A tripeptide composed of three different amino acids can be made in 6 different constitutions, and the tetrapeptide shown above (composed of four different amino acids) would have 24 constitutional isomers. When all twenty of the natural amino acids are possible components of a peptide, the possible combinations are enormous. Simple statistical probability indicates that the decapeptides made up from all possible combinations of these amino acids would total 20¹⁰!

Natural peptides of varying complexity are abundant. The simple and widely distributed tripeptide glutathione (first entry in the following table), is interesting because the side-chain carboxyl function of the N-terminal glutamic acid is used for the peptide bond. An N-terminal glutamic acid may also close to a lactam ring, as in the case of TRH (second entry). The abbreviation for this transformed unit is pGlu (or pE), where p stands for "pyro" (such ring closures often occur on heating). The larger peptides in the table also demonstrate the importance of amino acid abbreviations, since a full structural formula for a nonapeptide (or larger) would prove to be complex and unwieldy. The formulas using single letter abbreviations are colored red.

Thanks
Ruchi
Askiitians Faculty