Carbohydrates , Medical Biochemistry

1. Muhammad Zafar
BS MLT
MPhil Microbiology
Carbohydrates

2. Carbohydrates
 one of the four major classes of biomolecules along
with proteins, nucleicacids, and lipids.
 Carbohydrates are aldehyde or ketone compounds
with multiple hydroxyl groups.
 They make up most of the organic matter on Earth
because of their extensive roles in all forms of life.
First, carbohydrates serve as energy stores, fuels,
and metabolic intermediates. Second, ribose and
deoxyribose sugars form part of the structural
framework
of RNA and DNA.

3. Aldehyde ???
 Aldehyde, any of a class of organic compounds,
in which a carbon atom shares a double bond
with an oxygen atom, a single bond with a
hydrogen atom, and a single bond with another
atom or group of atoms (designated R in general
chemical formulas and structure diagrams).
 The double bond between carbon and oxygen is
characteristic of all aldehydes and is known as
the carbonyl group.

4. Ketones ???
 Ketone, any of a class of organic
compounds characterized by the presence of
a carbonyl group in which the carbon atom is
covalently bonded to an oxygen atom. The
remaining two bonds are to other carbon atoms
or hydrocarbon radicals (R).

5. Aldehyde vs ketone

6. Carbohydrates
 Third, polysaccharides are structural elements in
the cell walls of bacteria and plants. In
fact,cellulose, the main constituent of plant cell
walls, is one of the most abundant organic
compounds in the biosphere.
Fourth, carbohydrates are linked to many proteins
( Glycoproteins) and lipids ( Glycolipids).

7. Carbohydrates
 Carbohydrates are built from monosaccharides,
small molecules that typically contain from three
to nine carbon atoms and vary in size and in the
stereochemical configuration at one or more
carbon centers. These monosaccharides may be
linked together to form a large variety of
oligosaccharide structures.
 Stereochemistry is the study of
how molecules are affected by the way
their atoms are arranged in space. It is also
known as 3D chemistry as the
word stereo means three dimensional

8. Monosaccharides
 Monosaccharides Are Aldehydes or Ketones with
Multiple Hydroxyl Groups :
 Monosaccharides, the simplest carbohydrates,
are aldehydes or ketones that have two or more
hydroxyl groups; the
empirical formula of many is (C-H2O)n, literally a
"carbon hydrate." Monosaccharides are important
fuel molecules as
well as building blocks for nucleic acids. The
smallest monosaccharides, for which n = 3, are
dihydroxyacetone and d and l-glyceraldehyde

9. Monosaccharides
 They are referred to as trioses (tri- for 3).
Dihydroxyacetone is called a ketose because it
contains a keto group, whereas
glyceraldehyde is called an aldose because it
contains an aldehyde group. Glyceraldehyde has
a single asymmetric
carbon and, thus, there are two stereoisomers of
this sugar. d-Glyceraldehyde and l-
glyceraldehyde are enantiomers, or
mirror images of each other
 In stereochemistry, stereoisomerism, or spatial
isomerism, is a form of isomerism in which
molecules have the same molecular formula and
sequence of bonded atoms, but differ in the
three-dimensional orientations of their atoms in

10.  enantiomers
 each of a pair of molecules that are mirror images
of each other.

12.  the prefixes d and l designate the absolute
configuration.
Monosaccharides and other sugars will often be
represented here by Fischer projections
 Recall that, in a Fischer projection of a molecule,
atoms joined to an asymmetric carbon atom by
horizontal bonds are in front of the plane of the
page, and those joined by vertical bonds are
behind
 Fischerprojections are useful for depicting
carbohydrate structures because they provide
clear and simple views of the
stereochemistry at each carbon center.

13.  If the hydroxyl group on the 5th carbon is to the
right of the molecule is a D-sugar. If the hydroxyl
group on the 5th carbon is to the left of the
molecule is L-sugar.

14. Fischer projection
 The Fischer projection, devised by
Emil Fischer in 1891, is a two-dimensional
representation of a three-dimensional organic
molecule by projection. Fischer
projections were originally proposed for the
depiction of carbohydrates and used by chemists,
particularly in organic chemistry and biochemistry.

15.  Simple monosaccharides with four, five, six, and
seven carbon atoms are called tetroses,
pentoses, hexoses, and
heptoses, respectively. Because these molecules
have multiple asymmetric carbons, they exist as
diastereoisomers,
isomers that are not mirror images of each other,
as well as enantiomers. In regard to these
monosaccharides, the
symbols d and l designate the absolute
configuration of the asymmetric carbon farthest
from the aldehyde or keto group.

16. Disaccharide
 A disaccharide consists of two sugars joined by an O-
glycosidic bond. Three abundant disaccharides are
sucrose,
lactose, and maltose .
 Sucrose (common table sugar) is obtained
commercially from cane or beet.
 The anomeric carbon atoms of a glucose unit and a
fructose unit are joined in this disaccharide; the
configuration of this
glycosidic linkage is alpha for glucose and β for
fructose. Sucrose can be cleaved into its component
monosaccharides by the enzyme sucrase.
 A glycosidic bond or glycosidic linkage is a type of
covalent bond that joins a carbohydrate (sugar)
molecule to another group, which may or may not be
another carbohydrate

17. Disaccharide
 Lactose, the disaccharide of milk, consists of
galactose joined to glucose by a β-1,4-glycosidic
linkage.
 Lactose is hydrolyzed to these monosaccharides
by lactase in human beings and by b-
galactosidase in bacteria.
In maltose, two glucose units are joined by an a -
1,4 glycosidic linkage, as stated earlier. Maltose
comes from the
hydrolysis of starch and is in turn hydrolyzed to
glucose by maltase. Sucrase, lactase, and
maltase are located on the
outer surfaces of epithelial cells lining the small

18. Complex Carbohydrates Are Formed
by Linkage of Monosaccharides
 Because sugars contain many hydroxyl groups,
glycosidic bonds can join one monosaccharide to
another.
Oligosaccharides are built by the linkage of two or
more monosaccharides by O-glycosidic bonds .
 In maltose, for example, two d-glucose residues
are joined by a glycosidic linkage between the a -
anomeric form of C-1 on
one sugar and the hydroxyl oxygen atom on C-4
of the adjacent sugar. Such a linkage is called an
a -1,4-glycosidic
bond.

19.  The fact that monosaccharides have multiple
hydroxyl groups means that various glycosidic
linkages are possible.

20. Glycogen and Starch Are
Mobilizable Stores of Glucose
 Large polymeric oligosaccharides, formed by the
linkage of multiple monosaccharides, are called
polysaccharides.
Polysaccharides play vital roles in energy storage
and in maintaining the structural integrity of an
organism. If all of the monosaccharides are the
same, these polymers are called homopolymers.
The most common homopolymer in animal
cells is glycogen, the storage form of glucose.
glycogen is a very large, branched polymer of
glucose residues.

21.  Most of the glucose units in glycogen are linked by a -
1,4-glycosidic bonds.
 The branches are formed by a -1,6-glycosidic bonds,
present about once in 10 units .
 The nutritional reservoir in plants is starch, of which
there are two forms.
 Amylose, the unbranched type of starch,consists of
glucose residues in a -1,4 linkage. Amylopectin, the
branched form, has about 1 a -1,6 linkage per 30 a -
1,4 linkages, More than half the carbohydrate
ingested by human beings is starch. Both amylopectin
and amylose are rapidly hydrolyzed by a-amylase, an
enzyme secreted by the salivary glands and the
pancreas.

22. Cellulose, the Major Structural
Polymer of Plants

Cellulose, the other major polysaccharide of
glucose found in plants, serves a structural rather
than a nutritional role.
Cellulose is one of the most abundant organic
compounds in the biosphere. Some 1015 kg of
cellulose is synthesized and
degraded on Earth each year. It is an unbranched
polymer of glucose residues joined by β-1,4
linkages. The β
configuration allows cellulose to form very long,
straight chains. Fibrils are formed by parallel
chains that interact with
one another through hydrogen bonds. The α-1,4
linkages in glycogen and starch produce a very

23.  These differing consequences of the α and β
linkages are biologically important. The straight
chain formed by β linkages is optimal for the
construction of fibers having a high tensile
strength. In contrast, the open helix formed by α
linkages is well suited to forming an accessible
store of sugar. Mammals lack cellulases and
therefore cannot digest wood and vegetable
fibers.

25.  Thank you

26. Refrences
 McMurry, J., Begley, T.P. and Begley, T.,
2005. The organic chemistry of biological
pathways. Roberts and Company Publisher
 Carey, F.A. and Sundberg, R.J., 2007. Advanced
organic chemistry: part A: structure and
mechanisms. Springer Science & Business
Media.