What functional groups are found in proteins?

The functional groups that are found in proteins include: an amino, carboxyl, and an R group. The amino group (NH2) consists of a nitrogen bonded to two hydrogen atoms. It is present in alpha-amino acids, which are the building blocks of proteins. The carboxyl group consists of a carbon atom bonded to an oxygen atom by a double bond and to a hydroxyl group (-OH) through a single bond. The R group is the side chain and it varies among different amino acids. It determines interactions of each amino acid. The R group comes from the 20 amino acids which occur in proteins.