2. Proteins are the most abundant organic molecule of the
living system .
They occur in every part of the cell and constitute about
50% of the cellular weight.
The term protein is derived from a Greek word Proteios,
meaning holding the first place.
Function of protein:
1 Static( structural ) function: Certain protein perform
brick and mortar roles and are primarily responsible for
structure and strength of body.
these include collagen and elastin found in bone matrix,
vascular system and other organs α- keratin present in
epidermal tissue.
3. Dynamic function: These include protein acting as
Enzymes, hormones, blood clotting factor,
immunoglobulin, membrane receptors, storage protein ,
beside their function in genetic control, muscle
contraction, respiration etc.
Protein performing dynamic function are appropriately
regarded as the working horses of cell.
Composition of protein:
Carbon: 50 - 55%
Hydrogen: 06- 7.3%
Oxygen: 19 - 24%
Nitrogen: 13 - 19%
Sulfur: 0 - 4%
4. • Proteins on hydrolysis with concentrated HCl
for several hours yield L- α- Amino acid.
Therefore, protein are the polymer of L- α-
Amino acid.
• Standard amino acids: as many as 300 amino
acids occur in nature of these , only 20 known
as standard amino acid are repeatedly found in
the structure of protein, isolated from different
form of life animal, plant and microbial.
5. What is an amino acid?
• Twenty different kinds of amino acids are used by living
organisms to produce proteins
• An amino acid is a molecule containing an amine (-NH2) an
acid (-COOH) and a third chemical group (-R) that defines the
amino acid.
In glycine, the simplest amino acid, R is –H, or a hydrogen atom.
In alanine, R = -CH3. The R groups give specific properties to
each amino acid, and to the proteins composed of amino acids.
R
|
Structure of an amino acid: H2N – C – COOH
H
6. • Amino Acid:
COOH COOH
H NH2 H2N H
R
D- L-
naturally-occurring amino acids are
generally L-series
7. Classification of Amino Acids
H O
+ –
H3N C C O
R
• The amino acids obtained by hydrolysis of
proteins differ in respect to R (the side chain).
• The properties of the amino acid vary as the
structure of R varies.
16. Amino acid classification based on metabolic fat
• The carbon skeleton of amino acid can serve as a
precursor for the synthesis of glucose (glycogenic) , or fat
(ketogenic).
1. Glycogenic amino acid: these amino acid can serve as
precursors for the formation of Glucose or Glycogen.
e. g. Alanine, Aspartate, glycine, Metheionine
2. Ketogenic Amino Acid: fat can be synthesized from
these Amino acids. Two amino acid Leucine and Lysine.
3. Glycogenic and Ketogenic Amino Acid: the four amino
acids isoleucine, phenylalanine, trytophan, tyrosine are
precursor for the synthesis of glucose and fat.
17. Selenocystein – the 21st amino acid: it is found that the
active sites of certain enzymes /protein (selenoprotein)
e.g. Glutathion peroxidase, glycine reductase,
thioredoxine reductase. Selenocystein is an unusual
amino acid containing the trace element Selenium in
place of sulfur atom of cysteine.
Cysteine Selenocystein
19. Properties of Amino Acid
• Physical properties:
• Solubility: most of the amino acids are soluble in water
and insoluble in inorganic solvent:
• Melting point: amino acids are generally melt at higher
temperature , often above 200 C
• Taste : amino acids may be sweet (Gly, Ala, Val),
tasteless (Leu) or bitter (Arg, ILe).
• Monosodium Glutamate (Ajinomoto) is used as
flavouring agent in food industry, chinese foood to
increase taste and flavour.
20. R
CH
H2N CO2H
All DNA encoded aa are
CHO CHO
All are chiral,
except Glycine H OH HO H
R=H CH2OH CH2OH
D- L-
All DNA
encoded aa
CHO R
are usually L-
= = C
HO CH2OH H2N CO2H
H H
(S) - Glyceraldehyde (L) - Amino Acids
(-) - (-) -
27-
21. Amino acids as ampholytes: amino acid
containing both acidic (-COOH ) and basic (-
NH2) groups. They can donate proton and accept
proton.
22. Zwitterions or dipolar ion
• The name Zwitter derived from the German word
which mean hybrid. Zwitter ion is a hybrid
molecule containing positive and negative ionic
group.
• The amino acids rarely exists in a neutral form
with free carboxylic (-COOH ) and free amino
(-NH2) groups.
• In strongly acidic pH the amino acid are
positively charged, while in strongly alkaline pH
it is negatively charged.
• Each amino acid has a characteristics pH at which
it carries both positive and negative charge and
Exist as Zwitterions.
23. Isoelectric pH
• pH at which amino acids exist as the zwitterion
(neutral) and carries no net charge. Thus molecule is
electrically neutral.
• The pl value can be calculated by taking the average pKa
values corresponding to the ionizable groups. For
example leucine has two ionizable groups , and its pl
value can be calculated as follows.
24. • Leucine exists as cation at pH below 6 and anion at pH
above 6. at the ispelectric pH leucine is found as
Zwitterions .
• Titration curve of Amino acid: in the graphical
representation of Leucine titrarion at low pH , Leucine
exists in fully protonated forms as cation. As the titration
proceeds with NaOH, Leucine loses its protons and at
isoelectric pH its become Zwitterions. Further titration
results in formation of anionic form of Leucine.
29. Chemical properties
• Reaction due to –COOH group
1. Amino acid form salts (-COONa) with base, and Ester (-
COOR) with alcohol.
2. Decarboxylation: Amino acid undergo
deacarboxylation to produce amines.
this reaction assumes significance in the living cell due
to the formation of many biologically important
amine. These include histamine, tyramine, γ-amino
butyric acid from the amino acid histidine, tyrosine and
glutamate respectively.
3. Reaction with ammonia: the carboxyl group of
dicarboxylic amino acid reacts with NH3 to form amide.
Aspartic acid + NH3 Aspargine
30. Reaction due to NH2
4. The amino acid behave as bases and combine with acids
to form salts.
5. Reaction with ninhydrine: the α- Amino acidreact with
ninhydrine to form a purple , blue or pink color
complex(Ruhemann’s purple)
Amino acid + ninhydrine keto acid+NH3+
CO2+ Hydrindantin
Hydrindantin+ NH3+Ninhydrine
Ruhemann’s purple
6. Colour reaction
7. Transamination
8. Oxidative deamination
31. Non – standard amino acid
A. Amino acid derivatives in proteins: some of these
amino acid undergo specific modification after the
protein synthesis occurs. These derivatives of amino
acids are very important for protein structure.
• Collagen: the most abundant protein in mammals contain
4- hydroxyproline and 5- hydroxylysine.
• Histones: the protein found in association with DNA-
contain many methylated and phosphoraylated amino
acid.
• γ- Carboxyglutamic acid is found in certain plasma
protein involved in blood clotting.
32.
33.
34.
35. A mixture of amino acids can also be separated on the
basis of polarity
