Properties of amino acids: - Amino Acids have an Asymmetric Center - D and L stereoisomerism of amino acids - Acid-Base Properties of Amino Acids - Titration of amino acids - Absorption - Solubility - Chemical properties of amino acid

1. Properties of Amino Acids
Bio-Chemistry
Rahul SIR
Lecturer, Department of Med-Surg
Co-chair of the South East Asia Regional Hub within the
Challenger’s Committee at Nursing Now Challenge, London, UK

2. Specific learning objectives
🠶Properties of amino acids:
• Amino Acids have an Asymmetric Center
• D and L stereoisomerism of amino acids
• Acid-Base Properties of Amino Acids
• Titration of amino acids
• Absorption
• Solubility
• Chemical properties of amino acid

3. Properties of Amino Acids

4. Amino Acids have an Asymmetric Center
🠶 Optically active molecules have an asymmetry such that they are not
superimposable on their mirror image.
🠶 Cα atoms of all aa are asymmetric centers and optically active except
glycine, in which R=H two of the four substituents on α-carbon atoms are
hydrogen.
🠶 Cα is a chiral center, this carbon atom is attached to four different groups.

5. D and L stereoisomerism of amino acids
Fig.6.3: Marks' Basic Medical Biochemistry-A Clinical Approach, 2nd Edi

6. Acid-Base Properties of Amino Acids
🠶Amino acids in aqueous solution, contain weak acidic α-carboxyl
groups and weak basic α-amino groups
🠶 Charged and uncharged form of ionizable weak acid groups –COOH
and -NH3
+ exist in protonic equilibrium:
R-COOH R-COO- + H+
R- NH3
+ R- NH2
+ + H+

7. Cont--
🠶Henderson-Hasselbalch equation: Quantitative relationship between pH
and concentration of a weak acid (HA) and its conjugate base (A-).
🠶Derivation of Henderson-Hasselbalch equation: Consider release of a
proton by a weak acid (HA):
“salt” or conjugate base (A–) is ionized form of a weak acid (HA).

8. Cont--
🠶 Dissociation constant of acid (Ka): or
🠶By taking negative logarithm of both sides:
🠶Substituting pH = -log[H+] and pKa = -logKa obtain Henderson-
Hasselbalch equation:

9. Cont--
🠶Larger the Ka, the stronger acid, because most of HA has dissociated
into H+ and A–.
🠶Conversely, smaller the Ka, the less acid has dissociated and,
therefore, the weaker the acid.
🠶pKa values for a particular molecule are determined by titration.

10. Titration of an Amino Acids
• Carboxyl and amino group of glycine
titrated with a strong base (NaOH)
• 1st , at low pH2.34, -COOH group loses its
proton
• 2nd, at pH5.97,zwitterion/dipolar ion form
3
• 3rd, at pH9.60, NH + group loses its
proton.
• 4th,at pH12.0 titration complete
Fig3.10: Lehninger Principles of Biochemistry by David L Nelson, 6th Ed/

11. Cont--
🠶 Titration curves predict the electric charge of aa:
• For glycine, no ionizable group in its side chain, the isoelectric point
calculated by arithmetic mean of two pKa values:
• pKa for –COOH is pK1 is 2.34, whereas pKa for next pKa for -NH3
+ is
pK2 is 9.60
• pI=1/2 (pK1 + pK2) = 1/2 (2.34+9.60)=5.97

12. Cont--
and amino
🠶At physiological pH, carboxyl group exists as R-COO-
group as R-NH3
+
🠶Ex. Alanine:

13. Ultraviolet Spectra of Tyr, Phe and Trp
• Amino acids do not absorb visible
light
• Tyr, Phe, and Trp absorb high-
wavelength (250-290nm) UV light.
• Absorption of light by most proteins at
280nm used to detect presence of a
protein in solution
Fig.3.7. Harper’s I
llustrated Biochemistry 30th edition

14. Solubility
🠶Polar, uncharged R groups includes serine, threonine,
cysteine, asparagine, and glutamine.
• R groups of these aa more soluble in water, or more
hydrophilic, than those of nonpolar aa, because they contain
functional groups that form hydrogen bonds with water.

15. Chemical properties of amino acids
1. Reactions due to carboxyl group
🠶 Decarboxylation: Carboxyl group decarboxylated to give primary amines.
Removal of CO2 from aa with formation of amines. Ex. histidine to
histamine
https://www.slideshare.net/senchiy/amino-acids-metabolism-new-12281450
https://slideplayer.com/slide/7968638/

16. 🠶Formation of amides: Carboxyl group condense with amines to form
amides, and remove ammonia from brain
https://www.materialsworldmodules.org/resources/polimarization/4-condensation.html
https://www.tankonyvtar.hu/hu/tartalom/tamop412A/2011-0016_07_pharmacognosy_1/ch13s03.html

17. 🠶 Formation of Peptide bonds: Carboxyl group joins with amino group
of another aa to form peptide bond
Fig3.13: Lehninger Principles of Biochemistry by David L Nelson, 6th Ed/

18. Cont--
2. Reactions due to amino group
🠶 Amino group reacts with Co2 in alkaline pH to form carbamino
compound, serve to transport Co2 from tissues to lung by hemoglobin
(Hb)
Hb-NH2 + Co2 = Hb-NH-COOH (Carbamino Hb)
🠶 Amino group reacts with halides or acyl anhydrides, for ex. Reaction
of glycine to give hippuric acid, serves as a method for detoxification
of food additives and drugs to treat hyperammonemia

19. Cont--
🠶 Amino group reacts with fluorodinitrobenzene to form
dinitrophenyl aa, used to identify N-terminal aa in any peptide
chain and identify aa separated by paper chromatography

20. Cont--
• Transamination: α-NH2 group of one
aa is transferred to a α-ketoacid
resulting in formation of a new aa
and a new ketoacid
• Donor aa (I) becomes a new
ketoacid (I) after losing the α-NH2
group, and recipient ketoacid (II)
becomes a new aa (II) after
receiving the NH2 group
Text Book of Medical Biochemistry by Chatterjee & Rana Shinde, 8th Ed

21. Cont--
• α-amino group from L-amino acid is
transferred to α-carbon atom of α-
ketoglutarate, produced α-keto acid and
glutamate
• Transfer of amino groups from one carbon
skeleton to another is catalyzed by
aminotransferases
• All aminotransferases have prosthetic group,
which is pyridoxal phosphate (PLP),
coenzyme form of pyridoxine or vitamin B6
Fig18.4: Lehninger Principles of Biochemistry by David L Nelson

22. Cont--
• Oxidative Deamination: α-amino
group removed from aa to form
corresponding new aa and α-keto
acid and ammonia.
• Glutamic acid undergo oxidative
deamination
Fig18.7: Lehninger Principles of Biochemistry by David L Nelson
L-Glutamate γ semialdehyde

23. Cont--
3. Reactions due to side chain
🠶 Formation of disulfide bonds: -SH (sulfhydryl) group of two cysteine
can join together to form disulfide bond.
• It provide stability to protein structure by forming intrachain disulfide
bonds
🠶 Transmethylation: Terminal –S-CH3 group of methionine, after
activation into S-adenosylmethionine, serves as a major methyl
donor in methylation reactions
🠶 Reactions due to –OH group of tyrosine, serine and threonine:

24. Cont--
–OH group of serine and threonine in proteins is highly reactive and
helps in:
• Phosphorylation: Proteins are phosphorylated at their tyrosine,
serine and threonine residues by kinase using ATP
• Glycosylation: The –OH group join with carbohydrates to form O-
glycosidic bonds of glycoproteins
• N-Glycosidic linkages: Amide group of glutamine and asparagine link
with carbohydrates to form N-glycosidic bonds of glycoproteins

25. Classification of Proteins
1. Based on Solubility
• Albumins: Soluble in water and salt solutions
• Globulins: Soluble in salt solution but sparingly soluble in water
• Protamines: Soluble in 70-80% ethanol; usually rich in proline
• Protamines: Soluble in water, dilute acids and alkalies; rich in arginine
• Histones: Soluble in salt solutions
• Scleroproteins: Insoluble in water and salt solutions

26. Cont--
2. Based on Shape
• Fibrous Proteins: Appear like hair with long thin fibers. Ex. Keratin,
myosin etc.
• Globular Proteins: Appear like spherical globular drops. Ex. Insulin,
albumin, globulins etc.
3. Based on Functions
• Structural: Collagen, Keratin, myosin etc
• Enzyme: Pepsin, amylase, lipase etc
• Hormone: Insulin, glucagon etc
• Transport: Hb, Mb etc
• Storage: Ferritin, ceruloplasmin etc
• Protective: Antibodies

27. Cont--
3. Based on Composition
• Simple Proteins: Pure proteins and do not contain any non-protein
part, e.g. albumin, globulin, collagen etc.
• Conjugated Proteins: Contains covalently bound non-protein part
called prosthetic group which cannot be separated without loss of
activity of the proteins.
 Ex. Lipoproteins have lipids, glycoproteins have carbohydrates,
nucleoproteins have nucleic acid, metalloproteins have metal ions as
the non-protein part.

28. Properties of Proteins
1. Based on Denaturation: Loss of native form of protein with disruption
of its secondary, tertiary, and quaternary structure leading to changes
in its physical and chemical characteristics and loss of biological
activity
2. Solubility: Depends on its aa composition and 3D conformation,
because, to be soluble, surface amino acids of protein have to interact
with solvent molecules.

29. Cont--
3. Buffering action of proteins: Buffer solution resists change in its pH by
limited additions of acids or alkalis
• Buffer system composed of a strong acid and its salts with a weak
base or reverse
4. Precipitation: Stability of proteins depends on its capacity to interact
with solvent and its degree of hydration.
• Any force that destabilize this interaction leads to precipitation.
Methods for precipitation, salting out, immunoprecipitation etc.

30. Bonds stabilizing Proteins Structure
1. Peptide bond
2. Disulfide bond
3. Hydrogen bond
4. Electrostatic interaction
5. Van der Waal’s interaction
6. Hydrophobic interaction

31. Reference Books
edition.
1) Harper’s Illustrated Biochemistry-30th edition
2) Textbook of Biochemistry with Clinical Correlations. 4th
Thomas M. Devlin.
3) Biochemistry. 4th edition. Donald Voet and Judith G. Voet.
4) Biochemistry 7th edition by Jeremy M. Berg, John L. Tymoczko
and Lubert Stryer
5) Lehninger Principles of Biochemistry, 6th Ed.
6) Text Book of Medical Biochemistry by Chatterjee & Rana Shinde,
8th Ed.
31

32. Thank You
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