Obesity hormone peptide YY. Molecular model of part of the hormone peptide YY (or PYY3-36), a hormone that helps to regulate appetite. It is released by parts of the gastrointestinal tract after a meal, in levels proportional to the calorific content of the food. It has come under scrutiny as it has been found that injections of PYY3-36 can dramatically reduce appetite. This has made it a target for anti-obesity treatments. In this model, atoms are represented as balls, and are colour-coded: carbon, light blue; nitrogen, dark blue; oxygen, orange; and hydrogen, yellow. The strip shows the structure of the protein.