2. Provide the monomer units from which the
long polypeptide chains of proteins are
synthesized
L-amino acids and their derivatives
participate in cellular functions as diverse as
nerve transmission and the biosynthesis of
porphyrins, purines, pyrimidines, and urea.
Short polymers of amino acids called
peptides perform prominent roles in the
neuroendocrine system as hormones,
hormone-releasing factors,
neuromodulators, or neurotransmitters.
Biochemistry For Medics 7/5/2012 2
3. Each amino acid (except proline) has a
carboxyl group, an amino group and a
distinctive side chain bonded to the alpha
carbon atom. At physiological pH the
carboxyl group is dissociated forming the
negatively charged carboxylate ion(-COO-),
and the amino group is protonated(-NH3+)
Biochemistry For Medics 7/5/2012 3
4. Amino acids can be classified in 4 ways:
1. Based on structure
2. Based on the side chain characters
3. Based on nutritional requirements
4. Based on metabolic fate
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5. I. Aliphatic Amino Acids:
a) Mono-amino mono-carboxylic acids:
Simple amino acids: Glycine , Alanine
Biochemistry For Medics 7/5/2012 5
6. I. Aliphatic Amino Acids:
a) Mono-amino mono-carboxylic acids:
Branched chain amino acids: Valine,
Leucine and Isoleucine
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7. I. Aliphatic Amino Acids:
a) Mono-amino mono-carboxylic acids:
-OH group-containing amino acids: Serine
and Threonine
Biochemistry For Medics 7/5/2012 7
8. I. Aliphatic Amino Acids:
a) Mono-amino mono-carboxylic acids:
Sulfur-containing amino acids: Cysteine,
Cystine(Formed by linking of two cysteine
residues) and Methionine.
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9. I. Aliphatic Amino Acids:
a) Mono-amino mono-carboxylic acids:
Amide group-containing amino acids:
Glutamine and Asparagine
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10. I. Aliphatic Amino Acids:
a) Mono-amino di-carboxylic acids:
Aspartic acid and Glutamic acid
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11. I. Aliphatic Amino Acids:
a) Di- basic mono-carboxylic acids:
Arginine and Lysine
Biochemistry For Medics 7/5/2012 11
12. ii ) Aromatic amino acids-
Phenyl alanine and tyrosine
Biochemistry For Medics 7/5/2012 12
15. V. Derived Amino Acids:
Non-α-amino acids
e.g.: β-alanine, γ-amino butyric acid
(GABA), δ-amino Levulinic acid
Derived and Incorporated in tissue proteins:
e.g.: Hydroxy-proline, hydroxy-lysine
Derived but not incorporated in tissue
proteins:
e.g.: Ornithine, Citrulline, Homocysteine,
Argino succinic acid
Biochemistry For Medics 7/5/2012 15
16. A. Amino acids with a non-polar side-chain:
e.g.: Alanine, Valine, Leucine, Isoleucine,
Phenylalanine, Tryptophan, Proline
Each of these amino acids has a side chain that
does not bind or give off protons or participates in
hydrogen or ionic bonds.
Side chains of these amino acids can be thought
of as “Oily” or lipid like, a property that promotes
hydrophobic interactions.
Biochemistry For Medics 7/5/2012 16
17. B) Amino acids with a polar but uncharged side-chain:
e.g.: Glycine, Serine, Threonine, Tyrosine, Cysteine,
Asparagine and Glutamine.
These amino acids are uncharged at neutral pH,
although the side chains of cysteine and Tyrosine
can lose a proton at an alkaline pH.
Serine , Threonine and Tyrosine each contains a polar
hydroxyl group that can participate in hydrogen bond
formation.
Side chains of Asparagine and Glutamine
contain a carbonyl group and amide group, they can
also participate in hydrogen bond formation.
Biochemistry For Medics 7/5/2012 17
18. C) Amino acids with a charged side-chain:
a) Amino acids with a positively charged side-
chain:
The basic amino acids- Lysine, Arginine and
Histidine
b) Amino acids with a negatively charged side-
chain:
The acidic amino acids- Glutamic acid and
Aspartic acid
They are hydrophilic in nature.
Biochemistry For Medics 7/5/2012 18
19. I. Essential amino acids:
These amino acids cannot be synthesized in the body
and have to be present essentially in the diet.
Examples-Valine, Isoleucine, Leucine, Lysine,
Methionine, Threonine, Tryptophan and
Phenylalanine.
II. Semi-essential amino acids:
These amino acids can be synthesized in the body but
the rate of synthesis is lesser than the
requirement(e.g. during growth, repair or pregnancy)
Examples-Arginine and Histidine.
III. Non-essential amino acids:
These amino acids are synthesized in the body, thus
their absence in the diet does not adversely affect the
growth.
Examples- Glycine, Alanine, and the other remaining
amino acids.
Biochemistry For Medics 7/5/2012 19
20. The carbon skeleton of amino acids can be
used either for glucose production or for the
production of ketone bodies, Based on that
I. Both glucogenic and ketogenic amino
acids:
Isoleucine, Tyrosine, Phenylalanine and
Tryptophan
II. Purely Ketogenic amino acids:
Leucine and Lysine
III. Purely Glucogenic amino acids:
The remaining 14 amino acids are
glucogenic.
Biochemistry For Medics 7/5/2012 20
21. Of the over 300 naturally occurring amino
acids, 20 constitute the monomer units of
proteins. These 20 amino acids are called
the Primary or Standard amino acids.
Seleno cysteine is the 21st Amino Acid
The other are Pyroglutamate and Pyrolysine.
Biochemistry For Medics 7/5/2012 21
22. Each amino acid has three letter (code) and
one letter (Symbol) abbreviations-
Examples-1) Unique first letter
Cysteine- Cys- C
Histidine- His- H
2) Priority of commonly occurring amino
acids
Alanine- Ala- A (Preference over Aspartate)
Glycine- Gly-G (Preference over
Glutamate)
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23. 3) Similar sounding names- Some one letter
symbols sound like the amino acids they
represent- Example
Tryptophan – W (Twyptophan)
Phenyl alanine – F
4) Letters close to initial letter
Aspartate- Asx- B( near A)
Lysine Lys- K(near L)
Biochemistry For Medics 7/5/2012 23
24. Arginine- Guanidinium group
Phenyl Alanine- Benzene group
Tyrosine- Phenol group
Tryptophan- Indole group
Histidine- Imidazole group
Proline- Pyrrolidine
Proline has a secondary amino group,
hence it is an imino acid.
Biochemistry For Medics 7/5/2012 24
25. Physical properties-
Colorless
Crystalline
May be sweet(Glycine, Alanine, Valine),
tasteless(Leucine) or bitter(Arginine,
Isoleucine). Aspartame- An artificial
sweetener contains Aspartic acid and Phenyl
alanine.
Soluble in water, acids, alkalis but insoluble
in organic solvents
High melting point(More than 200 c)
0
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26. Amino acids can exist as ampholytes or
zwitterions in solution, depending upon pH of the
medium.
The pH at which the amino acids exist as
zwitterions, with no net charge on them is called
Isoelectric pH or Isoelectric point.
In acidic medium, the amino acids exist as cations
In alkaline medium , they exist as anions.
Due to no net charge, there is no
electrophoretic mobility at Isoelectric
pH.
Solubility and buffering capacity is
also minimum at Isoelectric pH
Biochemistry For Medics 7/5/2012 26
28. If HCl is added drop wise to am amino acid
solution, at a particular p H, 50 % of the
molecules are in the cationic form and 50%
are in the zwitterion form. This pH is
pK1(with regard to COOH)
If the titration is done from the Isoelectric
point with NaOH, molecules acquire the
anionic form. When 50 % of the molecules are
in the anionic form and 50% are in the
zwitterion form. This pH is pK2(with regard
to NH2)
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29. For mono amino
mono carboxylic
amino acids-
pI = pK1+pK2
-------------
2
The buffering
action is maximum
in and around
pK1or at pK2 but is
minimum at pI
Biochemistry For Medics 7/5/2012 29
30. The α carbon of each amino acid is attached to four
different groups and is thus a chiral or optically
active carbon atom.
Glycine is exceptional because there are two
hydrogen substituents at the α carbon, thus it is
optically inactive.
Amino acids with asymmetric centre at the α carbon
can exist in two forms, D and L forms that are mirror
images of each other and are called Enantiomers.
All amino acids found in proteins are of L-
configuration
D- amino acids are found in some antibiotics and in
bacterial cell walls.
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31. 1) Reactions due to amino group
2) Reactions due to carboxyl group
3) Reactions due to side chain
4) Reaction due to both amino and carboxyl
groups
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32. Oxidativedeamination-α amino group is removed
and corresponding α-keto acid is formed. α-keto
acid produced is either converted to glucose or
ketone bodies or is completely oxidized.
Transamination-Transfer of an α amino group
from an amino acid to an α keto acid to form a
new amino acid and a corresponding keto acid.
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33. Formation of carbamino compound
CO2 binds to α amino acid on the globin
chain of hemoglobin to form carbamino
hemoglobin
The reaction takes place at alkaline pH
and serves as a mechanism for the transfer
of Carbon dioxide from the tissues to the
lungs by hemoglobin.
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34. 1) Decarboxylation- Amino acids undergo
alpha decarboxylation to form
corresponding amines. Examples-
Glutamic acid GABA
Histidine Histamine
Tyrosine Tyramine
2) Formation of amide linkage
• Non α carboxyl group of an acidic amino
acid reacts with ammonia by condensation
reaction to form corresponding amides
Aspartic acid Asparagine
Glutamic acid Glutamine
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35. 1) Ester formation
OH containing amino acids e.g. serine,
threonine can form esters with
phosphoric acid in the formation of
phosphoproteins.
OH group containing amino acid can also
form: Glycosides – by forming
O- glycosidic bond with carbohydrate
residues.
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36. 2) Reactions due to SH group (Formation of
disulphide bonds)
Cysteine has a sulfhydryl group( SH) group and
can form a disulphide (S-S) bond with another
cysteine residue.
The dimer is called Cystine
Two cysteine residues can connect two
polypeptide chains by the formation of interchain
disulphide chains.
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37. 3)Transmethylation
The methyl group of Methionine can be
transferred after activation to an acceptor for
the formation of important biological
compounds.
Biochemistry For Medics 7/5/2012 37
38. Reactions due to side chains
4)Reactions due to both amino & carboxyl
groups
Formation of peptide bond
Biochemistry For Medics 7/5/2012 38
39. Incorporated in to tissue proteins
Niacin, Serotonin and melatonin are
synthesized from Tryptophan
Melanin, thyroid hormone, catecholamines
are synthesized from Tyrosine
GABA (neurotransmitter) is synthesized
from Glutamic acid
Nitric oxide, a smooth muscle relaxant is
synthesized from Arginine.
Act as precursors for haem, creatine and
glutathione, Porphyrins, purines and
pyrimidines.
Biochemistry For Medics 7/5/2012 39
40. S.No. Test Significance
1) Ninhydrin reaction Given by all Alpha amino acids
2) Xanthoproteic test Given by aromatic amino acids
3) Millon’s test Confirmatory test for Tyrosine
4) Biuret test Not given by free amino acids
5) Sakaguchi test Given by Arginine
6) Hopkins Cole reaction Confirmatory test for Tryptophan
7) Lead acetate test Given by cysteine and cystine but not
given by Methionine
8) Nitroprusside reaction Given by SH group containing amino
acids
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