Carbohydrates 1 PDF

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Summary

CarbohydratesMost abundant molecules on the planetComplex organic molecules formed from carbon dioxide and water by photosynthesisGood source of chemical energy(CH 2 O)n where n>Monosaccharides Colourless, crystalline solid  Water soluble  Most are sweet tasting (interaction between the sugars...

Description

Carbohydrates Most abundant molecules on the planet Complex organic molecules formed from carbon dioxide and water by photosynthesis Good source of chemical energy (CH2O)n where n>3 Monosaccharides     

Colourless, crystalline solid Water soluble Most are sweet tasting (interaction between the sugars and receptors) Exist in open chain and ring structures 2 families - Aldoses and Ketoses

Triose  

Simplest monosaccharide Minimum number of carbos = 3

Stereochemistry   

A molecule with n chiral centres have 2n stereoisomers Glyceraldehyde has 1 chiral centre so 21=2 stereoisomers As another C is added the number of stereoisomers increases by a factor of 2

Aldoses      

6 carbon sugar – hexose Carbonyl group at the end of the chain – aldehyde Reducing sugars Asymmetric (chiral) centres at C2, C3, C4 and C5 L and D configuration based on special orientation of C5 groups 16 possible isomers

Epimers   

Describes the orientations of 1 chiral centres only D-Galactose is the C-4 epimer of glucose D-Mannose is the C-4 epimer of glucose

Ketoses      

6 Carbon sugar Carbonyl position at the C2 position Reducing sugars (the proximity of the OH group to the carbonyl group means that another OH group has been oxidised to form another ketone Asymetric C3, C4 and C5 L and D configuration based on spatial orientation at C5 groups 8 possible isomers

Ring formation       

In aqueous solution all monosaccharides >5 carbons exist as cyclic structures Covalent bond formation between carbonyl group and an oxygen atom of a hydroxyl group Produces another asymmetrical chiral centre Anomers (α and β) α when OH group is in the downward position on new chiral centre β when OH group is in the upward position on new chiral centre C5 OH attacks positive carbon on carbonyl group

Drawing glucose

α

Drawing Fructose

Conformation of Pyranose Rings   

Tetrahedral geometry of saturated carbons produce non planar structures Axial substituents sterically hinder each other Chair form of β-D glucopyranose predominates as all axial positions are occupied by H atoms

Conformation of Furanose Rings   

Furanose rings like pyranose rings are not planar Conformation is known as envelope form In ribose either the C2 or C3 is out of the plane

Derivatisations     

OH groups replaced by substituents Addition of phosphate group e.g. ATP Addition of sulphate group e.g. Heparin – reduces blood clotting Addition of amine groups e.g. Glucosamine – found in connective tissue. Addition of Hydrogen or Alkyl groups e.g. Deoxyribose – found in DNA.

Glyosidic Bond Formation 

O-glyosidic bonds are formed when hydroxyl group reacts with the anomeric carbon

  

Very important in glycobiology Determines the structure of oligo and polysaccharide Non-reducing sugar

N-glyosidic Bonds  

Formed when nitrogen on e.g. nucleotides or protein react with the anomeric carbon Found in ATP

Oligosaccharides        

Consists of 2 monosaccharides linked together covalently by a covalent bond Short chains of monosaccharides (less than 20 residues) A few disaccharides occur naturally and are a source of energy Lactose (Galb1-4Glu) – found in milk Trehalose (Glua1-1aGlu) – found in insects Maltose (Glua1-4Glu) - hydrolysis of starch Sucrose (Glua1-2bFru) – found in plants Most oligosaccharides are associated with other biomolecules e.g. proteins and lipid

Maltose Glyosidic Bond Formation

Polysaccharides Most carbohydrates found in nature as polysaccharides and vary by:      

Monosaccharides units involved Chain length Type of bonds linking units Degree of branching Homopolymer-identical monosaccharides Heteropolymer-variable monosaccharides

Roles of Sugars 1. Energy source (Mono-, Disaccharides, Starch, Glycogen and ATP) 2. Structural (Cell Walls, Exoskeletons and Connective tissue)

3. Information (Glycoproteins, Glycolipids and Nucleic acids, recognition) Why so many roles?   

Diverse structural forms 3 different amino acids – 6 tripeptides 3 different hexoses – 1056 different trisaccharides

Energy Source    

Both plants and animals store energy as polysaccharides Plants store energy as starch Animals store energy as glycogen Easily metabolises to provide energy

Why store energy as a polysaccharides?      

Compact granule Bonding means less osmotic pull Little water associated with it Starch and glycogen are structurally very similar. Homopolymers of a-D Glucose. The a-D Glucose molecules are linked together by a1-4 linkages (favour globular structures).

Starch 1. Unbranched (amylose) 2. Branched (amylopectin) α1-6 linkage every 30 residues Glycogen 

α1-6 linkage every 10 residues

α-linkage 

Favours coiled structure

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 (various monosaccharides linked by b glycosidic bonds)

Cellulose      

Most abundant compound on earth Unbranched polymer Β formation allows for very long chains Parallel chain can interact by H bonding to form fibrils β1-4 glyosidic bond Fibres have a high tensile strength

Chitin 

β1-4 linkage

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Linear homopolymer composed of N-acetylglucosamine 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

Bacterial Cell Walls    

Sugar rich coating outside plasma membrane Gram positive bacteria retain iodine complex Gram-positive bacteria- cross-linked multi-layered proteoglycans, whereas gram negativeonly a single layer Penicillin (inhibits the enzyme that forms the peptidoglycan cross linkages)

Connective Tissue   

Bone, teeth, cartilage, skin, blood vessels Collagen- fibrous element – strength Proteoglycans- ground substance- elastic properties of joints

Proteoglycans         

E.g. Peptidoglycan 95% carbohydrate and 5% protein Polysaccharide chains- glycosaminoglycan (GAG) GAG are anionic polysaccharide chains made up of repeating disaccharide units. Monosaccharide + amino sugar = N-acetylglucosamine or Nacetylgalactosamine Glycosaminoglycans attached to Core Protein e.g. Aggrecan These units can attach to long filament of Hyaluronan to form enormous assemblies These can interact with fibrous matrix protein e.g. collagen Forms cross-linked meshwork giving strength and resilience

Glycoproteins and Glycolipids  

Oligosaccharides associated with proteins and lipids. (5% carbohydrate, 95 % other) Play important roles in recognition and communication

Sugar fingerprint     

Sugar residues of our cell surfaces unique to us- recognise self from non self Sugar allow much more verity of sugar than proteins do Very important for tissue typing Define blood groupings Mount immune response

Blood Groups   

Oligosaccharides are synthesised by specific enzymes – Glycosyltransferase Specific to sugars being linked Blood groups are good examples

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Common foundation knowns as the O antigen

Oligosaccharide Assembly 

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A and B antigens differ by 1 monosaccharide – N-acetylgalactose (A) or Galactose (B) linked a1,3 to O antigen Type A and B glycosyltransferase add specific sugars they differ by 4aa out of 354aa O phenotype is result of mutation resulting in no active glycosyltransferases AB is the universal recipient O is the universal donor

Lectins      

Proteins that recognise and bind specific carbohydrate structures Can be divided on the basis of their amino acid sequence and biochemical properties Serve a wide variety of cell recognition and adhesion processes Lectins recognise ‘old’ erythrocytes and remove them. Some peptide hormones circulatory lifetime influenced by oligosaccharide moieties. Microbial/Viral pathogens utilise lectins for adhesion or toxin entry (recognise specific oligosaccharides of host cells). Selectins are a family of lectins involved in cell-cell recognition and adhesion e.g. the movement of T-lymphocytes to a site of infection or inflammation

Influenza    

Hemagglutinin (lectin) recognises sialic acid residues on host surface glycoproteins.(H1-3 human, 16 so far) After penetrating the membrane neuramidase (sialidase) breaks the glycosidic bond to release the virus Without neuramidase, virus non-invasive Tamiflu and Relenza inhibit the enzyme...