Figure 1.

Amino acids, peptide bonds, polypeptides, and proteins. (a) Amino acid and peptide bond structure. The box indicates a peptide bond (-CO-NH-). (b–e) Examples of smaller bioactive peptide hormones also illustrating particular conformational aspects. (b) Met-enkephalin, a non-structured opioid penta-peptide. (c) Luteinizing hormone releasing hormone, a β-strand deca-peptide hormone. (d) Oxytocin, a small disulfide bridge-constrained uterus-contracting nona-peptide hormone. (e) Neuropeptide Y, a 36-amino acid peptide hormone containing an α-helix. Figure 1e is obtained from https://commons.wikimedia.org/wiki/File:Neuropeptide_Y.png.