1. Amino Acids are the building units of proteins
Amino Acids are the building units of proteins. Proteins are polymers of amino acids linked together by what is called “ Peptide bond” (see latter).
There are about 300 amino acids occur in nature. Only 20 of them occur in proteins.
Structure of amino acids:
Each amino acid has 4 different groups attached to α- carbon ( which is C-atom next to COOH). These 4 groups are : α-amino group, COOH group, Hydrogen
atom and side Chain (R)
Amino Acids R

2. Classification of amino acids
1- Chemical classification: According to number of COOH and NH2 groups i.e. according to net charge on amino acid and
A- Monobasic, monocarboxylic amino acids i.e. neutral or uncharged: R

3. According to chemical structure of R group, neutral amino acids are subclassified into:
I- Aliphatic, neutral amino acids: (6 amino acids)
1- Glycine (Gly): R= H
2- Alanine (Ala) R= CH3
3, 4, 5-Branched chain amino acids: valine, leucine and isoleucine
R is branched
Valine (Val): R= isopropyl group

4. Leucine (Leu): R= isobutyl gp
Isoleucine (Ile) R = is isobutyl
R is isobutyl in both leucine and isoleucine but branching is different: in leucine → branching occurs on γ carbon
in isoleucine→ branching occurs on β- carbon
6- Proline: In proline, amino group enters in the ring formation being α-imino group so proline is an α-imino acid rather than α-amino acid

5. Valine
Leucine
Isoleucine

6. II- Neutral Sulfur containing amino acids: 2 amino acids
1- Cysteine (Cys): R= CH2SH
What is cystine?
2- Methionine (Met):
III- Neutral, hydroxy amino acids: 2 amino acids
1- Serine (Ser): R= CH2OH
2- Threonine (Thr):

7. Formation of cystine
The bond S-S in cystine is called: disulfide bond

8. Serine
Threonine

9. Cysteine
Methionine

10. VI- Neutral aromatic amino acids: includes 3 amino acids
1- Phenyl alanine (Phe) :
It’s alanine in which one hydrogen of CH3 is substituted with phenyl group. So it’s called phenyl alanine.
2- Tyrosine (Tyr): - it is P- hydroxy phenyl alanine
- it is classified as phenolic amino acid

11. 3- Tryptophan: contains indol ring

12. Phenylalanine
Tyrosine
Tryptophan

13. VI- Neutral amino acids with amide group in R (2):
1- Aspargine (Asn):
2- Glutamine (Gln):

14. At physiological PH (7.4), in neutral amino acids, -COOH group is dissociated forming a negatively charged carboxylate ion (COO-) and amino group is protonated forming positively charged ion (NH3+) forming Zwitterion

15. B- Basic amino acids: contain two or more NH2 groups or nitrogen atoms that act as base i.e. can bind proton. At physiological pH, basic amino acids will be positively charged.
a- Lysine( Lys)
b- Arginine(Arg): contains guanido group:
c- Histidine (His): contains imidazole ring.
guanido

16. C- Acidic Amino acids:
e.g.
a- Aspartic acid (Asp) (aspartate),
b- Glutamic acid (Glu) (glutamate)
at physiological pH, acidic amino acids will carry negative charge

18. A- Neutral amino acids (15 amino acids)
Summary:
A- Neutral amino acids (15 amino acids)
Classified according to R into
- Aliphatic amino acids including glycine, alanine, valine, leucine, isoleucine and proline
Sulfur containing amino acids including: cysteine and methionine
Hydroxy amino acids including serine and threonine
Aromatic amino acids including phenylalanine, tyrosine and tryptophan
Amino acids containing amide group including aspargine and glutamine

19. Basic amino acids (3)
Lysine
Arginine
Histidine

20. Acidic amino acids (2)
Aspartic acid
Glutamic acid

21. At physiologic pH, neutral amino acids are present as Zwitterion (carry equal positive and negative charges) so it is neutral (not charged).
- At physiological pH, basic amino acids will be positively charged.
While acidic amino acids are negatively charged
So, Basic and acidic amino acids are called charged amino acids