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FOOD CHEMISTRY
BY
DR BOOMINATHAN Ph.D.
M.Sc.,(Med. Bio, JIPMER), M.Sc.,(FGSWI, Israel), Ph.D (NUS, SINGAPORE)
PONDICHERRY UNIVERSITY
II lecture
2/August/2012
Glycosidic Bonds
The anomeric hydroxyl and a hydroxyl of another sugar
or some other compound can join together, splitting out
water to form a glycosidic bond:
R-OH + HO-R'  R-O-R' + H2O
E.g., methanol reacts with the anomeric OH on glucose
to form methyl glucoside (methyl-glucopyranose).
H OH
H OH
H2O
H O
HO
HO
H
H
H
+
CH3-OH
H O
HO
HO
H
OH
H
OH
-D-glucopyranose
methanol
H
OH
OCH3
methyl--D-glucopyranose
Disaccharides:
Maltose, a cleavage
product of Starch
(e.g., amylose), is a
disaccharide with an
(1 4) glycosidic
link between C1 - C4
OH of 2 glucoses.
It is the  anomer
(C1 O points down).

6 CH2OH
H
5
O
H
OH G
4
OH
3
H
1
4
maltose
b
OH
H
O
H
4
2
OH
OH
5
O
H
OH
H
H
3
OH
2
6 CH2OH
H
1
H
H
1
H
3
O
H
OH
G
O
2
OH
5
O
H
OH
4
6 CH2OH
H
5
H
H
H
6 CH2OH
3
cellobiose
H
2
OH
1
H
OH
Cellobiose, a product of cellulose breakdown, is the otherwise
equivalent b anomer (O on C1 points up).
The b(1 4) glycosidic linkage is represented as a zig-zag, but
one glucose is actually flipped over relative to the other.
Other disaccharides include:
 Sucrose, common table sugar, has a glycosidic bond
linking the anomeric hydroxyls of glucose & fructose.
Because the configuration at the anomeric C of glucose
is  (O points down from ring), the linkage is (12).
The full name of sucrose is -D-glucopyranosyl-(12)b-D-fructopyranose.)
 Lactose, milk sugar, is composed of galactose & glucose,
with b(14) linkage from the anomeric OH of galactose.
 Its full name is b-D-galactopyranosyl-(1 4)--Dglucopyranose
CH2OH
H
O
H
OH
H
H
H
1
O
OH
6CH OH
2
5
O
H
4 OH
3
H
OH
H
H
H
H 1
O
H
OH
CH2OH
CH2OH
CH2OH
H
H
H
O
H
OH
H
O
O
H
H
O
H
OH
H
O
OH
2
OH
H
OH
H
OH
H
H
OH
amylose
Polysaccharides:
Plants store glucose as amylose or amylopectin, glucose
polymers collectively called starch.
Glucose storage in polymeric form minimizes osmotic
effects.
Amylose is a glucose polymer with (14) linkages.
The end of the polysaccharide with an anomeric C1 not
involved in a glycosidic bond is called the reducing end.
CH2OH
CH2OH
O
H
H
OH
H
H
OH
H
O
OH
CH2OH
H
H
OH
H
H
OH
H
H
OH
CH2OH
O
H
OH
O
H
OH
H
H
O
O
H
OH
H
H
OH
H
H
O
4
amylopectin
H
(16)
1
(14)
O
6 CH2
5
H
OH
3
H
CH2OH
O
H
2
OH
H
H
1
O
CH2OH
O
H
4 OH
H
H
H
H
O
OH
O
H
OH
H
H
OH
H
OH
Amylopectin is a glucose polymer with mainly (14)
linkages, but it also has branches formed by (16)
linkages. Branches are generally longer than shown above.
* Why branches?
The branches produce a compact structure & provide
multiple chain ends at which enzymatic cleavage can occur.
CH2OH
CH2OH
O
H
H
OH
H
H
OH
H
O
OH
CH2OH
H
H
OH
H
H
OH
H
H
OH
CH2OH
O
H
OH
O
H
OH
H
H
O
O
H
OH
H
H
OH
H
H
O
4
glycogen
H
(16) branches
1
O
6 CH2
5
H
OH
3
H
CH2OH
O
H
2
OH
H
H
1
O
CH2OH
O
H
4 OH
H
H
H
H
O
OH
O
H
OH
H
H
OH
H
OH
Glycogen, the glucose storage polymer in animals, is similar in
structure to amylopectin.
But glycogen has more (16) branches.
Highly Branched Structure:
The highly branched structure permits rapid glucose release
from glycogen stores, e.g., in muscle during exercise.
The ability to rapidly mobilize glucose is more essential to
animals than to plants.
Glycosaminoglycans
CH2OH
D-glucuronate
6COO
H
4
6

5
H
OH
3
H
H
2
OH
1
H
H
OH
O
O
H
4
O
H
5
3
H
2
1 O
H
NHCOCH3
N-acetyl-D-glucosamine
hyaluronate
Glycosaminoglycans (mucopolysaccharides) are
linear polymers of repeating disaccharides.
* The constituent monosaccharides tend to be modified,
with acidic groups, amino groups, sulfated hydroxyl and
amino groups, etc.
* Glycosaminoglycans tend to be negatively charged,
because of the prevalence of acidic groups.
Hyaluronate
CH 2OH
D-glucuronate
6

6COO
H
4
5
H
OH
3
H
H
2
OH
1
H
H
OH
O
O
H
4
O
H
5
3
H
2
1 O
H
NHCOCH 3
N-acetyl-D-glucosamine
hyaluronate
Hyaluronate (hyaluronan) is a glycosaminoglycan with a
repeating disaccharide consisting of 2 glucose derivatives,
glucuronate (glucuronic acid) & N-acetyl-glucosamine.
The glycosidic linkages are b(13) & b(14).
core
protein
heparan sulfate
glycosaminoglycan
transmembrane
-helix
cytosol
Proteoglycans are glycosaminoglycans that are covalently
linked to serine residues of specific core proteins.
The glycosaminoglycan chain is synthesized by sequential
addition of sugar residues to the core protein.
Glycosaminoglycans
* The most abundant heteropolysaccharides in the body are
glycosaminoglycans.
the
* These molecules are long unbranched polysaccharides containing a
repeating disaccharide unit.
* The disaccharide units contain either of two modified sugars--- Nacetylgalactosamine (GalNAc) or N-acetylglucosamine (GlcNAc) and a
uronic acid such as glucuronate or iduronate.
GAGs are highly negatively charged molecules, with extended
conformation that imparts high viscosity to the solution.
Glycosaminoglycans
Dermatan sulfate
composed of
L-iduronate
(many are sulfated)
+
GalNAc-4-sulfate
linkages is b (1, 3)
Glycosaminoglycans
* GAGs are located primarily on the surface of cells or in the
extracellular matrix (ECM).
* Along with the high viscosity of GAGs comes low
compressibility, which makes these molecules ideal for a
lubricating fluid in the joints.
* At the same time, their rigidity provides structural integrity to
cells and provides passageways between cells, allowing for cell
migration.
* The specific GAGs of physiological significance are hyaluronic
acid, dermatan sulfate, chondroitin sulfate, heparin, heparan
sulfate, and keratan sulfate.
Characteristics of GAGs
Localization
Comments
Hyaluronate
synovial fluid, vitreous humor,
ECM of loose connective tissue
large polymers, shock
absorbing
Chondroitin sulfate
cartilage, bone, heart valves
most abundant GAG
Heparan sulfate
basement membranes,
components of cell surfaces
contains higher acetylated
glucosamine than heparin
Heparin
component of intracellular
granules of mast cells
lining the arteries of the lungs,
liver and skin
more sulfated than heparan
sulfates
Dermatan sulfate
skin, blood vessels, heart
valves
Keratan sulfate
cornea, bone,
cartilage aggregated with
chondroitin sulfates
GAG
Some proteoglycans of the extracellular matrix bind
noncovalently to hyaluronate via protein domains called link
modules. E.g.:
• Multiple copies of the aggrecan proteoglycan associate
with hyaluronate in cartilage to form large complexes.
• Versican, another proteoglycan, binds hyaluronate in the
extracellular matrix of loose connective tissues.
CH 2OH
D-glucuronate

6COO
Websites on:
Aggrecan
Aggrecan &
versican.
6
H
4
5
H
OH
3
H
hyaluronate
H
2
OH
1
H
H
OH
O
O
H
4
O
H
5
3
H
2
1 O
H
NHCOCH 3
N-acetyl-D-glucosamine
N-sulfo-glucosamine-6-sulfate
iduronate-2-sulfate
CH2OSO3
H
H
COO
OH
O
O
H
O
H
H
OH
H
H
H
H
OSO3
O
H
NHSO3
heparin or heparan sulfate - examples of residues
Heparan sulfate is initially synthesized on a membraneembedded core protein as a polymer of alternating
N-acetylglucosamine and glucuronate residues.
Later, in segments of the polymer, glucuronate residues
may be converted to the sulfated sugar iduronic acid,
while N-acetylglucosamine residues may be deacetylated
and/or sulfated.
Heparin
Heparin, a soluble glycosaminoglycan found in
granules of mast cells, has a structure similar to that of
heparan sulfates, but is more highly sulfated.
When released into the blood, it inhibits clot formation
by interacting with the protein antithrombin.
Heparin has an extended helical conformation.
Charge repulsion by the many negatively charged groups
may contribute to this conformation.
Some cell surface heparan
sulfate glycosaminoglycans
remain covalently linked to
core proteins embedded in
the plasma membrane.
core
protein
heparan sulfate
glycosaminoglycan
transmembrane
-helix
cytosol
 The core protein of a syndecan heparan sulfate
proteoglycan includes a single transmembrane -helix,
as in the simplified diagram above.
 The core protein of a glypican heparan sulfate
proteoglycan is attached to the outer surface of the
plasma membrane via covalent linkage to a modified
phosphatidylinositol lipid.
Proteins involved in signaling & adhesion at the cell
surface recognize & bind heparan sulfate chains.
E.g., binding of some growth factors (small proteins) to
cell surface receptors is enhanced by their binding also to
heparan sulfates.
Regulated cell surface Sulf enzymes may remove sulfate
groups at particular locations on heparan sulfate chains
to alter affinity
N-sulfo-glucosamine-6-sulfate
iduronate-2-sulfate
for signal
CH2OSO3
H
proteins, e.g.,
O
H
H
growth factors. H COO O
H
Diagram
by Kirkpatrick &
Selleck.
OH
O
H
OH
H
H
H
OSO3
O
H
NHSO3
heparin or heparan sulfate - examples of residues
C
CH2OH
Oligosaccharides
that are covalently
attached to proteins
or to membrane
lipids may be linear
or branched chains.
O
H
H
OH
O
CH2
CH
NH
H
O
serine
residue
O H
OH
H
HN
C
CH3
b-D-N-acetylglucosamine
O-linked oligosaccharide chains of glycoproteins vary in
complexity.
They link to a protein via a glycosidic bond between a
sugar residue & a serine or threonine OH.
O-linked oligosaccharides have roles in recognition,
interaction, and enzyme regulation.
C
CH2OH
O
H
H
OH
O
CH2
CH
NH
H
O
serine
residue
O H
OH
H
HN
C
CH3
b-D-N-acetylglucosamine
N-acetylglucosamine (GlcNAc) is a common O-linked
glycosylation of protein serine or threonine residues.
Many cellular proteins, including enzymes & transcription
factors, are regulated by reversible GlcNAc attachment.
Often attachment of GlcNAc to a protein OH alternates
with phosphorylation, with these 2 modifications having
opposite regulatory effects (stimulation or inhibition).
CH2OH
O
O
H
H
OH
HN
C
HN
CH2
C
H
H
OH
H
HN
C
CH3
O
N-acetylglucosamine
Initial sugar in N-linked
glycoprotein oligosaccharide
Asn
CH
O
HN
HC
R
C
O
X
HN
HC
R
C
O
Ser or Thr
N-linked oligosaccharides of glycoproteins tend to be
complex and branched.
First N-acetylglucosamine is linked to a protein via the
side-chain N of an asparagine residue in a particular
3-amino acid sequence.
NAN
NAN
NAN
Gal
Gal
Gal
NAG
NAG
NAG
Man
Man
Man
Key:
NAG
NAG
Asn
N-linked oligosaccharide
Fuc
NAN = N-acetylneuraminate
Gal = galactose
NAG = N-acetylglucosamine
Man = mannose
Fuc = fucose
Additional monosaccharides are added, and the N-linked
oligosaccharide chain is modified by removal and addition
of residues, to yield a characteristic branched structure.
Many proteins secreted by cells have attached N-linked
oligosaccharide chains.
Genetic diseases have been attributed to deficiency of
particular enzymes involved in synthesizing or modifying
oligosaccharide chains of these glycoproteins.
Such diseases, and gene knockout studies in mice, have
been used to define pathways of modification of
oligosaccharide chains of glycoproteins and glycolipids.
* Carbohydrate chains of plasma membrane
glycoproteins and glycolipids usually face the outside of
the cell.
They have roles in cell-cell interaction and signaling, and
in forming a protective layer on the surface of some
cells.
Lectins
Lectins are glycoproteins that recognize and bind to specific
oligosaccharides.
Concanavalin A & wheat germ agglutinin are plant lectins
that have been useful research tools.
The C-type lectin-like domain is a Ca++-binding
carbohydrate recognition domain in many animal lectins.
Recognition/binding of CHO moieties of glycoproteins,
glycolipids & proteoglycans by animal lectins is a factor in:
• cell-cell recognition
• adhesion of cells to the extracellular matrix
• interaction of cells with chemokines and growth factors
• recognition of disease-causing microorganisms
• initiation and control of inflammation.
Examples of lectins
Mannan-binding lectin (MBL) is a glycoprotein found in
blood plasma.
It binds cell surface carbohydrates of disease-causing
microorganisms & promotes phagocytosis of these
organisms as part of the immune response.
Selectins
Selectins are integral proteins
of mammalian cell plasma
membranes with roles in
cell-cell recognition & binding.
selectin
lectin domain
outside
transmembrane
The C-type lectin-like domain
-helix
is at the end of a multi-domain
cytosol
cytoskeleton
extracellular segment
binding domain
extending out from the cell
surface.
A cleavage site just outside the transmembrane -helix
provides a mechanism for regulated release of some lectins
from the cell surface.
A cytosolic domain participates in regulated interaction
with the actin cytoskeleton.