2. Amino Acids, Peptides, Proteins
OBJECTIVES
• What are Amino acids
• Structure and naming of amino acids
• Standard and Non-standard amino acids
• What is peptide bond and peptides
• Ionization behavior of amino acids and peptides
• Structure and properties of peptides
3. AMINO ACIDS
ØAmino acids are organic acids containing an amine group, they are
the building units of proteins.
ØThere are about 300 amino acids occur in nature. Only 20 of them
occur in proteins.
4. Structure of amino acids:
ØEach amino acid has 4 different groups attached to α- carbon ( which is C-atom next to
COOH).
ØThese 4 groups are :
ØAmino Group,
ØCOOH Group
ØHydrogen Atom
ØSide Chain (R)
COOH
|
H2N—C — H
|
R side chain
6. • The most common amino acids are alpha-amino
(a-amino acids) acids and the most common amino
acids are the L- alpha-amino acids.
• Biochemical designation: start from a-carbon and
go down the R-group
Structure of amino acids:
8. At physiological PH (7.4), COOH group is dissociated forming a
negatively charged carboxylate ion (COO-) and H2N amino
group is protonated forming positively charged ion (NH3
+)
forming Zwitter ion
Amino Acids Physiological PH
9. • Both the –NH2 and the –COOH groups in an amino acid
undergo ionization in water.
• At physiological pH (7.4), a zwitterion forms
– Both + and – charges
– Overall neutral
– Amphoteric
• Amino group is protonated
• Carboxyl group is deprotonated
• Soluble in polar solvents due to ionic character
• Structure of R also influence solubility
Zwitterions
10. Classification of Amino acids
Amino acids are classified on following bases
1. On the basis of protein formation
2. On the basis of Side chain
3. Nutritional classification
4. On the basis of polarity
11. • The building blocks of proteins
• These 20 A.A. are the only A.A. coded for by DNA .
• Each amino acid (except proline) has a carboxyl group, an amino group, and a
distinctive side chain “R-group” bonded to the a -carbon atom.
• Proline has imino (NH)group
Standard & Nonstandard amino Amino Acids:
Nonstandard amino acids are not coded for protein synthesis
by DNA. eg. Ornithine, Citrulline
13. Classification on the basis of side chain
• Acidic side chain
• Basic side chain
• Neutral
– Aliphatic side chain
– Aromatic side chain
• Hetero cyclic
– Hydrocarbon side chain
– Hydroxylic side chain
– Sulphur – containing side chain
– Imino side chain
26. Classification of amino acids based on their
nutritional value
1-Essential amino acids ;They are those amino acids that cannot be synthesized in
the body thus they are essential in the diet.
Essential amino acids include the following amino acids ;
MATT VIL PHLY
2- Non-essential amino acids; They are those amino acids that can be synthesized in
the body thus they are non-essential in the diet.
27. 3-Classification according to the polarity of
the side chain R.
a)Non-polar amino acids.
Each of these amino acids has a non-polar side chain that does not bind or give
off protons or participate in hydrogen or ionic bonds.
b)Polar amino acids
1-Polar uncharged amino acids
These amino acids have zero net charge at neutral pH
2-Polar charged amino acids
Ø Polar acidic or positively charged amino acids.
The amino acids acid are proton donors
Ø Polar basic or negatively charged amino acids
The side chains of the basic amino acids accept protons
28.
29.
30. Peptide Bond
• Acid amide linkage.
• α-carboxyl group of one amino acid join with
the α-amino group of another amino acid.
• It is covalent linkage
C
H
C
R1
H2N
O
OH + C
H
C
R2
H2N
O
OH C
H
C
R1
H2N
O
N C
H
C
H
R2
O
OH + H2O
Peptide bond
31. Characteristic of Peptide bonds
• Peptide bonds are not broken by conditions that denature
proteins, such as heating or high concentrations of urea.
• Prolonged exposure to a strong acid or base at elevated
temperatures is required to hydrolyze these bonds non-
enzymatically.
• The peptide bond has a partial double-bond character, that is, it
is shorter than a single bond,
• It is rigid and planar.
• The peptide bond is generally a trans bond
32. Peptide bond formation
Make sure you clearly
indicate whether water
is a product or reactant!
Zwitterion form
Formation: dehydration reaction
Highlight the peptide bond
33. • More amino acids can be added using the same
condensation reaction
– oligopeptide/polypeptide/protein
Peptides and Proteins
Fig 3-14
R1 R2 R3 R4 R5
34. C
H
CO OHH2N
C H3
+
+ H2O
C
H
C OO HH2N
H
C
H
CO O HH2N
CH2O H
+
Glycine ( Gly )Alanine ( Ala ) Serine (Ser )
2C
H
CH2N
CH3
O
N
H
C
H
H
C
O
N
H
C
H
CH2OH
C
O
O H
Alanylglycylserine (Ala-Gly-Ser)
Peptide Bond
35. Clinical Importance
1. Synthesis of body proteins.
2. Synthesis of essential nitrogen compounds e.g ( Histamine,
DNA,RNA).
3. Synthesis of glucose, fat, energy.
4. Synthesis of hormones(Thyroid Hormones)
5. Synthesis of neurotransmitters(dopamine, epinephrine,
norepinephrine, serotonin, and histamine)
