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Carbohydrates
BIOMEDICAL IMPORTANCE

Carbohydrates are the most abundant organic molecules in nature.

The empiric formula for many of the simpler carbohydrates is (CH2O)n, hence the name
"hydrate of carbon."

They have a wide range of functions, including:
1.
providing a significant fraction of the energy in the diet of most organisms.
2.
acting as a storage form of energy in the body (glycogen).
3.
serving as cell membrane components that mediate some forms of intercellular
communication.
4.
Carbohydrates also serve as a structural component of many organisms, including the
cell walls of bacteria, the exoskeleton of many insects, and the fibrous cellulose of plants.

Diseases associated with carbohydrate metabolism include diabetes mellitus,
galactosemia, glycogen storage diseases, and lactose intolerance
Classification of carbohydrates :

Carbohydrates are classified as :
1. Monosaccharides are those sugars that contain one unit of carbogydrate and
cannot be hydrolyzed into simpler carbohydrates.
2. Disaccharides are condensation products of two monosaccharide units; examples
are maltose and sucrose.
3. Oligosaccharides are condensation products of three to ten monosaccharides. Most
are not digested by human enzymes.
4. Polysaccharides are condensation products of more than ten monosaccharide units;
examples are the starches and dextrins, which may be linear or branched polymers.

Monosaccharides can be classified according to the
number of carbon atoms they contain. Examples of some
monosaccharides commonly found in humans are listed
in Figure 7.1 .

Carbohydrates are chemically characterized as:

Carbohydrates with an aldehyde as their most oxidized
functional group are called aldoses.

Carbohydrates with a keto group as their most oxidized
functional group are called ketoses.

For example, glyceraldehyde is an aldose, whereas
dihydroxyacetone is a ketose.

Monosaccharides can be linked by glycosidic bonds to
create larger structures (Figure 7.3).
Table 14–1. Classification of Important Sugars
Trioses (C3 H6 O3 )
Tetroses(C4 H8 O4 )
Aldoses
Ketoses
Glycerose (glyceraldehyde)
Dihydroxyacetone
Erythrose
Erythrulose
Pentoses (C5 H10 O5 )
Ribose
Ribulose
Hexoses (C6 H12 O6 )
Glucose
Fructose
Heptoses (C7 H14 O7 )
Sedoheptulose
BIOMEDICALLY, GLUCOSE IS THE MOST IMPORTANT
MONOSACCHARIDE

Glucose is the major metabolic fuel of mammals and a universal
fuel of the fetus. It is the precursor for synthesis of all the other
carbohydrates in the body, including for storage glycogen; ribose
and deoxyribose in nucleic acids; galactose in lactose of milk, in
glycolipids, and in combination with protein in glycoproteins and
proteoglycans.
The Structure of Glucose Can Be Represented in
Three Ways
The straight-chain structural formula (aldohexose; Figure (A) .

a cyclic structure (Figure B).

The six-membered ring containing one oxygen atom is actually in
the form of a chair (Figure C).
Sugars Exhibit Various Forms of Isomerism
Isomers& epimers

Isomers are molecules that have the same molecular formula, but
have a different arrangement of the atoms in space. (different
structures). For example, fructose, glucose, mannose, and galactose
are all isomers of each other, having the same chemical formula
C6H12O6.
1. Epimers If two monosaccharides differ in configuration around only one
specific carbon atom (with the exception of the carbonyl carbon, they
are defined as epimers of each other. (Of course, they are also isomers!)
For example, glucose and galactose are epimers in C-4 structures differ
only in the position of the -OH group at carbon 4. [Note: The carbons in
sugars are numbered beginning at the end that contains the carbonyl
carbon that is, the aldehyde or keto group .

Glucose and mannose are C-2 epimers.

However, galactose and mannose are NOT epimers—they differ in the
position of -OH groups at two carbons (2 and 4) and are, therefore,
defined only as isomers
2. D and L isomerism:

A special type of isomerism is found in the pairs of structures
that are mirror images of each other. These mirror images are
called enantiomers, and the two members of the pair are
designated as a D- and an L-sugar.

The orientation of the —H and —OH groups around the
carbon atom adjacent to the terminal primary alcohol carbon
(carbon 5 in glucose) determines whether the sugar belongs to
the D or L series.

When the —OH group on this carbon is on the right , the
sugar is the D isomer; when it is on the left, it is the L isomer.

D-glucose and L-glucose are enantiomers.

Most of the monosaccharides in mammals are D sugars, and
the enzymes responsible for their metabolism are specific for
this configuration.
3. Cyclization of monosaccharides (Pyranose and
furanose)

Less than one percent of each of the
monosaccharides with five or more carbons exists in
the open-chain (acyclic) form.

Rather, they are predominantly found in a ring form, in
which the aldehyde (or ketone) group has reacted
with an alcohol group on the same sugar.

The ring structures of monosaccharides are similar to
the ring structures of either pyran (a six-membered
ring) or furan (a five-membered ring) (Figures 14–3 &
14–4).

For glucose in solution, more than 99% is in the
pyranose form.
Figures 14–3 Pyranose and furanose forms of glucose.
4. Anomeric carbon (Alpha and beta anomers) :

Anomeric carbon: Formation of a ring results
in the creation of an anomeric carbon at carbon
1 of an aldose or at carbon 2 of a ketose. These
structures are designated the α or β
configuration of the sugar, for example, α -Dglucose and β -D-glucose .These two sugars are
both glucose, but they are anomeric of each
other.
Enzymes can distinguished between these two
forms:

Glycogen is synthesized from α-D
glucopyranose Cellulose is synthesized from β D glucopyranose
Figures 14–4
5. Optical Activity

When a plane polarized light is passed through a solution containing
monosaccharides the light will either be rotated towards right or left.

This rotation is because of the presence of asymmetric carbon atom.

If it is rotated towards left- levorotatory (-)

If it is rotated towards right- dextrorotatory(+)
6. Aldose-ketose isomerism:
Fructose has the same molecular formula as glucose but differs in its structural
formula, since there is a potential keto group in position 2, the anomeric carbon of
fructose (Figures 14–4), whereas there is a potential aldehyde group in position 1,
the anomeric carbon of glucose (Figures 14–3).
Reducing sugars:

Sugars in which the oxygen of the anomeric carbon (the carbonyl
group) is free and not attached to any other structure, such sugars
can act as reducing agents and are called reducing sugars

A reducing sugar can react with chemical reagents (for example,
Benedict's solution) and reduce the reactive component, with the
anomeric carbon becoming oxidized.

[Note: Only the state of the oxygen on the anomeric carbon
determines if the sugar is reducing or nonreducing—the other
hydroxyl groups on the molecule are not involved.]
Many Monosaccharides Are Physiologically
Important

Derivatives of trioses, tetroses, and pentoses and of a seven-carbon sugar (sedoheptulose)
are formed as metabolic intermediates in glycolysis and the pentose phosphate pathway .

Pentoses are important in nucleotides, nucleic acids, and several coenzymes .

Glucose, galactose, fructose, and mannose are physiologically the most important hexoses.

In addition, carboxylic acid derivatives of glucose are important, including D -glucuronate (for
glucuronide formation and in glycosaminoglycans) and its metabolic derivative, L -iduronate
(in glycosaminoglycans)
Deoxy Sugars Lack an Oxygen Atom

Deoxy sugars are those in which one hydroxyl group has been
replaced by hydrogen. An example is deoxyribosein DNA.
Sugar derivatives

sugar alcohol - lacks an aldehyde or ketone; e.g., ribitol.

sugar acid - the aldehyde at C1, or OH at C6, is oxidized to a carboxylic acid; e.g.,
gluconic acid, glucuronic acid.
H
H
H
C
C
C
OH
OH
OH
CH2OH
D-ribitol
CHO
COOH
CH2OH
H
C
OH
HO
C
H
OH
H
C
OH
OH
H
C
OH
H
C
OH
HO
C
H
H
C
H
C
CH2OH
D-gluconic acid
COOH
D-glucuronic acid
Sugar derivatives
amino sugar - an amino group substitutes for a hydroxyl.
The amino sugars Are Components of Glycoproteins, Gangliosides, & Glycosaminoglycans
include D -glucosamine, a constituent of hyaluronic acid , D -galactosamine, a constituent of
chondroitin and D -mannosamine.
The amino group may be acetylated, as in N-acetylglucosamine.
Several antibiotics (eg, erythromycin) contain amino sugars, which are important for their
antibiotic activity.
CH2OH
CH2OH
O
H
H
OH
H
H
OH
H
OH
OH
H
NH2
-D-glucosamine
O
H
H
H
O OH
OH
H
N
H
C
CH3
-D-N-acetylglucosamine
Sugar derivatives
N-acetylneuraminate (N-acetylneuraminic acid, also called sialic acid) is often found as
a terminal residue of oligosaccharide chains of glycoproteins.
Sialic acid imparts negative charge to glycoproteins, because its carboxyl group tends
to dissociate a proton at physiological pH, as shown here.
H
O
H3C
C
O
NH
R
H
COO
H
R=
OH
H
HC
OH
HC
OH
CH2OH
OH
H
N-acetylneuraminate (sialic acid)
Glycosidic Bonds

Glycosides (glycosidic bond ) are formed by condensation between the hydroxyl group of
the anomeric carbon of a monosaccharide, and a second compound that may or may
not be another monosaccharide , splitting out water .

R-OH + HO-R'  R-O-R' + H2O

If the second group is a hydroxyl, the O-glycosidic bond is an acetal link because it results
from a reaction between a hemiacetal group (formed from an aldehyde) and another —
OH group.

If the hemiacetal portion is glucose, the resulting compound is a glucoside; if galactose, a
galactoside; and so on.

If the second group is an amine, an N -glycosidic bond is formed, eg, between adenine
and ribose in nucleotides such as ATP .
Naming glycosidic bonds:

Glycosidic bonds between sugars are named according to the numbers of the connected
carbons, and also with regard to the position of the hydroxyl group of the sugar involved in
the bond.

If this anomeric hydroxyl group is in the α configuration, the linkage is an α-bond. If it is in the β
configuration , the linkage is a β-bond.

Lactose, for example, is synthesized by forming a glycosidic bond between carbon 1 of a βgalactose and carbon 4 of
glucose. The linkage is, therefore , a β(1 —>4)
glycosidicbond .[Note: Because the anomeric
end of the glucose residue is not involved
in the glycosidic linkage it (and, therefore, lactose)
remains a reducing sugar.] .
Maltose, Sucrose, & Lactose Are Important
Disaccharides

The disaccharides are sugars composed of two monosaccharide residues linked by a
glycoside bond

The physiologically important disaccharides are maltose, sucrose, and lactose . Hydrolysis of
sucrose yields a mixture of glucose and fructose called "invert sugar" because fructose is
strongly levorotatory and changes (inverts) the weaker dextrorotatory action of sucrose.
POLYSACCHARIDES
Serve storage & structural functions
 polysaccharides

HOMO polysaccharides (all 1 type of monomer), e.g., glycogen, starch, cellulose, chitin

HETERO polysaccharides (different types of monomers), e.g., peptidoglycans,
glycosaminoglycans

Polysaccharides include the following physiologically important carbohydrates:

Starch is a homopolymer of glucose forming an -glucosidic chain, called a glucosan
or glucan.

It is the most important dietary carbohydrate in cereals, potatoes, legumes, and other
vegetables.
Starch and glycogen
Function: glucose storage
Starch -- 2 forms:
amylose (13–20%), which has a nonbranching
helical structure of α(1-> 4) linked glucose
residues (Figure A).
1.
CH2OH
O
H
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
H
O
OH
2
OH
H
OH
H
OH
H
OH
amylose
2.
amylopectin (80–85%), which consists of
branched chains of a(1-> 4) linked glucose
residues with a(1-> 6) linked branches (Figure B)
Starch and glycogen

Glycogen:
 branched
polymer of a(1-> 4) linked glucose residues with a(1-> 6)
linked branches
 like
amylopectin but even more highly branched and more compact
 branches
increase H2O-solubility
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
O
6 CH2
5
H
OH
3
H
CH2OH
O
H
2
OH
O
H
H
1
4
O
CH2OH
H
OH
H
H
H
H
O
OH
O
H
OH
H
H
OH
H
OH
Cellulose and chitin

Function: STRUCTURAL, rigidity important

Cellulose:
 homopolymer,
β(1-> 4) linked glucose residues
 cell walls of plants
CH2OH
O
H
H
OH
H
OH
H
1
O
H
H
OH
6CH OH
2
5
O
H
4 OH
3
H
H
H 1
2
OH
O
O
H
OH
CH2OH
CH2OH
CH2OH
H
H
O
O
H
OH
H
OH
O
H
O
H
OH
H
OH
OH
H
H
H
H
H
H
H
OH
cellulose

Chitin:
homopolymer,
β(1-> 4) linked N-acetylglucosamine residues
hard exoskeletons (shells) of arthropods (e.g., insects, lobsters and
crabs)
Metabolism of carbohydrates

The principal sites of dietary carbohydrate digestion are the mouth and intestinal
lumen. This digestion is rapid and is generally completed by the time the stomach
contents reach the junction of the duodenum and jejunum.

A. Digestion of carbohydrates begins in the mouth

The major dietary polysaccharides are of animal (glycogen) and plant origin (starch,
composed of amylose and amylopectin). salivary α-amylase acts briefly on dietary
starch in a random manner, breaking some a(1-»4) bonds.

Carbohydrate digestion halts temporarily in the stomach, because the high acidity
inactivates the salivary α-amylase.

B. Further digestion of carbohydrates by pancreatic enzymes occurs in the small
intestine

When the acidic stomach contents reach the small intestine, they are neutralized by
bicarbonate secreted by the pancreas, and pancreatic α-amylase continues the
process of starch digestion.

Final carbohydrate digestion by enzymes synthesized by the
intestinal mucosal cells

The final digestive processes occur at the mucosal lining of upper
jejunum .

isomaltase, cleaves the bond α(1-»6) in isomaltose

maltase cleaves maltose, both producing glucose.

sucrase cleaves sucrose producing glucose and fructose.

lactase (β-galactosidase) cleaves lactose producing galactose and
glucose. These enzymes are secreted through, and remain
associated with, the luminal side of the brush border membranes of
the intestinal mucosal cells.