Carbohydrates Note For Nurses Part V

Polysaccharides

• Polysaccharides are more complex substances. Some are polymers of
a single monosaccharide and are termed as Homopolysaccharides (Homoglycans),
e.g. starch, glycogen, etc.

• Some contain other groups other than carbohydrates such as
hexuronic acid and are called as Heteropolysaccharides (heteroglycans), e.g.
Mucopolysaccharides.

Homo-polysaccharides (Homoglycans)

1. Starch Starch is a polymer
of glucose, and occurs in many plants as storage foods. It may be found in the
leaves, and stem, as well as in roots, fruits and seeds where it is usually
present in greater concentration.

• Starch granules: Appear under
microscope as particles made up of concentric layers of material. They differ
in shape, size and markings according to the source .Starchy foods are mainstay
of our diet.

• Composition of starch granule: It consists of two polymeric units
of glucose called (i) Amylose and (ii) Amylopectin, but they differ in
molecular architecture and in certain properties.

• Solubility: Starch
granules are insoluble in cold water, but when their suspension is heated,
water is taken up and swelling occurs, viscosity increases and starch gels or
pastes are formed.

• Reaction with I2: Both the granules and the colloidal solutions
react with Iodine to give a blue color. This is chiefly due to amylose, which
forms a deep-blue complex, which dissociates on heating. Amylopectin solutions
are colored blue-violet or purple.

• Ester Formation: Starches are capable of forming esters with
either organic or inorganic acids.

• Hydrolysis of starch: It yields succession of polysaccharides of
gradually diminishing molecular size

2. Glycogen

Glycogen is the reserve carbohydrate of the animal, hence it is
called as animal starch. It has been shown to be present in plants which have
no chlorophyll systems, e.g. in fungi and yeasts. It is also found in large
amounts in oysters and other shell fish. In higher animals, it is deposited in
the liver and muscle as storage material which are readily available as
immediate source of energy. It is dextrorotatory with an [α] D 20° is very
rapid but ceases when the pH falls to 5.5 due to lactic acid formed from
glucose= +196° to +197°.

Structure:

Glycogens have a complex structure of highly branched chains. It is
a polymer of D-Glucose units and resemble amylopectin. Glucose units in main
stem are joined by α1 → 4 glucosidic linkages and branching occurs at branch
points by α1 → 6 glucosidic linkage. A branch point occurs for every 12 to 18
glucose units Formation of glycogen from glucose is called as Glycogenesis and
breakdown of glycogen to form glucose is called as glycogenolysis. Postmortem
glycogenolysis

3. Inulin It is a polymer of D-fructose and
has a low molecular weight (MW = 5000). It occurs in tubers of the Dehlia, in the
roots of the Jerusalem artichoke, dandelion and in the bulbs of onion and
garlic. It is a white, tasteless powder. It is levorotatory and gives no colour
with iodine. Acids hydrolyse it to D-fructose; similarly it is also hydrolysed
by the enzyme inulinase, which accompanies it in plants. It has no dietary
importance in human beings as inulinase is absent in human.

Biomedical Importance

• It is used in physiological investigation for determination of
the rate of glomerular filtration rate (GFR). • It has been also used for
estimation of body water (ECF) volume.

4. Cellulose Cellulose is a polymer
of glucose. It is not hydrolyzed readily by dilute acids, but heating with
fairly high concentrations of acids yields, the disaccharide Cellobiose and D-Glucose.
Cellobiose is made up of two molecules of D-Glucose linked together by
β-Glucosidic linkage between C1 and C4 of adjacent glucose units.

Biomedical Importance

Cellulose is a very stable insoluble compound. Since, it is the
main constituent of the supporting tissues of plants, it forms a considerable
part of our vegetable food. Herbivorous animals, with the help of bacteria, can
utilise a considerable proportion of the cellulose ingested, but in human
beings no cellulose splitting enzyme is secreted by GI mucosa, hence it is not
of any nutritional value. But it is of considerable human dietetic value that
it adds bulk to the intestinal contents (roughage) thereby stimulating
peristalsis and elimination of indigestible food residues.

5. Dextrins

When starch is partially hydrolysed by the action of acids or
enzymes, it is broken down into a number of products of lower molecular weight
known as dextrins (see hydrolysis of starch). They resemble starch by being
precipitable by alcohol, forming sticky, gummy masses.

Biomedical Importance

• Dextrin solutions are often used as mucilages (mucilages on the
back of the postage stamp)

• Starch hydrolysates consisting largely of dextrins and maltose
are widely used in infant feeding.

6. Dextrans It is a polymer
of D-Glucose. It is synthesised by the action of Leuconostoc mesenteroides, a
non-pathogenic gram +ve cocci in a sucrose medium. Exocellular enzyme produced
by the organisms bring about polymerisation of glucose moiety of sucrose
molecule, and forms the polysaccharide known as Dextrans. They differ from
dextrins in structure. They are made up of units of a number of D-Glucose
molecules, having α1 → 6, α1 → 4 or α1 → 3 glycosidic linkages, within each
unit and the units are joined together to form a network.

Clinical Aspect

Dextran solution, having molecular wt approx. 75,000 have been used
as Plasma Expander. When given IV, in cases of blood loss (haemorrhage), it
increases the blood volume. Because of their high viscosity, low osmotic
pressure, slow disintegration and utilisation, and slow elimination from the
body they remain in blood for many hours to exert its effect.

Disadvantage:

Only disadvantage is that it can interfere with grouping and
cross-matching, as it forms false agglutination (Roleux formfation). Hence
blood sample for grouping and cross-matching should be collected before
administration of dextran in a case of haemorrhage and blood loss, where blood
transfusion may be required.

7. Agar It is a homopolysaccharide. Made up
of repeated units of galactose which is sulphated. Present in seaweed. It is
obtained from them.

Biomedical Importance

• In human: Used as laxative in constipation. Like cellulose, it is
not digested, hence add bulk to the faeces (“roughage” value) and helps in its
propulsion.

• In microbiology: Agar is available in purified form. It dissolves
in hot water and on cooling it sets like gel. It is used in agar plate for
culture of bacteria.

Hetero-polysaccharides (Heteroglycans)

Mucopolysaccharides (MPS)

Jeanloz has suggested the name Glycosaminoglycans (GAG) to describe
this group of substances. They are usually composed of amino sugar and uronic
acid units as the principal components, though some are chiefly made up of
amino sugar and monosaccharide units without the presence of uronic acid.

Important ACcidic Hetro polysacheride

Hyaluronic Acid A sulphate
free mucopolysaccharide. It was first isolated from vitreous humour of eye.
Later it was found to be present in synovial fluid, skin, umbilical cord,
haemolytic streptococci and in rheumatic nodule. It occurs both free and
salt-like combination with proteins and forms so[1]called ground
substance of mesenchyme, an integral part of gel-like ground substance of
connective and other tissues.

Biomedical Importance

• The invasive power of some pathogenic organisms may be increased
because they secrete hyaluronidase. In the testicular secretions, it may
dissolve the viscid substances surrounding the ova to permit penetration of
spermatozoa.

• Clinically the enzyme is used to increase the efficiency of
absorption of solutions administered by clysis.

Sulphate containing heteropolysaccharides

Sulphate Containing Acid MPS 1. Keratan Sulphate (Kerato Sulphate)
A sulphate containing acid MPS. Found in costal cartilage, and cornea has been
isolated from bovine cornea. It has been reported to be present in Nucleus
pulposus and the wall of aorta.

2. Chondroitin Sulphates They are
principal MPS in the ground substance of mammalian tissues and cartilage. They
occur in combination with proteins and are called as Chondroproteins.

3. Heparin It is also
called α-Heparin. It is an anticoagulant present in liver and it is produced
mainly by mast cells of liver (Originally isolated from liver). In addition, it
is also found in lungs, thymus, spleen, walls of large arteries, skin and in
small quantities in blood.

Proteoglycans Chemistry And Functions
Chemistry

• Proteoglycans are conjugated proteins. Proteins called “core”
proteins are covalently linked to glycosaminoglycans (GAGs).

Any of the GAGs viz. hyaluronic acid (HA); keratan sulphates I and
II, chondroitin sulphates A, B, C, heparin and heparan sulphate can take part
in its formation.

• The amount of carbohydrates in proteoglycans is much greater
(upto 95%) as compared to glycoproteins.

Functions of Proteoglycans

• As a constituent of extracellular matrix or ground substance: Interacts with collagen and elastin

• Acts as polyanions: GAGS present
in proteoglycans are polyanions and hence bind to polycations and cations such
as Na and K. Thus attracts water by osmotic pressure into extracellular matrix
contributing to its turgor.

• Acts as a barrier in tissue:
Hyaluronic acid in tissues acts as a cementing substance and contributes to
tissue barrier which permit metabolites to pass through but resist penetration
by bacteria and other infective agents.

• Acts as lubricant in joints:
Hyaluronic acid in joints acts as a lubricant and shock absorbant.
Intraarticular injection of hyaluronic acid in knee joints is used to alleviate
pain in chronic osteoarthritis of knee joints.

• Role in release of hormone: Proteoglycans
like hyaluronic acid are present in storage or secretory granules, where they
play part in release of the contents of the granules.

• Role in cell migration in embryonic tissues: Hyaluronic acid is present in high concentration in embryonic
tissues and is considered to play an important role in cell migration during
morphogenesis and wound repair.

• Role in glomerular filtration:
Proteoglycans like hyaluronic acid is present in basement membrane (BM) of
glomerulus of kidney where it plays important role in chargeselectiveness of
glomerular filtration.

• Role as anticoagulant in vitro and in vivo: In vitro, heparin is used as an anticoagulant. 2 mg/10 ml of blood
is used. Most satisfactory anticoagulant as it does not produce a change in red
cell volume or interfere with its subsequent determinations.  In vivo, heparin is an important
anticoagulant. It binds with factor IX and XI, but its most important action is
with plasma antithrombin III. Binding of heparin to lysine residues in
antithrombin III produces conformational change which promotes the binding of
the latter to serine protease thrombin which is inhibited, thus fibrinogen is
not converted to fibrin

Four naturally occurring thrombin inhibitors in plasma are:

(i) Antithrombin III (75% of the activity)

(ii) α2-macroglobulin contributes remainder

(iii) Heparin cofactor II

(iv) α1-antitrypsin The last two shows minor activity.

• Role as a coenzyme: Heparin acts
in the body to increase the activity of the enzyme Lipoprotein lipase.

Heparin binds specifically
to the enzyme present in capillary walls, causing a release of the enzyme into
the circulation. Hence heparin is called as Clearing factor. • As a receptor of
cell: Proteoglycans like heparan sulphate are components of plasma membrane of
cells, where they may act as receptors and can participate in cell adhesion and
cell-cell interactions.

• Role in compressibility of cartilages: Chondroitin sulphates and hyaluronic acid are present in high concentration
in cartilages and have a role in compressibility of cartilage in weight
bearing.

• Role in sclera of eye: Dermatan
sulphate is present in sclera of the eye where it has an important function in
maintaining overall shape of the eye.

• Role in corneal transparency:
Keratan sulphate I is present in cornea of the eye and lie between the collagen
fibrils. It plays an important role in maintaining corneal transparency.

Biomedical Importance/Clinical Aspect Mucopolysaccharidoses: 

The mucopolysaccharidoses are a group of related disorders, due to
inherited enzyme defect,in which skeletal changes, mental retardation, visceral
involvement and corneal clouding are manifested to varying degrees.

Defect/defects in these disorders result in:

• Widespread deposits in tissues of a particular MPS

• In excessive excretion of MPS in urine.

 Reference:

Notes Made By The Help of “The Text Book of Medical Biochemistry By MN. Chatterjea 8th Edition“