Peptide hormones were among the earliest signals regulating multi-cellular organisms and play critical roles controling bodily functions from bacteria to yeast to man.
For example, peptide hormones from the intestine regulate all aspects of our interaction with food (hunger, satiety, motility) and control blood glucose levels (glucagon, GLP-1, GIP, etc.). While these roles are not yet completely understood, multiple important pharmaceuticals have resulted from early work in the area.
Similarly, peptide signals from fat cells (cytokines, chemokines, adipokines) regulate many of the actions that control energy storage and the inflammatory state related to diabetes.
In addition, peptide hormones from the brain coordinate "tone" in the body,modulating response to stress as well as many basic bodily functions (fluid and electrolyte balance, parturition, sexual cycles, growth, et al.
In short, research to generate peptide hormone analogs with much improved, marketable pharmaceutical action (drug-like) is a remarkably rich field for drug discovery.
