Aminoacids

ByDr. Sumbul Fatma

29th September, 2010

Types of proteins

Type Examples• Structural tendons, cartilage, hair, nails• Contractile muscles• Transport hemoglobin• Storage milk• Hormonal insulin, growth hormone• Enzyme catalyzes reactions in cells• Protection immune response

Amino acids

• Building blocks of proteins• 20 are present in mammalian proteins

Structure of amino acids

• Groups attached to α- carbon– a carboxyl group– an amino group– a side chain

Examples H

I

H2N—C —COOH I

H glycine

CH3 I

H2N—C —COOH I

H alanine

Zwitterions

Net charge is zero on the molecule

Isoelectric point

• The pH at which the molecule carries no net charge

• In acidic solution-cationic• In alkaline solution- anionic

Titration curve of glycine• pK1- pH at which 50%

of molecules are in cation form and 50% are in zwitterion form

• pK2- pH at which 50% of molecules are in anion form and 50% are in zwitterion form

• Buffering action is maximum around pK values and minimum at pI

• What if amino acid has more than two ionizable groups?

Titration curve of aspartic acid

Learning check-1

CH3 CH3

+H3N–CH–COOH H2N–CH2–COO–

(1) (2)

Select from the above structures

A. Alanine in base.

B. Alanine in acid.

Solution-1

CH3 CH3

+H3N–CH–COOH H2N–CH2–COO–

(1) (2)

Select from the above structures

A. Alanine in base- (2)

B. Alanine in acid – (1)

Classification on the basis of side chain

• Non-polar– Side chain does not bind or give off

protons– Promote hydrophobic interaction– In aqueous solution- clustered in the

interior– Helps giving a 3-d shape

Proline

• imino acid– Has a secondary amino group

Classification on the basis of side chain

• Polar– uncharged– Acids– Bases

• Hydrophilic

Polar uncharged amino acids

• Have zero net charge at neutral pH• Hydrophillic

Polar acidic amino acids

• Have a negative charge on the R-group

Polar basic amino acids

• Have a positive charge on the R-group

Learning check-2

Identify each as (1) polar or (2) nonpolar

A. NH2–CH2–COOH (Glycine)

CH3 | CH–OH

|

B. NH2–CH–COOH (Serine)

Solution-2

Identify each as (1) polar or (2) nonpolar

A.(2) NH2–CH2–COOH (Glycine)

CH3

| CH–OH |

B. (1) NH2–CH–COOH (Serine)

Optical properties of amino acids• Amino acids having asymmetric carbon atom

exhibit optical activity• The mirror image forms produced with

reference to the α-C atom are called D and L isomers

• L-amino acids – natural amino acids• D-amino acids are found in antibiotics (like

Gramicidin-S, Actinomycin-D and Valinomycin) and in plant and bacterial cell walls

• Glycine an exception- no optical activity

Stereoisomers/Enantiomers

Learning check-3

• All 20 amino acids exist in nature equally as both D and L stereoisomersTrue/False

Solution-3

• All 20 amino acids exist in nature equally as both D and L stereoisomers– False

Amino acid derivatives of importance

• Gamma amino butyric acid (GABA, a derivative of glutamic acid) and dopamine (from tyrosine) are neurotransmitters

• Histamine (Histidine) is the mediator of allergic reactions

• Thyroxine (Tyrosine) is an important thyroid hormone

Peptide bond

• Proteins are made by polymerization of amino acids through CO-NH bridge

• α-carboxyl group of one amino acid reacts with α-amino group of another amino acid

Proteins are made by controlled polymerization of amino acids

H2N CH C

R1

OH

O

H2N CH C

R2

OH

O

H2N CH C

R1

NH

O

CH C

R2

OH

O

peptide bond is formed

+ HOH

residue 1 residue 2

two amino acidscondense to form...

...a dipeptide. Ifthere are more itbecomes a polypeptide.Short polypeptide chainsare usually called peptideswhile longer ones are calledproteins.

water is eliminated

N or aminoterminus

C or carboxyterminus

Peptides

• 2 aa- dipeptide• 3-?• 4- ?• Upto 10- oligo peptide• 10-50- polypeptide• More than 50 - proteins

Learning check-4

What are the possible tripeptides formed from one each of leucine, glycine, and alanine?

Solution-4

Tripeptides possible from one each of leucine, glycine, and alanine

Leu-Gly-AlaLeu-Ala-GlyAla-Leu-GlyAla-Gly-LeuGly-Ala-LeuGly-Leu-Ala

References

• Lippincott’s Illustrated reviews: Biochemistry 4th edition – unit 1