapped
What is an anomeric position
- An anomeric carbon can be identified as the carbonyl carbon (of the aldehyde or ketone functional group) in the open-chain form of the sugar.
What form do sugars naturally exist
- Aldoess and ketoses (reducing sugars) can cyclise
- Pentoses (furanose) and hexoses (pyranose) particularly favoured
What is the Haworth representation
- Represents the cyclic structure of a saccharide
- Side groups point up or down
- Hemiacetal- OH reacts with aldehyde- forms C1- anomeric centre
- 1 O in the ring
What is the Mills representation
- Ring
- Shows OH groups with dash/wedged bonds
- In alpha glucose anomeric OH is coming out, then alternate dash and wedge for rest
What is chair representations
- Chair like configuration
2.
Describe glucose chair like configuration when anomeric position is in beta form
- All OH groups are equatorial
- Anomeric position OH is pointing up and equatorial
- Pyran ring- 6 membered
- Beta-D-glucopyranose
Describe glucose chair like configuration when anomeric position is in the alpha form
- OH in anomeric position is axial pointing down
- Alpha-D-glucopyranose
- Alpha OH always pointing down
What happens to the 5 position OH
- CH2OH is opposite to side to what the 5OH would have been
How is the hemiacetal formed
- Reversible reaction with water
- R-C=O (R-C=OH+)reacts with HO-R
- Aldehyde is protonated -
What are the 5 forms of glucose in order of most common
- Beta-pyranose- most common
- Alpha-pyranose
- Beta-furanose
- Alpha-furanose
- open ring
Describe order of frequency of the forms of glucose
- Alpha-pyranose is less common than beta as has axial hydroxide group- less stable
- Is more common than expected due to anomeric effect
- 6-Membered rings are more stable than furanose
- Open ring form is least common- aldehyde is unstable
Describe what acetals are
- Much harder to reverse than hemi-acetals
- When both OH groups have R group attached
- Require specific enzymes or stronger reagents (e.g. acids/bases)
- In sugar acetals are called glycosides
- The bond from C1 to the OR group is called the glycosidic bond
- What formed when two sugars join
How is an acetal formed
- R-C=OH+ + 2HO-R’
- 2 Eq of alcohol
What is added to a saccharide to form a glycosidic bond
- R’OH - can be another saccharide
- H+ or enzyme
Describe how disaccharide is formed
- 2 saccharides can join together with OH of anomeric bond in one sugar and OH of another carbon on other sugar
- 4 other reactive OH groups
What are sugar acids
- Gluconic acids- can’t cyclise as no aldehyde
- Glucuronic acid- has aldehyde and acid so can cyclise
- Formed from oxidation of aldehyde or primary alcohol group
What are sugar alcohols
- Produced by reduction of aldoses or ketoses
- Sorbitol is formed from glucose and mannitol from mannose
- Contain no aldehyde so can’t form rings
What are alditols
- A class of an acyclic polyol derived from an aldose by reduction of the carbonyl functional group
What are 3 examples of alditols
- Ribitol is a constituent of flavin co-enzyme
- Glycerol and myo-inositol are components of lipids
- Xylitol is a sweetner
What are amino sugars
- When one or more hydroxyl group of monosaccharides are replaced by amino groups the products are amino sugars
- D-glucoasmine, D-galactasamine
- The amino groups are sometimes acylated
What is L-ascorbic acid
- Vitamin C
- A water soluble vitamin
- Structure resembles monosaccharides
What are deoxysugars
- Sugars that contain one oxygen atom less than in the parent molecule
- D-2-deoxyribose- part of DNA
What are sugar phosphates
- Phosphate derivatives of monosaccharides found in all living cells
- Important intermediates in carbohydrate metabolism
What are glycoproteins
- Many proteins are functionalised by sugars post synthesis in the cell- glycosylation
- Glycoproteins can be N-linked or O-linked
- The rest of the sugar is then attached
What is glycation
- Uncontrolled addition of sugars to proteins
- Important in Alzheimer’s and diabetes
Give a major example of glycoproteins
- Erythropoietin hormone (EPO)
- Secreted by kidneys
- Stimulates production of red blood cells
- Glycosylation increases stability and bioactivity
- Can be used to treat anaemia
What are 3 other examples of glycoproteins
- Bacterial cell walls
- Cartilage
- Heparin
Describe bacterial cell walls
- Alternating N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM) with peptide cross-linkages
Describe cartilage
- Long, brush-like polysaccharides attached to central protein
- Acts as a shock-absorber due to movement of water under pressure
Describe heparin
- Polysaccharide that helps prevent blood clotting
