2.3.1—Amino acids and peptide bonds

Amino acids are the monomers of proteins. Every amino acid has the same basic arrangement around a central carbon: an amine group, a carboxyl group, a hydrogen atom and a variable R group. The R group changes the amino acid’s properties, but it is not the part that forms the peptide link.

1. Position the carboxyl group of one amino acid beside the amine group of another.
2. Remove the hydroxyl group from the carboxyl group and a hydrogen from the amine group; together they form one water molecule.
3. Join the remaining carbonyl carbon to the nitrogen, creating a covalent peptide bond and a dipeptide.
4. Repeat the condensation reaction to join more amino acids and form a polypeptide chain.
5. In hydrolysis, add water across a peptide bond to break the chain back into smaller peptides or amino acids.

The sequence, number and types of amino acids in a polypeptide determine which interactions can occur as it folds. Peptide-bond formation therefore creates the covalent chain that later supports protein structure and function; hydrolysis reverses the joining process.

A polypeptide is a chain of amino acids joined by peptide bonds, but it is not automatically a functional protein until it folds or assembles correctly. When locating a peptide bond, look for the carbon–nitrogen link next to the carbonyl carbon; do not mistake an R group for the peptide bond.