Amino Acids Amino Acids are the building units of proteins. Proteins are polymers of amino acids...

Amino Acids

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)

R

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

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

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

Valine

Leucine

Isoleucine

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):

Formation of cystine

The bond S-S in cystine is called: disulfide bond

Serine

Threonine

Cysteine

Methionine

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

3- Tryptophan: contains indol ring

Phenylalanine

Tyrosine

Tryptophan

VI- Neutral amino acids with amide group in R (2):

1- Aspargine (Asn):

2- Glutamine (Gln):

• 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

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

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 .

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

Lysine

Arginine

Histidine

Basic amino acids (3)

Aspartic acid

Glutamic acid

Acidic amino acids (2)

-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