Carbohydrate Structure and Functions
Classification
Carbohydrates are important energy sources in our body Carbohydrates:
- Polyhydroxy aldehydes or ketones or substances that give these compounds on hydrolysis
Monosaccharides
- Carbohydrate that cannot be hydrolysed to a simpler carbohydrate - General Formula: Cn H2O n , where n=3 to 8
Oligosaccharides play a key role in process takes place in the surface of cell - Cell-cell interaction
- Immune recognition
- May be joined to non-sugar molecules Polysaccharides are essential structural components
- Cellulose – Major component of grass and trees - They are either linear or branched
Monosaccharides
Aldoses Ketoses
Organic group Aldehyde Ketone
Position of Carbonyl group END of carbon chain ANY OTHER position
Stereochemistry
Carbohydrates have stereochemical property, they can be classified under D or L configuration by looking at its Fischer Projection
In nature, more D-sugars are found. D, L system classification
- If the OH group of the glyceraldehyde is on the LEFT, then it is in L configuration, and vice versa. - For amino acids, when the NH3+ group is on the LEFT in Fischer projection, then it is in L-configuration. - Based on chiral carbon farthest from carbonyl group (penultimate carbon)
Diastereomers are the groups of compound which are neither mirror image nor superimpossible to its stereoisomers.
Epimers are sugars which only differ in 1 chiral carbon
Number of stereoisomers : 2n, n=number of chiral carbon/stereocentres
Cylic Structures
- Occurs in aldotetroses and all monosaccharides with 5 or more carbons
- They exists almost entirely as 5- and 6- cyclic hemiacetals or hemiketals in aqueous solution
- They form when C=O group bond in aldehyde and ketone with –OH is alcohol
- Six- membered ring: pyranose - Five-membered ring: furanose
- Haworth projection
o Anomeric Carbon (*) on the right
o Hemiacetal or hemiketal oxygen are at the back right
- Furanose close to being planar while pyranose are more stable in chair conformation
Formation of Haworth projection from Fischer projection:
- From a fischer carbohydrate, join the first carbon with the OH group of second- last carbon - Bend the bonds to the left
- For L-sugar, put CH2OH below the plane - For D-sugar put CH2OH above the plane
* (anomeric carbon)
Trans = α
Cis= β
OH group on the left = β OH group on the right = α
Monosaccharide Derivatives
- Amino sugars
o One or more hydroxyl groups replaced by amino group o They often condensed with acetic acid
- Deoxy sugars
o One or more hydroxyl groups replaced by hydrogens (i.e. remove oxygen) o Occurs in DNA (Deoxyribonucleic acid)
- Lactones
o Oxidation products of carbonyl group of cyclic form of monosaccharides o A sugar is classified as reducing sugar when it can be oxidized
- Alditols
o Products after reduction from carbonyl groups of OH groups o Reducing agents: NaBH4
- Glycosides
o Carbohydrates in which OH of anomeric carbon is replaced by O-R o Glycosidic bond is the bond between the anomeric carbon AND O-R o Furanosides, Pyranosides
Disaccharides
Glycosidic bonds in Disaccharides
- O-glycosidic bond
o Condensation reaction between anomeric carbon and oxygen atom of OH group - N-glycosidic bond
o Between Anomeric carbon and Nitrogen atom - Nomenclature
o Position of anomeric carbon ( α or β)
o Position of carbon linked by the glycosidic bond
- End of chain with free anomeric carbon is the reducing end, the other which have formed glycosidic can no longer be oxidized!
Maltose
- D-glucose + D-glucose
- Joined by α(14) glycosidic bond - Malt, Milo, etc
Lactose
- D-Glucose + β-D-Galactose - Joined by β(14) glycosidic bond - Milk, etc
Sucrose
- β-D-Fructose + D-glucose
- Joined by αβ(12) glycosidic bond
Polysaccharides
Starch
- Serve as energy storage in plants - Polymer of α-D-Glucose residues
o Consists of amylase (straight) and amylopectin (branched) - Amylose
o Linear, unbranched chain with 4000 D-glucose residues joined by α(14) glycosidic bond o Forms HELIX in water (6 residues per turn)
o Iodine can also fit in the helix to form dark-blue starch-iodine complex - Amylopectin
o Highly branched, 24-30 residues of D0glucose joined by α(14) and branched out by α(16) glycosidic bond
- Can be digested by amylase
o Catalyse hydrolysis of α(14) glycosidic bonds o α-amylase
Endoglycosidase that catalyses anywhere along the chain o β-amylase
Exoglycosidase which only catalyse from non-reducing ends - Debranching enzymes – hydrolyses amylopectin
Glycogen
Storage polymer in animals
- Mainly in liver (10% liver mass) and skeletal muscle(2% muscle mass) - Similar to amylopectin, but glycogen has more branches
- Branches in every 8-12 residues - Glycogen phosphorylase
o Catalyses α(14) bond from non-reducing ends Glucose-1-phosphate o More reducing endsmore monomers more energy
Cellulose
- Major structural components of plant cell wall
- LINEAR polymer with approximately 2800 D-glucose residues o β(14) glycosidic bond
o Fully extended conformation with alternating 180o flips
o Extensive intra and inter molecular hydrogen bonding between chains - ONLY cellulose can hydrolyse the glycosidic bond
Chitin
- Major structural unit for exoskeleton
- Also found in cell walls of algae, fungi, and yeasts
- Similar to cellulose, but monomer is N-acetyl-β-D-glucosamine - Joined by β(14) glycosidic bond
Peptidoglycans
- Rich in Gram-Positive bacteria - Forms bacterial cell wall
- Heteropolysaccharides: Two monomers
o N-acetyl-β-D-glucosamine (NAG) (as in Chitin) o N-acetylmuramic acid (NAM)
- Linked by β(14) glycosidic bond
- The COO- terminal of NAM will be joined to an 4 amino acids
o In Staphylococcus aureus, the amino acid is a D- amino acids (normally we will see L-amino acids, but here is the exception)
o In Staphylococcus aureus, each tetrapeptide is further cross-linked to adjacent tetrapeptide by pentapeptide of five
Glycine
residues - The reason which they have D-amino acid is that it isresistant to proteases (enzyme which hydrolyses them)
- Lysozyme (special proteases) can catalyse hydrolysis of β(14) bond between NAG – NAM (NOT NAM— NAG)
180o flip
H-bond
Glycosaminoglycans
- Polysacharrides with repeating disaccharide units o One monosaccharides is an amino sugar
o The other is a negative charged due to the presence of sulphur or COO- group o Examples: Heparin, Hyaluronic acid, Chondroitin sulphate and keratan sulphate - Heparin
o Natural anticoagulant – prevent blood clotting - Hyaluronic acid
o Component of vitreous humour of eye and lubricating fluid of joints
- Chondroitin sulphate and keratan sulphate
o Components of connective tissue, especially cartilages
Glycoproteins
- Proteins that contain carbohydrate residues covalently linked to polypeptide chain by glycosidic bonds - Almost all secreted membrane associated proteins are glycoproteins (added at ER or Golgi bodies)
o All antibodies are glycoproteins O-linked
(only can be added to Ser/Thr)
N-linked
(Only can be added to Asn)
- The presence of carbohydrate groups in protein causes: o Increased solubility in proteins (OH group)
o Changes in protein folding (Change in biological activities in proteins) o Protection from proteolytic enzymes (Carbohydrates blocks amino group) o Facilitates biological recognition
- Biopharmaceuticals manufacturing
o Cells produces glycosidase, so when we want to take out glycoproteins, many times the proteins are degraded due to removal of glycosidic bond by glycosidase
Glycolipids
- Membranes of animal plasma cells (sphingolipids)
- Large number of relatively small covalently- bound carbohydrates - ABO blood group
o Small lipid bound carbohydrates determine blood groups
A antigen (NAG)
B-antigen (Galactose)
- Carbohydrates facilitate biological recognition
o ABO blood group, degradation of old proteins, contact with neighbouring cells (contact inhibition) , infection by viruses, etc
