Protiens and polypeptide

Protein &Primary Structure

Learning Objectives:

1. Draw the condensed structural formula for a peptide and give its name.
2. Describe the primary structure for a protein.

Peptide bond is an amide bond that forms when the ¬COO- group of one amino acid reacts with the ¬NH3+ group of the next amino acid. The linking of two or more amino acids by peptide bonds forms a peptide. An O atom is removed from the carboxylate end of the first amino acid, and two H atoms are removed from the ammonium end of the second amino acid, which produces water. Two amino acids form a dipeptide, three amino acids form a tripeptide, and four amino acids form a tetrapeptide. A chain of five amino acids is a pentapeptide, and longer chains of amino acids Form polypeptides.

We can write the amidation reaction for the formation of the dipeptide glycylalanine (Gly–Ala, GA) (see Figure 2.1) during the amidation, the O atom removed from the carboxylate group combines with two H atoms from the ¬NH3+ group to produce H2O.
The amino acid written on the left, glycine, has a free ¬NH3+ group, which makes it the N-terminal amino acid. In the peptide, the amino acid written on the right, alanine, has a free ¬COO- group, which makes it the
C-terminal amino acid. The dipeptide forms when the carbonyl group in glycine bonds to the N atom in the ¬NH3 + group of alanine.