Elementary plant Biochemistry and Biotechnology (2+1)

Isomerism

• All amino acids except proline, found in protein are α-amino acids because NH₂. group is attached to the α-carbon atom, which is next to the COOH group.
  
• Examination of the structure of amino acids reveals that except glycine, all other amino acids possess asymmetric carbon atom at the alpha position.
• Because of the presence of asymmetric carbon atom, amino acids exist in optically active forms.
• For example, in the steric configuration for serine, the carboxyl group is written on the top, while the amino group is written to the left in the case of L-serine and to the right in the case of D-serine This distinction will hold good for all the amino acids having asymmetric carbon atoms.

• 'D' and 'L' do not refer to the optical rotation, but to the steric configuration of amino group to the right and left side of the carboxyl group. The direction of optical rotation of amino acid is indicated by the symbol + or -, which follows the designation 'D' or 'L'. The steric configuration and optical rotation of an amino acid may be simultaneously expressed as D (+) or D (-) and L (+) or L (-). L-forms are more common than D-forms and most of the naturally occurring amino acids are L-amino acids.