What are peptides ?
Peptides are small proteins. Peptides and proteins differ only in their length: Proteins >100 amino acids Peptides 2-100 amino acids Peptides and proteins are macromolecules, i.e. long molecules made from small subunits. They can be imagined as chains of pearls, composed of differently shaped beads. The “pearls” are the 20 proteinogenic amino acids (amino acids that are used in the cell to build peptides and proteins). They are linked together in any combination, i.e. the number of possible combinations is virtually infinite.
The amino acids may be repeated in any frequency, so for example, glycine (Gly) in the protein collagen is found in every third position in the chain: …-Xaa (mostly Pro (L-proline)-Yaa-Gly-… Apart from water, our body consists mainly of proteins. Although sharing the same structural principle, they have a huge variety of functions (see Table 2). Proteins are also a vitally important constituent of our diet. What makes this variety possible? Through variation in the amino acid building blocks and in length, macromolecules can be “constructed” with the most diverse properties imaginable!
Biologically active peptides are, for instance, hormones or toxins (see p. 23). The majority of our hormones are peptides of widely varying length: TRH (thyrotropin-releasing hormone) is a tripeptide (it consists of 3 amino acids). LHRH (also known as GnRH (gonadotrophin-releasing hormone) is a peptide made from 10 amino acids (decapeptide). Calcitonin consists of 32 amino acids. pTH (parathormone), with its 84 amino acids, is almost a protein. Insulin consists of 2 chains of peptides, one composed of 30 amino acids and one of 21 that are linked to each other via disulfide bridges (see p. 23). In general, peptide hormones are produced by specialized cells, released into the blood and transported to the target organ. The cells to be stimulated have receptors that recognize the hormones and bind them specifically. These receptors are specialized proteins that are embedded in the cell membrane. The binding of the hormone acts as a signal for the cell and the desired effect is triggered.
• 2 amino acids = dipeptide
• 3 amino acids = tripeptide
• 4 amino acids = tetrapeptide
• 5 amino acids = pentapeptide etc.
• a few amino acids (2-20) = oligopeptide
Endogenous proteins and their functions Examples Occurrence of the protein Requirements/function Myosin, actin Muscle tissue Flexible molecule, contractible Collagen (the most common protein in the body) Connective tissue, tendons, skin Molecule stable in shape, stretch-resistant Hemoglobin, albumines Blood Transporter molecule, soluble Enzymes such as the digestive enzyme trypsin in the small intestine (trypsin is a “protease”, which splits peptides and proteins) Throughout the body Enzymes = catalysts Hormones such as thyrotropin hormone (TSH) (stimulates the thyroid glands) Throughout the body Hormones = messengers Antibodies (immunoglobulins) Blood Immune defense 7 With the 20 amino acids that can occur several times in the peptide, even with short peptides many combinations are possible, e.g. approx. 3.2 million different pentapeptides! Even dipeptides can be biologically active: Leu-Trp* and other dipeptides lower blood pressure. Acetyl-Asp-Glu (NAAG) is an important neurotransmitter, a substance that mediates the transmission of signals between nerve cells. The peptides found in the body are usually obtained by enzymatic splitting or “cleavage” of proteins