Question 21.3: Identify the individual amino acids used to form the followi...
Identify the individual amino acids used to form the following tripeptide. What is the name of the tripeptide?
Analysis
• Locate the amide bonds in the peptide. To draw the structures of the individual amino acids, break the amide bonds by adding water. Add \mathrm{O}^{-} to the carbonyl carbon to form a carboxylate anion -\mathrm{COO}^{-} and 2 H’s to the N atom to form -\mathrm{NH}_3{ }^{+} .
• Identify the amino acids by comparing with Table 21.2 .
• To name the peptide: [1] Name the C-terminal amino acid. [2] Name the other amino acids as substituents by changing the -ine (or -ic acid ) ending to the suffix – yl. Place the names of the substituent amino acids in order from left to right.
| Table 21.2 The 20 Common Naturally Occurring Amino Acids | |||||
| Neutral Amino Acids | |||||
| Name | Structure | Abbreviations | Name | Structure | Abbreviations |
| Alanine |  |
Ala A | Phenylalanine* |  |
Phe F |
| Asparagine |  |
Asn N | Proline |  |
Pro P |
| Cysteine |  |
Cys C | Serine |  |
Ser S |
| Glutamine |  |
Gln Q | Threonine* |  |
Thr T |
| Glycine |  |
Gly G | Tryptophan* |  |
Trp W |
| Isoleucine* |  |
Ile I | Tyrosine |  |
Tyr Y |
| Leucine* |  |
Leu L | Valine* |  |
Val V |
| Methionine* |  |
Met M | Arginine* |  |
Arg R |
| Aspartic acid |  |
Asp D | Histidine* |  |
His H |
| Glutamic acid |  |
Glu E | Lysine* |  |
Lys K |
| Essential amino acids are labeled with an asterisk (*). | |||||
To determine the amino acids that form the peptide, work backwards. Break the amide bonds in red that join the amino acids together. This forms leucine, alanine, and tyrosine.
The tripeptide is named as a derivative of the C-terminal amino acid, tyrosine, with leucine and alanine as substituents; thus, the tripeptide is named leucylalanyltyrosine.