Carbohydrates

• Carbohydrate


  Sugar molecules made up of carbon, hydrogens and oxygens
• Roles of Sugar

  • Source of energy/stored fuels
  • Provide structure to cells and organisms
  • Major component on cell surface
  • Influence function of proteins
  • Specific recognition interactions
• Aldose


  Contains an aldehyde group (CHO)
• D - and L- stereoisomers

  Mirror images of each other - naturally occuring sugars are D
• Ketose


  Contains a ketone group (CO)
• Fisher Projection

  • Provide clear view of stereochemistry at each carbon centre
  • Horizonal lines: project out of paper
  • Vertical lines: project into paper
• Epimers


  Sugars that differ in configuration only around one carbon atom
• Chair Form


  Most energetically favourable over boat form
• Glycosidic Linkages


  • Hemiacetal of one links with -OH of another and condenses with the elimination of H2O to form a glycosidic bond
  • 11 different possibilities (since b1->a1 is the same as a1->b1)
• Naming Glycosidic Linkages

  1. Involved C-1 (hemiacetal) of first sugar

  2. Determine anomer of C-1 (a -> hydroxyl pointing down, b -> hydroxyl pointing up)

  3. Determine which carbon the left sugar bound by

  4. Zig-zag bond can be used so Haworth projections remain correct when aligned horizontally
• Branched Structures

  • Structural function: unbranched homopolysaccharides (cellulose & chitin)
  • Storage function: branched polysaccharides (starch, glycogen)
• Oligosaccharide Ends

  • Nonreducing end: anomeric carbon without OH group (unable to open)
  • Reducing end: anomeric carbon with OH group (able to open)
• Cellulose

  • Tough, water insoluble glucose polymer found in all cell walls
  • Glucose residues linked by (B 1->4) glycosidic bonds
  • Chains of cellulose interact to form stable supramolecular fibres of great strength
• Chitin

  • Main component of exoskeleton in arthropods
  • N-acetylglucosamine residues linked by (B 1->4) glycosidic bonds
  • Chains of chitin interact to form extended fibres
• Starch & Glycogen

  • Homopolymer of glucose residues
  • Glucose residues linked by (a 1->4) glycosidic bonds
  • Open helix structure rather than linear structure
• Glycoproteins


  • Carbohydrate groups covalently attached to proteins
  • Short oligosaccharide chains
  • Important functions in recognition
• Carbohydrate Linking

  1. Via Asparagine residues (N-glycans): Sugars are linked to amide side chain (NH)

  2. Via Serine or Threonine residues (O-glycans): Sugars are linked to the oxygen atom in the side chain (OH)
• ABO Groups

  • Carbohydrates attached to glycoproteins on surfaces of red blood cells
  • O -> Gal-GlcNAc-Gal-Fuc
  • A -> Gal-GlcNAc-Gal-Fuc & GalNAc
  • B -> Gal-GlcNAc-Gal-Fuc & Gal
• Gene Influence: ABO blood group

  1. Gene encodes a glycosyltransferase -> three alleles

  2. A allele: encodes an active glycosyltransferase (GalNAc)

  3. B allele: encodes an active glycosyltransferase (Gal)

  4. O allele: encodes a dead enzyme - no active glycosyltransferase
• Cell Recognition Using Carbohydrates

  • Lectins: broad family of proteins which recognise carbohydrates
  • C-type Lectins: large class involved in Calcium binding -> which acts as a bridge between protein & sugar through direct interaction with sugar hydroxyl groups
  • Selectins are C-type -> involved in cell-cell adhesion
• Relenza


  Anti-flu drug is a sialic acid analogue -> binds to active site on influenza
• Glucose Transporters

  • GLUT1: found in all cell types, Kt = 3
  • GLUT2: liver, pancreatic islets, intestine, Kt = 17
  • GLUT3: brain (neuronal), testis (sperm) - will always be working closest to Vmax due to low Kt, Kt = 1.4
  • GLUT4: muscle, fat, heart, Kt = 5
• Monosaccharide: basic unit of carbohydrates

  A) ketone

  B) aldehyde

  C) ketose

  D) aldose

  
• Disaccharide

  A) glucose

  B) glucose

  C) alpha

  D) maltose

  
• Disaccaride

  A) glucose

  B) galactose

  C) beta

  D) lactose

  
• Sugar Ends

  A) nonreducing

  B) reducing

  
• Polysaccharide

  A) cellulose

  B) glucose

  C) glucose

  D) glucose

  E) glucose

  
• Polysaccharide

  A) chitin

  B) glucosamine

  C) N-acetyl group

  
• Polysaccharide

  A) Starch & glycogen

  B) glucose

  C) glucose

  D) glucose

  E) glucose

  F) open helix

  
• Protein Glycosylation

  A) N-linked

  B) Asn

  
• Protein Glycosylation

  A) O-linked

  B) Ser

  
• ABO Groups: carbohydrates attached to glycoproteins on surfaces of red blood cells

  • ABO are glycosphingolipids

  A) O

  B) A

  C) B

  
• Selectins are C-type -> involved in cell-cell adhesion

  • Bind WBCs to sites of injury and allow movement of cells from bloodstream to site of infection
  • To infect host cell -> virus need to bind to receptors on cell -> receptors are sometimes carbohydrates
  • Influenza recognises sialic acid residues on cell-surface glycoproteins
  • Relenza: anti-flu drug is a sialic acid analogue -> binds to active site on influenza

  A) receptor

  B) salic acid

  C) neuraminidase

  D) receptor

  E) salic acid

  F) neuraminidase inhibitors

  G) relenza

  H) neuraminidase

  
• Glycosyltransferase: catalyse the biosynthesis of oligosaccharides by transfer of a monosaccharide from a nucleotide sugar to a carbohydrate acceptor
• Sucrose = glucose + fructose