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CARBOHYDRATES IICARBOHYDRATES II
Lecture Contents…Lecture Contents…
the disaccharides sucrose, maltose, lactose, cellulose
the oligosaccharides N-linked, O-linked
the polysaccharides homopolysaccharides heteropolysaccharides
ObjectivesObjectives after you have studied this chapter, you should be able to
answer these questions: what are the general structural features of oligosaccharides
and polysaccharides? what structural properties of cellulose? what account for its
widespread occurrence in plants? what are the most common homopolysaccharides and
heteropolysaccharides and what are their functions? what structural properties distinguish proteoglycans from
glycoproteins? what are the functions of proteoglycans and glycoproteins?
The DisaccharideThe Disaccharide 2 monosaccharides joined together to form a glycoside bond
glycosidic bond: bond bet. one anomeric C of a monosaccharide with a
hydroxyl grp of another monosaccharide cat. by an enzyme glycosidase via a condensation
reaction various types: (12); (13); (14) (16) & , β
configurations
crystalline, water soluble, sweet tasting e.g.: sucrose, lactose, maltose, cellobiose
How is a glycosidic bond formed?
Different types of glycosidic bondDifferent types of glycosidic bond
-linkages
-linkage
Alpha, where the C1 OH that is formed ends up on the same side of the ring as the -OH of C2
Beta, where the C1 OH that is formed ends up
on the opposite side of the ring from the -OH of C2
(11)
(12)
(13)
(14)
(16)
Common disaccharides:Common disaccharides:
sucrose
lactose
maltose
cellobiose
also known as table sugar
extracted from sugar cane, sugar beet
made up of glucose and fructose with (12) glycosidic bond
a nonreducing sugar
common sweetener – less sweet than fructose
hydrolysis by sucrase/invertase
-D-glucopyranosyl (12) -D-fructofuranoside
also known as milk sugar -makes up around 2-8% of the solids in milk
made up of galactose and glucose with (14) glycosidic bond
a reducing sugar
hydrolysis by lactase – (lacking this enzyme can cause lactose intolerance)-D-galactopyranosyl (14) -D-
glucopyranose
also known as malt sugar
intermediate product of starch hydrolysis
present in germinating grain, in a small proportion in corn syrup
made up of 2 molecules of glucose with (14) glycosidic bond
a reducing sugar
hydrolysis by maltase
-D-glucopyranosyl (14) -D-glucopyranose
a degradation product of cellulose
made up of 2 molecules of glucose linked by (14) glycosidic bond
a reducing sugar
does not freely exists in nature-D-glucopyranosyl (14) -D-
glucopyranose
The OligosaccharideThe Oligosaccharide consists of between 2 - 20 monosaccharides can be found linked to polypeptide/lipid in glycoprotein &
glycolipid functions:
glycoprotein – immune cell recognition, improve stability of protein folding, sperm-egg interaction
glycolipid - provide energy, markers for cell recognition 2 types of glycans:
N-linked – after N-glycosylation at Asn O-linked – after O-glycosylation at hydroxylysine,
hydroxyproline, Ser or Thr
Oligosaccharides linked to polypeptides forming glycoproteins
polypeptide chain
SerAsn
oligosaccharides
N-linked O-linked
The diversity of glycoproteins
Lipid part
Carbohydrate part
The PolysaccharideThe Polysaccharide store energy or as structural components
homopolysaccharide formed from one type of monosaccharide
e.g.: starch, glycogen, cellulose, chitin
heteropolysaccharide formed from 2/more types of
monosaccharide e.g.: glycosaminoglycan, peptidoglycan
StarchStarch main food storage for plants consists of:
-amylose – linear glucose polymer linked by (14) bonds
amylopectin – glucose units are linked in a linear way with (1→4) bonds; branching takes place with (1→6) bonds occurring each 24 to 30 glucose units
degradation enzymes: amylases
-glucosidase starch phosphorylase debranching enzyme
Homopolysaccharide
Amylose of starch
(a) The D-glucose residues of amylose linked through (14) glycosidic bonds
(b) The amylose polymer forms a left-handed helix
GlycogenGlycogen main animal food storage basic structure similar to amylopectin
but more branches, every 8-12 glucose residues
more compact degradation:
amylases glycogen phosphorylase
glycogen debranching enzyme
Homopolysaccharide
CelluloseCellulose
main component of plant cell wall linear polymer of glucose – 15000 residues
linked by β(14) linkage H-bonded structure form parallel microfibril –
strong & water resistant cross-linked with matrix containing other
polysaccharides & lignin – resistant to pressure
degradation – celullase
Homopolysaccharide
ChitinChitin
main component of arthropod’s exoskeleton as well as algae/fungi cell wall
similar to cellulose except: every C2–OH grp replaced by acetamide polymer of N-acetyl-D-glucosamine
bonded by β(14) linkage 3 types of microfibril: , β, degradation: chitinase
Homopolysaccharide
Struture of chitin…
Comparison between struture of cellulose and chitin…
GlycosaminoglycanGlycosaminoglycan
or mucopolysaccharide major structural importance in vertebrate
animals – e.g. proteoglycan in connective tissues, skin
polymers of repeating disaccharide units (i.e. N-acetylgalactosamine or N-acetylglucosamine or one of their derivatives) with either sulfate or carboxylate grps
5 main classes: hyaluronic acid, chondroitin sulfate, dermatan sulfate, heparin/heparan sulfate & keratan sulfate
Heteropolysaccharide
PeptidoglycanPeptidoglycan cross-linked complex of polysaccharides and peptides found in the cell
walls of bacteria protects bacteria from osmotic lysis framework: polysaccharide component – linear chains of alternating β(14)-linked
N-acetylglucosamine (NAG) & N-acetylmuramic acid (NAM) peptide component – D-amino acid containing tetrapeptide forms amide
linkage with NAM’s lactic acid residue neighbouring parallel peptidoglycan chains are covalently cross-linked
thru tetrapeptide side chains
Heteropolysaccharide
The peptidoglycan monomer of a gram-(+) bacterium: e.g. Staphylococcus aureus
The peptidoglycan monomer of a gram-(-) bacterium: e.g. Escherichia
coli
Comparison between gram-(-) and gram-(+) bacteria peptidoglycan framework
a. The peptidoglycan monomer
b. The peptidoglycan structure
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