Peptide hormones or protein hormones are hormones whose molecules are peptides or proteins, respectively. The latter have longer amino acidchain 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 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 to act within their nuclei. Like all peptides and proteins, peptide hormones and protein hormones are synthesized in cells from amino acids according to mRNA transcripts, which are synthesized from DNAtemplates inside the cell nucleus. Preprohormones, peptide hormoneprecursors, are then processed in several stages, typically in the endoplasmic reticulum, including removal of the N-terminalsignal sequence and sometimes glycosylation, resulting in prohormones. The prohormones are then packaged into membrane-bound secretory vesicles, which can be secreted from the cell by exocytosis in response to specific stimuli. These prohormones often contain superfluous amino acid residues that were needed to direct folding of the hormone molecule into its active configuration but have no function once the hormone folds. Specific endopeptidases in the cell cleave the prohormone just before it is released into the bloodstream, generating the mature hormone form of the molecule. Mature peptide hormones then travel through the blood to all of the cells of the body, where they interact with specific receptor on the surfaces of their target cells. Some neurotransmitters are secreted and released in a similar fashion to peptide hormones, and some "neuropeptides" may be used as neurotransmitters in the nervous system in addition to acting as hormones when released into the blood. When a peptide hormone binds to a receptor on the surface of the cell, a second messenger appears in the cytoplasm, which triggers signal transduction leading to the cellular responses. Some peptide/protein hormones also interact with intracellular receptors located in the cytoplasm or nucleus by an intracrine mechanism.
