Peptide hormones or protein hormones are hormones whose molecules are peptides, or proteins, respectively. The latter have longer amino acids chain lengths than the former. These hormones have an effect on the endocrine system of animals, including humans. Most hormones can be classified as either amino acid–based hormones (amine, peptide, or protein) or steroid hormones. The former are water-soluble and act on the surface of target cells via second messengers; the latter, being lipid-soluble, move through the plasma membranes of target cells (both cytoplasmic  and nuclear) to act within their nuclei. The four most biologically active peptides are:

i.                    Insulin

ii.                  Oxytocin

iii.                Vasopressin

iv.                Angiotensin II

Who determined the structure of insulin? 

Frederic Sanger

Frederic Sanger determined the structure of insulin, for which he got Nobel prize in 1958.