protein

Biochemistry

Peptide and proteins

Prof. Dr.
Abdulhussien M. Aljebory
Babylon University
College of Pharmacy
Peptides and Proteins
20 amino acids are commonly found in protein.
These 20 amino acids are linked together through “peptide bond forming peptides and proteins (what’s the difference?).
- The chains containing less than 50 amino acids are called “peptides”, while those containing greater than 50 amino acids are called “proteins”.

Peptide bond formation:
?-carboxyl group of one amino acid (with side chain R1) forms a covalent peptide bond with ?-amino group of another amino acid ( with the side chain R2) by removal of a molecule of water. The result is : Dipeptide ( i.e. Two amino acids linked by one peptide bond). By the same way, the dipeptide can then forms a second peptide bond with a third amino acid (with side chain R3) to give Tripeptide. Repetition of this process generates a polypeptide or protein of specific amino acid sequence.
- Each polypeptide chain starts on the left side by free amino group of the first amino acid enter in chain formation . It is termed (N- terminus).
- Each polypeptide chain ends on the right side by free COOH group of the last amino acid and termed (C-terminus).
Examples on Peptides:
1- Dipeptide ( tow amino acids joined by one peptide bond):
Example: Aspartame which acts as sweetening agent being used in replacement of cane sugar. It is composed of aspartic acid and phenyl alanine.
2- Tripeptides ( 3 amino acids linked by two peptide bonds).
Example: GSH which is formed from 3 amino acids: glutamic acid, cysteine and glycine. It helps in absorption of amino acids, protects against hemolysis of RBC by breaking H2O2 which causes cell damage.