Peptides are large polypeptide chains comprised of up to 50 amino acids that can be classified by function and also by synthesis. Some common types of peptides classified by function include hormones, neuropeptides, and alkaloids. When classified by synthesis, peptides can be ribosomal, nonribosomal, and peptonic. The classification system of peptides is considered an imperfect science because one peptide can belong to multiple groups simultaneously and scientists continue to debate when a peptide should be classified as a protein, or a protein as a peptide. Many scientists agree that a peptide does not conform easily to a specific pattern, whereas a protein is more distinct in nature to conformation.
Hormones, one of the most common classifications of peptide in the human body, are specific messenger molecules that are used in cellular communication. Hormones are identified as such because they are secreted and synthesized by specialized teams of cells, called endocrine glands. After secretion, hormone peptides travel to target organs where they act. A hormone’s shape is specifically related to receptors on the corresponding target organ’s cell membranes. For example, the hormone peptides glucagon and insulin have specific receptor sites in the liver that help them to control blood sugar levels.
Neuropeptides are widespread in the body’s central and peripheral nervous system and have specific inhibitory and excitatory functions. Neuropeptides work in much of the same way as neurotransmitters like serotonin and dopamine. One of the most well-known class of neuropeptides are endorphins. Endorphins are thought of as the body’s endogenous painkillers, often compared to the drug morphine.
Part of the function of endorphins includes the inhibition of another neuropeptide, Substance P, which transmits pain signals from the peripheral nervous system to receivers in the central nervous system. Sometimes, neuropeptides can work as hormones within certain systems of the body as well.
In regard to the classification of peptides due to their synthesis, most are the ribosomal type; this peptide type is synthesized when the messenger ribonucleic acid (mRNA) on the cell is translated. During translation, a chemical process occurs where one carboxyl group on an amino acid couples with another amino acid to create the beginning of an amino acid chain. Ribosomal peptides are often made up of 30 to 40 amino acids joined together. The nonribosomal type of peptide is synthesized when there are enzymatic catalysts present. Peptonic peptides are rare and are formed during digestion processes in the body.