Carbohydrates

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What is a Carbohydrate?

  • Most abundant organic molecule on the planet!
  • Simple carbohydrates: (C.H20)n where n>=3
  • Can also contain N, S or P
  • Monosaccharide = 1 sugar unit
  • Can link with other sugar units with glycosidic bonds
  • Disaccharide = 2 units
  • Oligosaccharide = app. 2-10 units
  • Polysaccharides = >=10 units
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Function of Carbohydrates - Metabolism

SOURCE AND STORAGE OF ENERGY 

Energy is trapped in photosynthesis as carbohydrates

Carbohydrate is main metabolic fuel

Polysaccharides such as starch and glycogen store energy

STARTING POINT FOR BIOSINTHESIS OF OTHER MACROMOLECULES

Carbon skeleton for macromolecules.

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Function of Carbohydrates - Structure

CELL ORGANELLES

Plant cell walls - cellulose

Bacterial cell walls - murein

Insect exoskeletons - chitin

Extracellular matrix - mucopolysaccharides

CELL-CELL RECOGNITION AND SIGNALLING

Glycoproteins and glycoplipids

IMMUNE SYSTEM

Key for recognition as foreign

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Stereoisomerism

Asymmetric carbon: has 4 different groups bound

Chrial: 4 different groups that can be organised to create mirror images

Enantiomers: stereoisomers that are mirror images of each other

Diastereoisomers: stereoisomers that are not mirror images (more than one chiral centre) and have different physical and chemical properties

Epimer: diastereoisomers that differ in configuration around 1 carbon only

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Oligo- and Poly- saccharides

Oligosaccharide: 2-10 monosaccharides joined by glycosidic bonds

Polysaccharide: More than 10 monosaccharides

Homo-polysaccharide: longer chains of all the SAME type of monosaccharide units

Hetero-polysaccharide: longer chains built up from MORE THAN ONE TYPE of monosaccharide unit

Storage polysaccharide: storage of chemical energy, e.g. starch and glycogen

Structural polysaccharide: role in structure of the cell, e.g. cellulose in plant cell walls

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Storage polysaccharides

Glucose cannot be stored as a monomer due to osmotic pressure.

Properties of storage molecules:

  • easily made and broken down
  • compact

Animals use glycogen

Plants use starch. Starch consists of two glucose polymers:

  • Amylopectin
  • Amylose
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Amylopectin

Homopolymer of glucose with a glycosidic link between alpha(1-4) and alpha(1-6)

Branches every 24-30 glucose molecules

Very large molecule - 10^6 glucose molecules

Helix and branched structures - compact

Branches - lots of sites for degradation

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Glycogen

Homopolymer of glycose with glycosidic link between alpha(1-4) and alpha(1-6) 

Branches every 8-12 glucose molecules

Highly branched - easily broken down by glycogen phosphorylase to give glucose-1-PO4

Debranching enzyme for alpha(1-6) branch points

Liver glycogen maintains blood glucose levels (~100g)

Muscle glycogen used in strenuous activity (~400g)

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Structural polysaccharides

Properties: 

  • Insoluble
  • Strong
  • Rigid

CELLULOSE

  • Homopolymer of glucose with beta(1-4) links
  • Up to 15000 glucose units
  • Found in all plants and fungi
  • Forms long straight chains
  • Every other glucose residue flipped 180 degrees
  • Extended chain
  • Parallel chains closely packed
  • Intra- and intechain H-bonding gives strength
  • Bundles of chains stack to form large rigid fibres
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Chitin - homopolysaccharide

Major structural component of insect and crustacea exoskeletons.

Cell walls of fungi and algae

Linear polymer of N-acetyl glucasamine with beta(1-4) links

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Glycosaminoglycans (mucopolysaccharides)

Long unbranched chains of repeating disaccharide units.

One sugar is N-acetyl glucosamine or N-acetyl galactosamine

Many carboxyl and sulphate groups - many negative charges

Chain extended, highly hydrated - very viscous, slimy mucous like consistency, elastic

Found in:

  • Ground substance of extracellular spaces
  • Synovial fluid of joints
  • Vitreous humor of the eye
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Hyaluronic acid

Ground substance of connective tissue and synovial fluid

Repeating disaccharide unit

D-glucuronate B(1-4) n-acetyl-glycosamine

High negative charge

Highly hydrated

Viscosity changes with shear force - shock absorber

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Glycoproteins

Found:

  • On the cell surface
  • In exported proteins e.g. extracellular matrix and blood

Distinguish between N-linked (N-X-S/T) and O-linked (S/T)

Involved in folding, signalling, recognition, increasing solubility

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Proteoglycan: Bottlebrush Model

Cartilage network of collagen fibres linked to mucopolysaccharide

Collagen provides strength and proteoglycans give resistance to compression

As cartilage compressed:

  • Water squeezed out
  • Negative charges exposed
  • Repel each other and resist further compression
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Glycosylation

Key in folding exported proteins

Possible outcomes of glycosylations:

  • Protein is ready for use
  • Protein needs folding
  • Protein is terminally misfolded
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