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Carbohydrates
Biochem and medical genetics
| Question | Answer |
|---|---|
| What is glycobiology | Study of carbohydrates and their role in cell biology outside of metabolism |
| What are glycoconjugates | Formed when mono- oligo- or polysaccharides are attached to proteins or lipids |
| What are glycans | The sugars of glycoproteins and glycolipids |
| What are lectins | proteins that specifically bind to sugar molecules |
| 3 classes of Glycoconjugates | Those attached to lipids Those attached to proteins through a Nitrogen atom (N-linked glycan). These are highly controlled and synthesised in the ER and Golgi Those attached to proteins through and Oxygen atom (O-linked glycan). |
| Where are glycoconjugates found | At the extracellular surface of the plasma membrane. Glycoproteins are secreted into biological fluids, such as serum and also make up the insoluble extracellular matrix Made inside the cell, in the lumen of the ER and Golgi |
| Summary of functions of glycoconjugates | Intrinsic-providing structural components e.g. cell walls and extracellular matrix. Modifying protein properties e.g. solubility and stability Extrinsic-directing trafficking of glycoconjugates. Mediating cell adhesion, interactions. Mediating signalling |
| Monosaccharides | Glycoconjugates are composed of monosaccharides with related chemical structures. Most common are hexoses with 6 carbon atoms. Can contain several alcohol groups and one aldehyde or ketone group. No. of Carbons varies between 3 and 7 |
| Examples of monosaccharides | Glucose - a 6 carbon sugar, an aldose (contains an aldehyde group) Fructose - a 6 carbon sugar, a ketose (contains a ketone group) |
| Stereochemistry of sugars | Sugars contain many chiral centres. Hexoses contain 4 chiral centres, which can exist in the L or D configuration (usually D) so there are 16 possible hexoses. D glucose and D galactose vary by carbon 4 only (they are epimers) |
| Ring chemistry - Pyranose | Hexoses are usually found in a ring form known as a pyranose configuration.This is caused by reaction of the 5-hydroxyl group with the 1 aldehyde group to create a hemiacetal.C1 is the anomeric carbon and exists in 2 anomeric configurations, alpha or beta |
| Ring chemistry - Furanose | Comes from the reaction of a ketone with an alcohol creating a hemiketal. This consists of 4 carbon and 1 oxygen atom with the anomeric carbon to the right of oxygen (C2). Carbons 1 and 6 are exocyclic e.g. in fructose |
| Shape of sugar rings | Not usually planar formations. There are 2 possible formations: chair or boat. Chair is usually more stable due to the lack of steric interference between groups at the apical points. The ring may close with the Oh pointing down (aloha) or up (beta) |
| Relationships between common Hexoses | Many are derived from glucose. Epimerisation at C2 yields mannose, while epimerisation at C4 yields galactose. Substitution of the 2-OH of glucose/galactose with an acetylated amino group yields N-acetylglucosamine/galactosamine. All commonly in D form |
| Physiological modifications of sugars | Addition of amine e.g. glucosamine and galactosamine Sulphur groups e.g. heparan sulphate (using N-deacetylase and N-Sulphotransferase). Sulphate is more negative so provides more bonding opportunities Usually C2 is modified |
| Common monosaccharides in humans | Trioses - glyceraldehyde and dihydroxyacetone Pentoses - ribose and deoxyribose Hexoses - glucose, galactose and fructose |
| Reactions of sugar molecules | A sugar can act as a reducing agent is it has a free aldehyde or ketone group, Ketoses must tautomerize before acting as reducing sugars. The test for reducing sugars is used to test blood glucose levels. |
| What is a reducing sugar | Any sugar with an open chain form with an aldehyde or free hemiacetal group. These reduce another compound and are oxidised. The carbonyl carbon is oxidised to a carboxyl group |
| Blood tests using reducing sugars | Small numbers of sugars will have open ring structures. These are oxidised, reducing their concentration. This shifts equilibrium so more sugars develop the open ring structure and can react until all the sugar has reacted. |
| How does the blood test work | Fehling's solution was used as a diagnostic test for diabetes/ The copper (II) complex oxidises the aldehyde to a carboxylate ion, and in the process copper (II) is reduced to copper (I). Red copper (I) oxide precipitates out the mixture |
| Modern blood glucose testing (glucometers) | Use glucose oxidase, which converts glucose to hydrogen peroxide and D-gluconolactone. Electronic meters use an electrode to take up electrons released providing a current which is measured and is proportional to the concentration of glucose |
| Modern blood glucose testing (colorimetric strips) | Use glucose oxidase, which converts glucose to hydrogen peroxide and D-gluconolactone. Use a peroxidase to give a colour change upon reaction with the hydrogen peroxide produced in the reaction. |
| Key disaccharides | Glucose alpha(1,2) beta fructose (sucrose) Glucose alpha (1,4) glucose (maltose) Galactose beta (1,4) glucose (lactose) |
| Roles of carbohydrates | Structural - cell walls, extracellular matrix Fuel source - glycogen stores, glucose transport, glycolysis, oxidation Signalling molecules - cell-cell recognition (lectins), site of bacterial/viral entry, self recognition e.g. blood groups |
| Polysaccharides | Multiple monosaccharides can be combined to form long chain polymers. Individually sugars are soluble in water, but as a polymer they move out of solution due to reduced H bond capacity and therefore reduced interaction with water |
| Cellulose | A structural polysaccharide found in plant cell walls. A homopolymer of glucose liked by 1,4 beta glycosidic bonds. Linear with a high tensile strength due to H bonds between fibres.. Cellulase is required to hydrolyse the bonds (not found in mammals) |
| Chitin | A structural polysaccharide. A homopolymer of N-acetyl glucose linked by beta 1,4 glycosidic bonds. Forms the exoskeleton in insects. Degraded buy chitinases |
| Structural carbohydrates in humans | Carbohydrates are present in the extracellular matrix to maintain ionic concentration gradients. GAGs are carbohydrates attached to ECM proteins to form proteoglycans. These have a net negative charge which attract sodium ions and water |
| Proteoglycans | Heavily O-glycosylated proteins giving strength to the ECM. Sugars attached to proteoglycans are large so provide a structural scaffold. Monosaccharide units arranged in linear chains of alternating residues of amino sugar and hexose derivatives |
| Proteoglycan structure | Ability to resist compression comes from a highly organised molecular super structure. Protein core of the major cartilage proteoglycan aggrecan organises GAGs attached at intervals along the chain. |
| Cartilage proteoglycans - globular domains | Globular protein domains off O-glycosylated area link the chain to hyaluronic acid (G1 and G2). C-terminal domain contains epidermal growth factor and C-Lectin which can bind to other matrix sugar molecules (G3) |
| Role of proteoglycans | Ability to aggregate to provide resistance is due to their highly hydrated state. Water binding is mediated by sugar molecules and sulphate residues attached to GAGs. These act as shock absorbers, releasing water slowly under pressure to act as a cushion |
| Linkage to proteins | Sugars are linked to proteins via asparagine (N-linked) or serine/threonine (O-linked) Sugar linked to protein is normally N-acetylglucosamine in the Beta linkage. N-linked glycans have a common core but different extensions |
| 3 stages of N-linked glycosylation | Formation of a lipid linked precursor oligosaccharide En bloc transfer of the oligosaccharide to the protein (Ans-X-Ser/Thr sequon) Processing of the oligosaccharide (often added when folding the protein) |
| Process of N-linked glycosylation | Lipid precursor is synthesised first in the cytoplasm then in the lumen of the ER. Transfer of the core structure occurs in the lumen during translation. Processing includes removal of some sugar molecules (trimming) followed by addition of new sugars |
| Trimming of Glycans | Occurs in the lumen of ER and Golgi. Done via glycosidases. Exoglycosidases cleave sugars from the non-reducing end e.g. glycosidase 1 removes terminal alpha 1,2 linked glucose. Mannosidases remove mannose linked alpha 1,2. |
| What are high mannose oligosaccharides | Glycans containing between 5 and 9 mannose sugars |
| What are complex glycans built on | A core consisting of 3 mannose residues and 2 GlcNAc residues |
| ABO blood groups | Structure of terminal sugars on N-linked glycans attached to Band 3 determine blood groups. Antigenic glycans are built up on the ends of polylactosamine chains by the action of Fuctosyltransferase to generate the H antigen. |
| What antigens do each blood group have | O - only H antigen A - GalNAc residue is added to the terminal galactose B - a Gal residue is appended AB - both modifications are present Hh blood group - individuals lacking gene for fucosyltransferase make antibodies for the H antigen |
| 2 types of O-linked glycosylation | Two major groups: Proteoglycans and Mucins (and in some antibody hinge regions) |
| What are mucins and what do they do | Large polymers of carbohydrates which hold water in the body (in digestive/intestinal tracts) Addition of sialylated glycans to serine and threonine residues results in regions of strong negative charge that give mucins the capacity to bind lots of water |
| How is mucin glycosylated | Based on simple set of core structures in which N-linked acetylgalactosamine is linked to serine or threonine side chains with a single beta 1,3 linked gal residue to form core 1. O-linked glycans attached to mucin consist of disialylated form of core 1 |
| Where are mucins present | In transmembrane proteins that contain globular domains. They serve to protect the domains away from the surface e.g. low density lipoprotein receptor. Also mediated cell-cell interactions due to interactions between receptors and O-linked sugars |
| How is O-linked glycosylation performed | Uses glycotransferases All sugars are added sequentially with the first GalNAc residue attached to Ser/Thr No simple targets, O-linked glycosylation is random Specificity comes from recognition of the target protein by oligosaccharide transerase |
| Glycolipids | Sugars attached to lipid molecules. When build on ceramide are known as glycosphingolipids. Roughly divided into two groups depending on whether first sugar attached to the sphingosine is galactose or glucose |
| Divisions of glycosphingolipids | Galatosphingolipids Glycosphingolipids Gangliosides Neolactosides Globosides |
| Glycolipid anchors | Anchor proteins to lipid membranes by covalent attachment to fatty acids and lipids Glycolipids attached to proteins are known as glycosylphosphatidylinositol anchors |
| Structure of glycolipid anchors | Glycan bridging protein to lipid attached to the inositol head group of the lipid. Ethanolamine linked to the 3rd mannose by a phosphodiester bond and the exposed amine group forms an amide bond with the carbonyl group of the C terminal in the protein |
| Carbohydrate energy stores | Major polysaccharide in animals is glycogen. Efficient for packing glucose without osmotic effects. Homopolymer of glucose residues linked by alpha 1,4 glycosidic bonds. Branch points at 8-10 residue spacing with branches joined by alpha 1,6 bonds |
| Starch | Two components: Amylose (10-30%) unbranched and Amylopectin (70-90%) branched. Amylopectin branched at 12-25 residue spacing. Branches are longer the glycogen (20-25 units) |
| How are starch and glycogen broken down | In the cell - broken down from the 4 end via phosphorylase (more branches = faster breakdown) In digestion - pancreatic and salivary amylases hydrolyse alpha 1,4 glycosidic bonds in starch, isomaltase alpha 1,6 links |
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