Carbohydrates and sugars

1. BIOCHEMISTRY
• Biochemistry is the study of chemical
processes in living organisms.
• Biochemistry deals with the structures,
functions and interactions of cellular
components such as : carbohydrates, lipids,
proteins and nucleic acids: the biomolecules.

2. CARBOHYRATES

3. Carbohydrates are one of the four major classes of
biomolecules along with proteins, nucleic acids and
lipids.
What are carbohydrates?
Carbohydrates are polyhydroxy aldehydes or ketones
or compounds which yield these on hydrolysis.

4. Uses of Carbohydrates
They make up most of the organic matter on Earth because
of their extensive roles in all forms of life.
Some of the uses of carbohydrates are:
• Carbohydrates serve as energy stores, fuels and metabolic
intermediates.
• Ribose and deoxyribose form part of the structural
framework of RNA and DNA.
• Polysaccharides are structural elements in cell walls of
bacteria and plants. Cellulose the main constituent of plant
cell walls is one of the most abundant organic compound in
biosphere.
• Carbohydrates are linked to many proteins and lipids where
they play key roles in mediating interactions among cells
and interactions between cells and other elements in the
cellular environment.

5. Types of Carbohydrates
There are three major classes of carbohydrates:
1. Monosaccharides: Monosaccharides or simple sugars
are small molecules that contain three to nine C-atoms.
The most abundant monosaccharide in nature is D-
glucose.
2. Oligosaccharides: These consists of short chains of
monosaccharide units joined together by covalent
bonds. Of these most abundant are the disaccharides
which have two monosaccharide units, for e.g. sucrose
which consists of D-glucose and D-fructose.
3. Polysaccharides: These consists of long chains having
hundreds or thousands of monosaccharide units. For
e.g. cellulose, starch.

6. MONOSACCHARIDES
• Monosaccharides, the simplest carbohydrates
are aldehydes or ketones that have two or
more hydroxyl groups.
• The empirical formula for many is (CH2O)n.
• Are important fuel molecules as well as
building blocks of nucleic acids.

7. • The smallest monosaccharide for which n=3
are dihydroxyacetone and D- and L-
glyceraldehyde.
Ketose
Aldoses, enatiomers b’cos
one asymmetric C-atom.

10. • D-Fructose is the most abundant ketohexose.
• Sugars differing in configuration at a single
asymmetric center are called epimers.
• D-Glucose and D-mannose are epimeric at C-2
while D-glucose and D-galactose are epimeric
at C-4.

11. • The predominant forms of monosaccharides
are not open chains. Rather the open chain
forms of these sugars cyclize into rings.
Similarly, C-1 aldehyde in the open chain form of
glucose reacts with C-5 hydroxyl
group to form an intramolecular hemiacetal. The
resulting six membered cyclic hemiacetal is called
pyranose b’cos of its similarity to pyran.

13. • For a ketohexose such as fructose, the C-2
keto group in the open chain form of fructose
reacts with a hydroxyl group within the same
molecule to form an intramolecular hemiketal.
• The keto group can react with either C-6
hydroxyl or C-5 hydroxyl to give a six
membered and five membered ring resp. The
five membered ring is called furanose.

15. Anomers
• An additional asymmetric center is generated on
formation of hemiacetal or hemiketal. For e.g. for
glucose , C-1, the carbonyl carbon becomes an
asymmetric center in the ring form. Thus two ring
structures can be formed :
α-D-glucopyranose and β-D-glucopyranose.
• The C-1 carbon atom is called the anomeric
carbon and the α- and the β- forms are called
anomers.
• The specific rotation of α-D-glucopyranose is +
112ο while that of β-D-glucopyranose is + 18 ο.

16. Mutarotation
• When α-D-glucose is dissolved in water its
specific rotation gradually changes with time and
reaches a stable value of 52.7°. Similarly, when β-
D-glucose is dissolved in water its specific
rotation also reaches 52.7°. This change in
rotation is called mutarotation. This is due to
formation of an equilibrium mixture of 1/3rd α
anomer, 2/3rd β anomer and very small amount
of straight chain compound. Thus, α and β forms
are interconvertible in aqueous solution.

18. Pyranose form dominates in fructose solutions while furanose form dominates in many
Fructose derivatives.

19. Glycosidic Bonds
• Just as simple hemiacetals react with another
molecule of alcohol to form an acetal, so can
the hemiacetal form of sugar react with a
molecule of an alcohol to form an acetal.
These compounds are known as glycosides.
• This new bond is called glycosidic bond.

22. Oligosaccharides
•Complex carbohydrates are formed by linkage of
monosaccharides.
•Oligosaccharides are obtained by linkage of two or
more monosaccharides by O-glycosidic bon.

26. A thermodynamically unfavorable reaction
can be driven by a favorable reaction
• How are specific pathways constructed for
individual reactions?
A pathway must satisfy two minimum criteria:
1. The individual reaction must be specific
2. The entire set of reactions that constitute the
pathway must be thermodynamically
favorable
The function of enzymes is to provide the
specificity.

27. • A reaction will only occur if the free energy
change ∆G is negative.
A + B C+D
∆G for the formation of products C and D from
substrates A and B is given by:
∆G ο’ is the standard free energy change.

28. • Another important thermodynamic fact is that
the overall free energy change for a series of
coupled reactions is equal to the sum of free
energy changes for individual steps. Consider
the following reactions:

29. Thus, a thermodynamically unfavorable
reaction can be driven by a
thermodynamically favorable reaction to
which it is coupled. In the above example the
reactions are coupled by shared intermediate
B. Thus, metabolic pathways are formed by
coupling of enzyme catalyzed reactions such
that the overall free energy of the pathway is
negative.

30. ATP: The universal currency of free
energy in the biological pathways
• Metabolism is facilitated by the use of common
energy currency , adenosine triphosphate (ATP).
• Part of the free energy derived from the oxidation
of foodstuff and light is converted into ATP.
• ATP is a highly accessible molecule, which acts as a
free energy donor in most energy requiring
processes such as motion, active transport or
biosynthesis.
• In fact, most catabolism consists of reactions that
extract energy from carbohydrates and fats and
convert it into ATP.

31. What is ATP?
• ATP is a nucleotide consisting of adenine, a ribose
and a triphosphate unit. The active form of ATP is
usually a complex of ATP with either Mg2+ or Mn2+
• ATP is an energy rich molecule because its
triphosphate unit contains two
phosphoanhydride bonds.
• A large amount of free energy is liberated when
ATP is hydrolyzed to adenosine diphosphate and
orthophosphate (Pi) (ADP) or adenosine
monophosphate (AMP) and pyrophosphate (PPi).

35. The precise ∆G ο’ for these reactions depends
upon the ionic strength of the medium and on
the concentration of Mg2+ and other metal
ions. Under typical cellular concentrations, the
actual ∆G for these hydrolyses is
approximately -50kJmol-1 (-12 kcalmol-1).

36. • The free energy liberated in the hydrolysis of
ATP is used to drive the reactions that require
an input of free energy such as muscle
contraction.
• ATP is synthesized from ADP and Pi when fuel
molecules are oxidized in chemotrophs, or
when light is trapped by phototrophs. This
ATP-ADP cycle is the fundamental mode of
energy exchange in biological systems.

37. • Some biosynthetic reactions are driven by
hydrolysis of nucleoside triphosphates that are
analogous to ATP: guinosine triphosphate (GTP),
uridine triphospahe (UTP), cytosine
triphospahe(CTP) .
• Enzymes catalyze the transfer of terminal
phosphoryl group from one nucleotide to another.
• Phosphorylation of nucleoside monophosphates is
catalyzed by nucleoside monophospahte kinases
while the phosphorylation of nucleoside
diphosphates is catalyzed by nucleoside
diphosphate kinases.

38. • Although all of the nucleotide triphosphates
are energetically equivalent , ATP is
nonetheless the primary cellular energy
carrier.

39. • Cells maintain high levels of ATP by using
oxidizable substrates and light as a source of
free energy to synthesize the molecule.
• In the cells the hydrolysis of an ATP molecule in
a coupled reaction changes the equilibrium
ratio of products to reactants by a very large
factor of 108. More generally the hydrolysis of n
ATP molecules changes the equilibrium ratio of
a coupled reaction or a series of reaction by
order of 108n .

40. What makes ATP particularly efficient
phosphoryl-group donor?
• Let us compare the standard free energy of
hydrolysis of ATP with that of a phospahte
ester such as glycerol-3-phosphate:

41. 1. Resonance stabilization: ADP and particularly Pi
have greater resonance stabilization than ATP.
2. Electrostatic repulsion: At pH 7, the triphosphate
unit of ATP carries about four negative charges.
They repel each other because they are in close
proximity. The repulsion between them is reduced
when ATP is hydrolyzed.
3. Stabilization due to hydration: More water can
bind more effectively to ADP and Pi than can bind
to the phosphoanhydride part of ATP, stabilizing
ADP and Pi by hydration.

43. ATP has phosphoryl transfer potential that is
intermediate among the biologically
important phosphorylated molecules. This
intermediate position enables ATP to function
efficiently as a carrier of phosphoryl groups.

44. • ATP serves as the principal immediate donor
of free energy in the biological systems rather
than as a long-term storage form of free
energy. In a typical cell, an ATP molecule is
consumed within a minute of its formation.
• Motion, active transport, signal amplification
and biosynthesis can take place only if ATP is
continually regenerated from ADP.
• The generation of ATP is one of the primary
roles of catabolism.

46. How is the energy released in oxidation of
carbon compounds converted into ATP?
Let us consider oxidation of glyceraldehyde-3-
phospahte. Oxidation to acid will release
energy.

47. However, the oxidation does not take place
directly. Instead, the carbon oxidation generates
an acyl phosphate, 1,3-bisphosphoglycerate. The
electrons released are captured by NAD+.

48. 1,3-Bisphosphoglycerate has high phosphoryl-
transfer potential. Thus, cleavage of 1,3-BPG
can be coupled to the synthesis of ATP.
The energy of oxidation is initially trapped as a
high-phosphoryl-transfer-potential compound
and then used to form ATP.

49. Ion gradient across the membranes provide an
important effective means of storing free
energy. Indeed, the electrochemical potential
of ion gradients across membranes, produced
by the oxidation of fuel molecules or by
photosynthesis , ultimately powers the
synthesis of most ATP in the cells.

51. Stages in generation of free energy from
oxidation of foodstuffs
1. Large molecules in food are broken down into smaller
units. This process is digestion. Proteins are broken down
into 20 amino acids, polysaccharides into simple sugars
while fats are hydrolyzed into glycerol and fatty acids.
2. These numerous molecules are broken down into a few
simple units. Most of them: sugars, fatty acids, glycerol
and some amino acids are converted into the acetyl unit
of acetyl Coenzyme A. Some ATP is generated at this
stage.
3. ATP is produced from complete oxidation of the acetyl
unit of acetyl CoA. This stage consists of citric acid cycle
and oxidative phosphorylation, which are the final
common pathways in the oxidation of fuel molecules.

54. Activated Carriers of Electrons for Fuel
Oxidation
In aerobic organisms, the ultimate electron
acceptor in the oxidation of fuel molecules is O2.
However, electrons are not transferred directly
to O2. Instead, fuel molecules transfer electrons
to special carriers, which are either pyridine
nucleotides and flavins. The reduced form of
these carriers then transfer their high potential
electrons to O2.
1. Nicotinamide adenine dinucleotide is a major
electron carrier in the oxidation of fuel
molecules.

55. The other major carrier in the oxidation of
fuel molecules is the coenzyme flavin
adenine dinucleotide. FAD is the electron
acceptor in following type of reactions:

57. 2. The electron donor in most reductive biosyntheses is
NADPH, the reduce form of nicotinamide adenine
dinucleotide phosphate (NADP+).
NADPH is almost exclusively used for reductive biosynthesis
while NADH is used for the generation of ATP.

58. 3. An activated carrier for two carbon fragments:
Coenzyme A is a carrier of acyl groups.

62. 4. Group transfer reactions play a variety of roles.

63. 5. Hydrolytic reactions cleave bonds by addition
of water.

64. 6. Functional groups maybe added to double bonds to
make single bonds or removed from single bonds to
make double bonds. The enzymes that catalyze these
reactions are called lyases.

65. Metabolic processes are regulated in three
principal ways:
• Through control of the amount of enzymes
• Their catalytic activities
• Their accessibility to enzymes