Structural Carbohydrates

• Basic carbohydrate uni
• Usually 2:1 ratio H:O
• 2 stereoisomers = L and D (D natural)

• a and b glucose are created by nucleophilic attack
• Cyclic structures called glucopyranomes
• Conversion of glucose to diff forms in equilibrium (open chain 3% in solution)

Glycans - monosaccharides joined in condensation reactions

Two types of glycosidic linkages:

a or B:

Major structual carbohydrates in plants - strong

Linear polymer of D-B-glulose connected by B1->4 bonds

Chair structre representation:

Organised into microfibrils - many cellulose chains with 7,000-18,000 glucose molecules each

Structural carbohydrate in plant cell walls

B1->4 polymers of D-xylopyranose with sidegroups of various monosaccharides

Very good at cross-linking (branched)

Plant cell walls

Mixture of branched polysaccahrides rich in a1->4 linked D-galacturonic acid

Gelling agent in jams/marmalades

Some red algae have it within their cell walls

D-galactose with B1->4 that hase an ether bridge connecting C3 and C6.

Very complex with lots of substitutions

Found in animal exoskeletons e.g. arthropods

Polymer of N-acetylglucosamine with B1->4 linkages

In extended fibres similar to cellulose

Animals

Polymers with glucaronic acid with either N-aceylgalactosamine or N-acetylglucosamine

Either a or B linkages

Important as a matrix to hold together protein components

• Contain peptidoglycan
• Gram positive bacteria (iodine complex gets trapped in layera)
  • Thick multi-layered peptidoglycan wall at surface
• Gram negative bacteria 
  • Thin single-layered petidoglycan chain between membranes in periplasmic space
  • Simpler cell wall
  • Linked by terapeptides to one another
• Some antibiotics prevent final crosslinking by transpeptidase = broken cell wall
• Peptidoglycan:
• Composed of strictly alternating N-acetylglucosamine and N-acetylglmuramic acid (NAM)
• Linked by terapeptides of L-Ala-D-Glu-L-Lys-D-Ala (contain 2 D amino acids - rare in nature)

• Glycosylation - attatchment of oligosaccharides to proteins (post translational mod.)
• Proteins passing through secretory pathway (ER and golgi) form glycoproteins.
  • Most cytoplasmic proteins are not
• N-linked glycosylation (ER) - common
  • Attatchment of N-linked oligosaccharide by oligosccaryl transferase
  • Precursor held in ER membrane by diolichol (lipid) 
  • Occurs as protein is translated
• O-linked glycosylation (golgi)
  • Very simple structures
  • Linked to serine, threonine or hydroyproline
• Functions of glycosylation:
  • Helps to fold in the ER (can stop aggregation)
  • Helps direct protein to destination - lechins guide to golgi 
  • Helps function - improve stability and solubility (can also function in signalling)

GPI anchor to caboxyl group helps binding to lumeral/extracellular surfaces of membranes