In alanylglycyl leucyl alanyl valine, the number of peptide linkages is ___________.

Solution

Chemistry ma’am · 4 steps

Count the amino acids from the name first. The peptide bonds are always one fewer.

1. Read the name. Each "-yl" ending marks one amino acid joined to the next, and the last name keeps its full form. Alanyl, glycyl, leucyl, alanyl, valine gives 5 amino acids: Ala, Gly, Leu, Ala, Val.
2. Peptide bond. A peptide linkage is the amide bond -CO-NH⁻. It forms when the -COOH of one amino acid joins the -NH₂ of the next and a molecule of water is lost. Each bond joins exactly two neighbouring amino acids.

   Ala - Gly - Leu - Ala - Val

3. Count the joins. In a straight chain of n amino acids there are n - 1 gaps between neighbours. Here that is 5 - 1 = 4.
4. So. This pentapeptide has 4 peptide linkages.

The common slip: 5. Counting the amino acids instead of the bonds between them gives 5. Five residues in a chain need only 4 links.

The idea to keep. A linear peptide with n amino acid residues has n - 1 peptide bonds. Each "-yl" in the name is one residue, and the last residue keeps its full name.

Answer from the NTA final answer key. Solution by PrepWiser.