Two amino acids can bond covalently forming a substituted amide bond, called a peptide bond:
To achieve this, the alpha carboxyl group of an amino acid has reacted with the alpha amino group of the other amino acid, and a water molecule has been lost:
Let's see it using the molecular models
Between the two amino acids, glycine and serine, the glycyl-serine dipeptide is formed:
covalently bonds carbonyl
of the first amino acid with the amino
of the second one.