Fundamentals of Biochemistry 2e

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Pabio 551 03/04/08 Leo Stamatatos Ph.D.
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Carbohydrates
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• Also called Saccharides
• The most abundant biological molecules
• Compounds of the generic formula (C.H2O)n (n3)
• Can also contain other atoms (N, S)

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Functions of Carbohydrates

• Energy - Fuel (glucose, ATP).
• - Storage (glycogen, starch)
• Structural elements - cell wall of bacteria,
  fungi, plants.
• - exoskeletons of arthropod.
• Metabolic intermediates amino acid, nucleotide
  synthesis
• Information storage/transfer DNA, RNA
• Cell-cell recognition/ signaling
• Immunomodulation

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Monosaccharides
- Derived from CO2 and H2O by photosynthesis -
They are classified according to (A) their
carbonyl group aldehydes or ketones
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Stereoisomers (or spatial isomers)
Asymmetric Centers or Chiral Centers a carbon
atom with four different constituents
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D aldoses of 3-6 carbons
Epimers different configuration around one C
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D ketoses of 3-6 carbons
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Cyclization of monosaccharides
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Anomeric carbon
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The two anomers freely interconvert in aqueous
solution At equilibrium D-glucose is a mixture
of the b anomer (63.6) and the a anomer (36.4)
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Sugars are Conformationally Variable
Chair conformation
Equatorial -Predominates -Position of bulkier
substituents
Axial
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Sugar Derivatives
Oxidation of the aldehyde group of aldoses
Aldonic acids
Uronic acids
Oxidation of the primary alcohol group of aldoses
Oxidation can be chemical or enzymatic
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Aldoses and ketoses can be reduced
Component of lipids
Component of flavin coenzymes
Sweetener
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Deoxy sugars one OH group is replaced by H
One of few L sugars of polysaccharides
Part of DNA
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Amino Sugars
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Constituent of glycoproteins and glycolipids.
Also known as sialic acid
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Polysaccharides
- Also known as glycans monosaccharides linked
together by glycosidic bonds
- Homopolysaccharides and heteropolysaccharides
- Form both branched and linear polymers (differ
from NA and proteins)

• Description of polysaccharides includes the
  identities, anomeric forms, linkages (exo- and
  endo-glycosidases) of all its component
  monosaccharide units

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Disaccharides
In milk
O-b-D-galactopyranosyl-(1 4)-D-glycopyranose
The most abundant disaccharide
O-a-D-glucopyranosyl-(1 2)-fructofuranoside
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Structural Polysaccharides
Composition similar to storage polysaccharides,
but small structural differences greatly
influence properties

• Cellulose is the most abundant natural polymer on
  earth.
• Accounts for over half of the carbon in the
  biosphere
• Cellulose is the principal strength and support
  of trees and plants.
• Chitin - exoskeletons of crustaceans, insects and
  spiders, and cell walls of fungi.

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Up to 15,000 D-glucose residues
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Storage Polysaccharides
Starch and Glycogen
Starch - A mixture of glycans (principal energy
source of plants) - Deposited in the
cloroplasts in the form of insoluble granules -
Composed of a-Amylose and Amylopectine
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Helical conformation because of the nature of the
a (1 4) bond Different than cellulose
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Up to 106 molecules of glucose residues mainly
a(1 4),but has branched molecules via a(1 6)
every 24-30 glucose residues.
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Digestion of Starch Salive amylase, randomly
hydrolyses the a(1 4) bonds Small Intestine
pancreatic amylase mixture of disaccharides
(maltose) and trisaccharides (maltotriose), and
oligosaccharides (dextrines) containing the a(1
6) branches (hydrolyzed by a-glucosidases and a
debranching enzyme). The monosaccharides are
then absorbed by the intestine and transported to
the blood stream
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Glycogen -Storage polysaccharide of animals
(skeletal muscle and liver) -Structurally it
resembles Amylopectin, but is more
branched It is degraded by glycogen
phosphorylase (cleaves the a(1 4) bonds) and a
glycogen debranching enzyme (cleaves the a(1 6)
bonds).
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Glycosaminoglycans -Part of the connective
tissue -Unbranched polysaccharides Alternating
Uronic Acid and hexosamine residues
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Glycoproteins

• Proteoglycans
• Bacterial cell walls
• Glycosylated proteins

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Proteoglycans (cartilage)
Hundreds
Up to 100 proteins
4,000 to 40,000 Å length
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Bacterial cell walls
Gram positive cell wall
100 500 Ao
Peptidoglycan Murein linear chain of
alternating N-acetyl-glucosamine (NAG) and
N-acetylmuramic acid (NAM). Each NAM bears a
tetrapeptide composed of alternating L and D
amino acids. Adjacent glycan chains are cross
linked into sheets by bonds between the
tetrapeptides. Teichonic acids polymers of
ribose or glycerol
Gram negative cell wall
Bilayer with usual phospholipids in the inner
leaflet but with LPS on the outer leaflet LPS is
toxic endotoxin Lipid A, core polysacharide, O
antigen polysaccharide side chains. LPS are
antigenic determinants
Murein
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The tetrapeptide of S. aureus
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Penicillin
Lysozyme
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Glycosylated proteins
N-linked oligosaccharides
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Synthesis of N-linked oligosaccharides
D2
D3
D1
ER
ER/Golgi
Golgi
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O-linked oligosaccharides
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• Functions of Oligosaccharides
• Structural role (proteins)
• Quality control during protein synthesis
• Mediate recognition events (lectins, selectins)
• Antigenic determinants (ABO group)

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