CHAPTER 7
CARBOHYDRATES
MEDICAL BIOCHEMISTRY
Mr. Genaro Alderite, MSERM

Carbohydrates
- are the most abundant biological
molecules
-They are composed of C, O, H
according to the formula (C•H2O)n
where n ≥3
- are carbon compounds that contain
large quantities of hydroxyl groups.

I
(CH2O)n or H - C - OH
I

Outlined Functions of Carbohydrates:
1. Structural
-Main skeletal support-cellulose, chitin
-Network skeletal support- alginate,
pectin
2. Energy storage
-starch (amylose and amylopectin)
-glycogen
3. Transport function
-Glycoproteins

4. Regulatory function
- Some glycoproteins
5. Catalytic functions
- Some glycoproteins
6. Antigen-antibody interactions
- Some glycoproteins
7. Filtration barriers
- Gylycoproteins and proteoglycans

8. Cell differentiation
9.Cell lubrication
10. Components of cellular
membrane
11. Components of clotting proteins
- Fibrinogen
12. Components of protective cellular
coats

Carbohydrates are among the
most abundant compounds on
earth. They are normally broken
down into five major
classifications of carbohydrates:
-Monosaccharides
-Disaccharides
-Oligosaccharides
-Polysaccharides
-Nucleotides

Monosaccharides -are classified by
their number of carbon atoms :
Name Formula Relevant
Ex:
triose C3H6O3 Glyceraldehyde
tetrose C4H8O4 Erythrose
pentose C5H10O5 Ribose, Ribulose
hexose C6H12O6 Glucose, Galactose
heptose C7H14O7 Sedoheptulose
octose C8H16O8 Sialic acid

Monosaccharide
•aldose:contains an aldehyde group
•ketose:contains a ketone group
C
C OHH
C HHO
C OHH
C OHH
CH2OH
D-glucose
OH
C HHO
C OHH
C OHH
CH2OH
CH2OH
C O
D-fructose

Monosaccharides
- The common monosaccharides are
glucose, fructose, and galactose.

1. Glucose
-the main sugar metabolized by
the body for energy.
-The D-isomer of glucose
predominates in nature and it is
for this reason that the enzymes
in our body have adapted to
binding this form only.
-the concentration of glucose in
the bloodstream usually falls
within a narrow range of 70 to
115mg/100 ml of blood.

-Sources of glucose include
starch, the major storage form of
carbohydrate in plant.
Glucose

2. Galactose
-nearly identical to glucose in
structure except for one hydroxyl
group on carbon atom number four
of the six-sided sugar.
-Since it differs in only one
position about all six asymmetric
centers in the linear form of the
sugar, galactose is known as an
epimer of glucose.

- Galactose is not normally
found in nature in large quantities,
however it combines with glucose
to form lactose in milk.

3.Fructose -a structural isomer of
glucose, meaning it has the same
chemical formula but a completely
different three-dimensional
structure.
-The main difference is that
fructose is a ketone in its linear
form while glucose is an aldehyde.

-Sources of fructose include fruit,
honey and high-fructose corn syrup.

Fructose as a straight-chain carbohydrate

Disaccharides, meaning "two
sugars", are commonly found in
nature as sucrose, lactose and
maltose.
-formed by a condensation
reaction where one molecule of
water condenses or is released
during the joining of two
monosaccharides. The type of
bond that is formed between the
two sugars is called a glycosidic
bond.

CONDENSATION REACTION RESULTING IN GLYCOSIDIC BONDS OF
MALTOSE

Lactose is a disaccharide formed
through the condensation of
glucose and galactose. The bond
formed between the two
monosaccharides is called a beta
glycosidic bond .

GLUCOSE + GALACTOSE = GLUCOSE
Lactose: is found exclusively in the milk of mammals and
consists of galactose and glucose in a β–(1,4) glycosidic bond.

• Lactose or milk sugar occurs in the
milk of mammals - 4-6% in cow's milk
and 5-8% in human milk. It is also a by
product in the the manufacture of
cheese.
• Lactose intolerance is the inability to
digest significant amounts of lactose,
the predominant sugar of milk. This
inability results from a shortage of the
enzyme lactase, which is normally
produced by the cells that line the
small intestine. Lactase breaks down
the lactose, milk sugar, into glucose
and galactose that can then be
absorbed into the bloodstream.

Sucrose is found in common table
sugar and is composed of glucose
and fructose linked via a 1-2 alpha
glycosidic bond.

Sucrose
-is an excellent preservative
because it has no "reducing end"
or reactive group like the other
sugars.
-Other natural sources of sucrose
are found in plants such as sugar
cane, sugar beets, and maple
syrup

Maltose
-is the final disaccharide and consists
of two glucose molecules joined by an
alpha glycosidic bond.
-is an interesting compound because of
its use in alcohol production. Through a
process called fermentation, glucose,
maltose and other sugars are converted
to ethanol by yeast cells in the absence
of oxygen.

-Through an analogous process,
muscle cells convert glucose into
lactic acid to obtain energy while
the body operates under
anaerobic conditions.
-Although maltose is uncommon in
nature, it can be formed through
the breakdown of starch by the
enzymes of the mouth.

GLUCOSE + GLUCOSE= MALTOSE

Oligosaccharides
-Important oligosaccharides are
raffinose and stachyose.
-Composed of repeating units of
galactose, glucose and fructose,
these oligosaccharides are of
nutritional importance because
they are found in beans and
legumes.

-Because of their unique glycosidic bonds,
raffinose and stachyose cannot be broken
down into their simple sugars.
-Therefore, they cannot be absorbed by
the small intestine and are often
metabolized by bacteria in the large
intestine to form unwanted gaseous
byproducts.
-Commercial enzyme preparations such as
Beano can be consumed before a meal
rich in beans and legumes in order to aid
the small intestine in the breakdown of
these oligosaccharides.

Polysaccharides
- (Complex Carbohydrates)
- are usually monomers and consist
of thousands of repeating glucose
units. Naturally, they allow for the
storage of large quantities of
glucose.
Starch-the major storage form of
carbohydrate in plants and has two
different types: amylose (straight
chain polymer) and amylopectin
(highly branched).

These differences account for the
fact that amylopectin can form
stable starch gels which are able
to retain water while amylose is
unable to do so.
- Amylopectin is often used by
manufacturers to produce many
different kinds of thick sauces and
gravies.
-Sources of starch include
potatoes, beans, bread, pasta, rice
and other bread products

Glycogen
- is a highly branched polymer of
glucose that is the main storage
form of carbohydrate in humans.
- is stored in the liver and muscle
where it is synthesized and
degraded depending upon the
energy requirements of the body.

Indigestible forms of
polysaccharides are known as
dietary fiber and come in many
different forms including cellulose,
hemicellulose, pectin, gum and
mucilage.
-Cellulose is by far the most
abundant biochemical compound
on the earth because it forms part
of the structure of many plants.

-Cellulose and most forms of
hemicellulose are insoluble
fibers.
-Pectin, gum and mucilage are all
soluble fibers and readily
dissolve or swell when mixed
with water.

Nucleotides
Other sugars of importance are
found in nucleotides such as
deoxyribonucleic acid (DNA) and
ribonucleic acid (RNA). Both RNA
and DNA are five sided cyclic
sugars; however, RNA has one
more hydroxyl group than DNA.
Glucose-6-phosphate, an
intermediate in the breakdown of
glucose for energy, can be used for
the synthesis of these compounds.

Glycoproteins
- these are proteins with
carbohydrates attached to their
peptide portion.
- has the backbone which is
essentially protein but the
carbohydrate part of the molecule
takes the form of oligosaccharide
chains which are pendant and
covalently bound to the protein
chain.

Important glycoproteins in the body:
1. Glycophorin- located in the red
cell membrane
- measures the quantity of
sialic acid in the red blood cells
as the sign of aging.
2. Follicle stimulating hormone
3. Luteinizing hormone
4. Thyroid stimulating hormone

5.Blood group glycoproteins

6. Antibodies
a. Interleukin 1- inflammation and
response to injury
b. Interleukin 2- mediates
proliferation of T cells.
c.Interleukin 3- Effects early stage
of hematoporetic cell
differentiation
d. Interleukin 4-
Induces proliferation of B cells
e. Interleukin 5- stimulates
differentiation of B lymphocytes