11.2 The common amino acids that make up proteins have similar structures. (a) Each amino acid consists of a central carbon atom (C_α) attached to (1) an amino group (NH₃⁺); (2) a carboxyl group (COO⁻); (3) a hydrogen atom (H); and (4) a radical group, designated R. (b) Amino acids are joined together by peptide bonds. In a peptide bond (pink shading), the carboxyl group of one amino acid is covalently attached to the amino group of another amino acid.