Peptides are classified into several types based on their production mechanisms. One key class is ribosomal peptides, which are synthesized from the translation of mRNA. These peptides often serve as hormones and signaling molecules, including tachykinin peptides, vasoactive intestinal peptides, opioid peptides, pancreatic peptides, and calcitonin peptides. Some ribosomal peptides, such as microcins, function as antibiotics and are produced by specific organisms. Ribosomal peptides typically undergo proteolysis, which involves the breakdown of proteins into smaller peptides or amino acids, to achieve their mature form.
On the other hand, nonribosomal peptides are synthesized by specific peptide-synthesizing enzymes, rather than by ribosomes. These peptides are often cyclic, although linear forms are also common, and are known for their complex structures. Nonribosomal peptides are prevalent in plants, fungi, and unicellular organisms, with glutathione being the most notable example, crucial for antioxidant defenses in aerobic organisms.
Milk peptides are another class, derived from milk proteins either through enzymatic breakdown by digestive enzymes or by proteinases from lactobacilli during milk fermentation. Peptones, which are peptides obtained from the proteolytic digestion of animal milk or meat, are used in laboratories as nutrients for cultivating bacteria and fungi.
Additionally, peptide fragments are typically the result of enzymatic degradation in controlled laboratory settings, but they can also occur naturally due to environmental degradation. These classifications highlight the diverse origins and functions of peptides in biological systems.