BC10M: Introductory Biochemistry, 2005 Semester 2'

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BC10M Introductory Biochemistry, 2005 Semester 2.

• Friday 25 Feb.
• Lecture 17
• Structures functions of common carbohydrates
• Andrew Pearson

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Basic units called monosaccharides  are simple
sugars such as glucose and fructose. The
structures of monosaccharides can be drawn in
different ways, to highlight different
aspects   the Fischer projection is best for
showing the stereoisomerism of the various
substituents but the Howarth projection is more
useful to biochemists because it approximates the
3-D structures and helps explain stabilities.  
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Fischer projections
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Properties of monosaccharides.  Containing many
hydroxyl groups sugars interact easily with
water they are hydrophilic
water-soluble. Containing many hydroxyl groups
sugars are highly oxidised and can be easily
hydrolysed into fragments. Containing many
hydroxyl groups sugars are easily derivatised
to give related molecules, including
aminosugars, acetylated sugars, dimers,
oligomers and polymers.
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The relationship between the various isomers of
sugars is not complicated to biochemists
Biomolecules like sugars are selected because
of their 3-D structure, and the wrong isomer
simply does not fit into binding sites.   The
distinction between D- and L- glucose is not
something that many biochemists concern
themselves with nowadays some bacterial
products contain the odd L-sugar, all others are
D-.
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Precise 3dimensional fit between a substrate and
enzyme.
Ionic interaction
Hydrophobic Interaction
H-bond
Adapted From Voet Voet
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Disaccharides two monosaccharides joined
by O-glycosidic bond. Three occur naturally in
stable form, Lactose in breast milk (sugar
transport) Sucrose in plant sap (plant transport
sugar) Trehalose in insect haemolymph (for
transport) and as an osmoprotectant in fungi all
others shown in books are digestion products of
something bigger.
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Disaccharides
maltose
trehalose
sucrose
cellobiose
lactose
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Glycogen, in animals, is similar but has
a greater degree of branching.
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Cellulose is a linear homopolymer of glucose, but
unlike amylose the O-glycosidic bond is beta in
cellulose alpha in amylose.
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Cellulose note the H-bonds.
A tetrasaccharide from heparan sulphate note the
substituted sugars.
Repeating unit of chitin from insects and
crustaceans
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The raffinose series at the end of a number of
unusual plant storage polysaccharides. Our
intestinal digestive enzymes are unable to cleave
stachyose into smaller units but anaerobes in
our lower intestines can..
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The difference in size between proteoglycans whi
ch are giant macromolecules and glycoproteins,
which are small soluble protein
macromolecules.
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Just so stories how furanosides and pyranosides
got their names
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• Functions of Monosaccharides
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• broken down for energy (by either fermentation
  or respiration)
• interconverted/carbon skeleton used for
  biosyntheses
• solvation of hydrophobes in aqueous transport
  media such as serum
• transport form of energy in animal serum, plant
  sap and intracellularly in general.  

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• Oligosaccharides
• short branched heteropolymeric chains of
• monosaccharides, often derivatised,
• with high informational content in their normal
• hydrated state.
• Functions
• markers for outer surface of cell membranes
• protein targeting markers in endoplasmic
  reticulum system
• prevention of membrane protein flip-flop
• formation of glycocalyx

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From Lehninger
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Polysaccharides long chains of
monosaccharides, sometimes branched, either
Homopolymers (only one type of monosacch.)
(e.g. cellulose, glycogen, amylose,
amylopectin) or heteropolymers (more than one
monosacch.) (e.g. glycosaminoglycans, algal cell
wall gels, peptidoglycan, chitin)
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• Functions
• energy reserves in plants
• (starch and some of the flatulogenic polymers)
• energy reserves in animals
• glycogen
• structural in all except animals
• chitin, celluloses, peptidoglycan
• lubrication and shock-absorbing
• in synovial fluids, connective tissues

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CHO function table