Two amino acids can get covalently linked by an amide bond, called peptide bond:
This bond is formed when the alpha carboxyl group of an amino acid reacts with the alpha amino group of the other amino acid, releasing a water molecule. For example, three amino acids can join in a tripeptide:
The alpha carbons of each amino acid alternate with the peptide bonds, forming the “backbone” of the polypeptide.
(the sequence is always given starting by the amino acid whose amino group is free, called the N-terminal amino acid; in this case, it is Lys).
backbone and sidechains.