A tetrapeptide is a peptide
Peptides are short polymers of amino acid monomers linked by peptide bonds. They are distinguished from proteins on the basis of size, typically containing less than 50 monomer units. The shortest peptides are dipeptides, consisting of two amino acids joined by a single peptide bond...

 consisting of four amino acids joined by peptide bond
Peptide bond
This article is about the peptide link found within biological molecules, such as proteins. A similar article for synthetic molecules is being created...

s. Many tetrapeptides are pharmacologically active, often showing affinity and specificity for a variety of receptors in protein-protein signaling. Present in nature are both linear and cyclic tetrapeptides; tetrapeptides may be cyclized by a fourth peptide bond or other covalent bonds.

Examples of tetrapeptides are:
  • Tuftsin
    Tuftsin is a tetrapeptide produced by enzymatic cleavage of the Fc-domain of the heavy chain of immunoglobulin G. It is produced primarily in the spleen.-Function:Its biological activity is related primarily to the immune system function....

     (L-threonyl-L-lysyl-L-prolyl-L-arginine) is a peptide related primarily to the immune system
    Immune system
    An immune system is a system of biological structures and processes within an organism that protects against disease by identifying and killing pathogens and tumor cells. It detects a wide variety of agents, from viruses to parasitic worms, and needs to distinguish them from the organism's own...

  • Rigin (glycyl-L-glutaminyl-L-prolyl-L-arginine) is a tetrapeptide with functions similar to those of tuftsin.
  • Postin (Lys-Pro-Pro-Arg) is the N-terminal tetrapeptide of cystatin C
    Cystatin C
    Cystatin C or cystatin 3 , a protein encoded by the CST3 gene, is mainly used as a biomarker of kidney function. Recently, it has been studied for its role in predicting new-onset or deteriorating cardiovascular disease...

     and an antagonist of tuftsin.
  • Endomorphin-1
    Endomorphins are two endogenous opioid peptides. Endomorphin-1 and endomorphin-2 are tetrapeptides with the highest known affinity and specificity for the μ opioid receptor...

     (H-Tyr-Pro-Trp-Phe-NH2) and endomorphin-2
    Endomorphins are two endogenous opioid peptides. Endomorphin-1 and endomorphin-2 are tetrapeptides with the highest known affinity and specificity for the μ opioid receptor...

    (H-Tyr-Pro-Phe-Phe-NH2) are peptide amides with the highest known affinity and specificity for the μ opioid receptor
    Mu Opioid receptor
    The μ-opioid receptors are a class of opioid receptors with high affinity for enkephalins and beta-endorphin but low affinity for dynorphins. They are also referred to as μ opioid peptide receptors. The prototypical μ receptor agonist is the opium alkaloid morphine; μ refers to morphine...

  • Morphiceptin
    Morphiceptin is a tetrapeptide that is aselective μ-opioid receptor agonist. It is derived from β-casomorphin and has over 1,000 times selectivity for μ- over δ-opioid receptors. When administered intracerebroventricularly, morphiceptin had an analgesic ED50 of 1.7 nmol per animal...

     (H-Tyr-Pro-Phe-Pro-NH2) is a casomorphin
    Casomorphins are peptides, i.e., protein fragments, derived from the digestion of milk protein casein. The distinguishing characteristic of casomorphins is that they have an opioid effect....

     peptide isolated from β-casein
    Casein is the name for a family of related phosphoprotein proteins . These proteins are commonly found in mammalian milk, making up 80% of the proteins in cow milk and between 60% and 65% of the proteins in human milk....

  • Gluten exorphine
    Gluten exorphine
    Gluten exorphins are a group of opioid peptides which are formed during digestion of the gluten protein. It has been hypothesized that people with autism and schizophrenia have abnormal leakage from the gut of these compounds, which then pass into the brain and disrupt brain function. This is...

    s A4 (H-Gly-Tyr-Tyr-Pro-OH) and B4 (H-Tyr-Gly-Gly-Trp-OH) are peptides isolated from gluten
    Gluten is a protein composite found in foods processed from wheat and related grain species, including barley and rye...

  • Tyrosine-MIF-1 (Tyr-Pro-Leu-Gly-NH2) is an endogenous opioid modulator.
  • Tetragastrin (N-((phenylmethoxy)carbonyl)-L-tryptophyl-L-methionyl-L-aspartyl-L-phenylalaninamide) is the C-terminal tetrapeptide of gastrin
    In humans, gastrin is a peptide hormone that stimulates secretion of gastric acid by the parietal cells of the stomach and aids in gastric motility. It is released by G cells in the antrum of the stomach, duodenum, and the pancreas...

    . It is the smallest peptide fragment of gastrin which has the same physiological and pharmacological activity as gastrin.
  • Kentsin (H-Thr-Pro-Arg-Lys-OH) is a contraceptive peptide first isolated from female hamster
    Hamsters are rodents belonging to the subfamily Cricetinae. The subfamily contains about 25 species, classified in six or seven genera....

  • Achatin-I (glycyl-phenylalanyl-alanyl-aspartic acid) is a neuroexcitatory tetrapeptide from giant African snail
    Giant African snail
    The East African land snail, or giant African land snail, scientific name Achatina fulica, is a species of large, air-breathing land snail, a terrestrial pulmonate gastropod mollusk in the family Achatinidae....

     (Achatina fulica).
  • Tentoxin
    Tentoxin is a natural cyclic tetrapeptide produced by phytopathogenic fungus Alternaria alternata. It selectively induces chlorosis in several germinating seedling plants. Therefore, tentoxin may be used as a potential natural herbicide....

     (cyclo(N-methyl-L-alanyl-L-leucyl-N-methyl-trans-dehydrophenyl-alanyl-glycyl)) is a natural cyclic tetrapeptide produced by phytopathogenic fungi from genus Alternaria
    Alternaria is a genus of ascomycete fungi. Alternaria species are known as major plant pathogens. They are also common allergens in humans, growing indoors and causing hay fever or hypersensitivity reactions that sometimes lead to asthma...

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