Question

why is insulin considered as a protein when it only has 49 amino acids

Answer 1

 is a peptide hormone produced by beta cells of the pancreatic islets, and it is considered to be the main anabolic hormone of the body.[5] It regulates the metabolism of carbohydrates, fats and protein by promoting the absorption of, especially, glucose from the blood into liver, fat and skeletal muscle cells.[6] In these tissues the absorbed glucose is converted into either glycogen via glycogenesis or fats(triglycerides) via lipogenesis, or, in the case of the liver, into both.[6] Glucose production and secretion by the liver is strongly inhibited by high concentrations of insulin in the blood.

Answer 2

The human insulin protein is composed of 51 amino acids, and has a molecular mass of 5808 Da. It is a dimer of an A-chain and a B-chain, which are linked together by disulfide bonds.  hence it is a protein.