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What Are Peptides?

Peptides are short chains of amino acids joined by amide bonds, shorter than the chains usually called proteins. They include hormones the body makes itself, synthetic analogues of those hormones and fragments of larger proteins, and many of them work by binding a receptor on the surface of a cell. The formal definition and naming rules are set out on the peptide concept page and in the lesson linked below.[1],[2]

This page groups peptides into classes, describes how they act at receptors and how they are investigated from the laboratory bench to human trials, and sets research peptides beside approved peptide drugs. It ends with a map of the PepGenex Science compound library by research area.

Last reviewed 2026-09-26

What types of peptides are there?

Peptides can be grouped in more than one way at once. By length, the IUPAC-IUB nomenclature calls chains shorter than roughly 10 to 20 residues oligopeptides and longer ones polypeptides, and it notes that the word protein is usually kept for sequences beyond about 50 residues.[1]

By origin and structure, four groups recur across the published literature:

  • Endogenous peptide hormones: peptides the body produces as chemical signals. Growth hormone-releasing hormone (GHRH) is one; in 1992 Mayo cloned its pituitary receptor and showed that cells expressing it bind GHRH with high affinity and specificity.[3]
  • Synthetic analogues: an endogenous sequence changed on purpose. Semaglutide carries two amino acid substitutions relative to human GLP-1 (Aib8 and Arg34) and a derivatized lysine at position 26, changes its developers describe as raising albumin affinity and securing stability against degradation. Semax, the heptapeptide Met-Glu-His-Phe-Pro-Gly-Pro, is described as an analogue of the adrenocorticotropin fragment 4-10.[4],[5]
  • Fragments of larger proteins: a short stretch taken from a longer chain. Thymosin beta-4 is a 43-amino-acid polypeptide, and the key ingredient of TB-500 is its 17-23 sequence, LKKTETQ, carrying an added acetyl group at the N-terminus.[6],[7],[8]
  • Metal complexes: a peptide bound to a metal ion. GHK-Cu is the copper(II) complex of the tripeptide glycyl-L-histidyl-L-lysine, whose structure in solution was reported in 1982.[9]

Which receptor families do peptides act on?

Many peptide hormones signal through G protein-coupled receptors, a family whose members cross the cell membrane seven times. Mayo identified seven potential membrane-spanning domains in the cloned GHRH receptor, and Zhang and colleagues place the GLP-1 receptor in class B of the family.[3],[10]

The receptor for growth hormone secretagogues, cloned in 1996 from pituitary and hypothalamus of swine and humans, is also a G protein-coupled receptor. Raun and colleagues reported that ipamorelin, like GHRP-6, acts through a GHRP-like receptor, a conclusion drawn from profiling with GHRP and GHRH antagonists.[11],[12]

How do peptides work at receptors?

Rang describes receptors as the molecules responsible for chemical recognition, the step that links a signal arriving at a cell to a response inside it. A peptide that binds a receptor and switches it on in this way is an agonist.[2]

Structures show what switching on looks like. In the 2017 cryo-electron microscopy structure of a peptide-bound GLP-1 receptor, the peptide is held between the N-terminal domain and the transmembrane bundle of the receptor, and a bend in the sixth transmembrane helix makes room for the G protein Gs. In cells expressing the GHRH receptor, GHRH raised intracellular cAMP production.[10],[3]

Some synthetic peptides are built to act on more than one receptor. Tirzepatide is studied as a dual agonist at the GIP and GLP-1 receptors, and retatrutide as a triple agonist at the GLP-1, GIP and glucagon receptors; structural studies of both describe how each engages its receptors.[13],[14]

Binding does not always mean activation. In Kenakin's survey of 380 antagonists at 73 G protein-coupled receptor targets, 322 behaved as inverse agonists, which reduce a receptor's constitutive activity, and 58 as neutral antagonists.[15]

How are peptides researched?

Laboratory work outside a living organism, in cultured cells, membranes or purified receptors, measures binding and signalling under controlled conditions. Duval and colleagues caution that flat two-dimensional cultures can produce cell behaviour that departs appreciably from the response seen in living tissue.[16]

Animal studies add a whole organism, with limits on how far results carry over. Surveying 150 compounds, Olson and colleagues found that animal toxicity studies had flagged 71% of the human toxicities later seen in clinical development when rodent and non-rodent species were counted together, and 43% for rodents alone.[17]

Human studies of an investigational new drug in the United States run under an investigational new drug application: 21 CFR 312.20 requires a sponsor that intends to conduct such a clinical investigation to submit one, and not to begin until it is in effect.[18]

21 CFR 312.21 describes three phases that generally follow one another but can overlap. Phase 1 is the first introduction into humans, generally 20 to 80 subjects; phase 2 is controlled study of effectiveness for a particular indication in usually no more than several hundred subjects; phase 3 expands to several hundred to several thousand subjects to gather the further information needed for an overall evaluation of the drug.[19]

How do research peptides differ from approved peptide drugs?

In the United States a new drug is marketed under an application reviewed by FDA; 21 CFR part 314 sets out the procedures and requirements for those applications and for FDA's review of them.[20]

A shared name or sequence does not make two materials the same product. Lian and colleagues report that synthetic peptides can carry structural modifications introduced by their starting materials, their manufacturing process or their storage, which is why each material is characterized in its own right.[21]

How a label, a regulation and a product's distribution relate is covered on the research use only concept page. This page does not state the regulatory status of any product.

Which peptides does the PepGenex Science library cover?

The compound profiles in the PepGenex Science library are grouped by research area below; each profile records the compound's identity and the published records it is cited to.

  • GLP-1 and incretin research: Semaglutide, Tirzepatide, Retatrutide and Cagrilintide.
  • Growth hormone axis: Sermorelin, Tesamorelin, CJC-1295, CJC-1295 (no DAC), Ipamorelin, AOD-9604 and IGF1-LR3.
  • Repair and tissue research: BPC-157, TB-500, Thymosin beta-4, GHK-Cu, LL-37, KPV and Thymosin alpha-1.
  • Cognitive peptides: Semax, Selank and DSIP.
  • Mitochondrial peptides: MOTS-c, Elamipretide (SS-31), NAD+, 5-Amino-1MQ and Glutathione.
  • Receptor and cell-signalling research: PT-141, Epithalon and FOXO4-DRI.

Where can researchers access PepGenex research materials?

Research materials are available through Research Access.

Limitations

The classes on this page overlap: one peptide can be a synthetic analogue of an endogenous hormone and a G protein-coupled receptor agonist at the same time, and the length boundaries are conventions rather than chemical limits.[1]

A receptor finding from a cultured cell or a solved structure is a finding about that experimental system, and an animal result is a finding about that species.[16],[17]

This page describes peptides in general from published and regulatory sources. It does not describe any PepGenex material, it does not state the regulatory status of any product, and it contains no preparation or handling information.

References

  1. IUPAC-IUB Joint Commission on Biochemical Nomenclature. Nomenclature and symbolism for amino acids and peptides. Recommendations 1983. Biochem J. 1984;219(2):345-373 (section 3AA-11). PMID 6743224 · DOI 10.1042/bj2190345 · PMC1153490
  2. Rang HP. The receptor concept: pharmacology's big idea. Br J Pharmacol. 2006;147 Suppl 1:S9-S16. PMID 16402126 · DOI 10.1038/sj.bjp.0706457 · PMC1760743
  3. Mayo KE. Molecular cloning and expression of a pituitary-specific receptor for growth hormone-releasing hormone. Mol Endocrinol. 1992;6(10):1734-1744. PMID 1333056 · DOI 10.1210/mend.6.10.1333056
  4. Lau J, Bloch P, Schäffer L, et al. Discovery of the Once-Weekly Glucagon-Like Peptide-1 (GLP-1) Analogue Semaglutide. J Med Chem. 2015;58(18):7370-7380. PMID 26308095 · DOI 10.1021/acs.jmedchem.5b00726
  5. Dolotov OV, Karpenko EA, Inozemtseva LS, et al. Semax, an analog of ACTH(4-10) with cognitive effects, regulates BDNF and trkB expression in the rat hippocampus. Brain Res. 2006;1117(1):54-60. PMID 16996037 · DOI 10.1016/j.brainres.2006.07.108
  6. Philp D, Huff T, Gho YS, et al. The actin binding site on thymosin beta4 promotes angiogenesis. FASEB J. 2003;17(14):2103-2105. PMID 14500546 · DOI 10.1096/fj.03-0121fje
  7. Esposito S, Deventer K, Goeman J, et al. Synthesis and characterization of the N-terminal acetylated 17-23 fragment of thymosin beta 4 identified in TB-500, a product suspected to possess doping potential. Drug Test Anal. 2012;4(9):733-738. PMID 22962027 · DOI 10.1002/dta.1402
  8. Ho EN, Kwok WH, Lau MY, et al. Doping control analysis of TB-500, a synthetic version of an active region of thymosin β4, in equine urine and plasma by liquid chromatography-mass spectrometry. J Chromatogr A. 2012;1265:57-69. PMID 23084823 · DOI 10.1016/j.chroma.2012.09.043
  9. Freedman JH, Pickart L, Weinstein B, et al. Structure of the glycyl-L-histidyl-L-lysine--copper(II) complex in solution. Biochemistry. 1982;21(19):4540-4544. PMID 6291585 · DOI 10.1021/bi00262a004
  10. Zhang Y, Sun B, Feng D, et al. Cryo-EM structure of the activated GLP-1 receptor in complex with a G protein. Nature. 2017;546(7657):248-253. PMID 28538729 · DOI 10.1038/nature22394 · PMC5587415
  11. Howard AD, Feighner SD, Cully DF, et al. A receptor in pituitary and hypothalamus that functions in growth hormone release. Science. 1996;273(5277):974-977. PMID 8688086 · DOI 10.1126/science.273.5277.974
  12. Raun K, Hansen BS, Johansen NL, et al. Ipamorelin, the first selective growth hormone secretagogue. Eur J Endocrinol. 1998;139(5):552-561. PMID 9849822 · DOI 10.1530/eje.0.1390552
  13. Sun B, Willard FS, Feng D, et al. Structural determinants of dual incretin receptor agonism by tirzepatide. Proc Natl Acad Sci U S A. 2022;119(13):e2116506119. PMID 35333651 · DOI 10.1073/pnas.2116506119 · PMC9060465
  14. Li W, Zhou Q, Cong Z, et al. Structural insights into the triple agonism at GLP-1R, GIPR and GCGR manifested by retatrutide. Cell Discov. 2024;10(1):77. PMID 39019866 · DOI 10.1038/s41421-024-00700-0 · PMC11255275
  15. Kenakin T. Efficacy as a vector: the relative prevalence and paucity of inverse agonism. Mol Pharmacol. 2004;65(1):2-11. PMID 14722230 · DOI 10.1124/mol.65.1.2
  16. Duval K, Grover H, Han LH, et al. Modeling Physiological Events in 2D vs. 3D Cell Culture. Physiology (Bethesda). 2017;32(4):266-277. PMID 28615311 · DOI 10.1152/physiol.00036.2016 · PMC5545611
  17. Olson H, Betton G, Robinson D, et al. Concordance of the toxicity of pharmaceuticals in humans and in animals. Regul Toxicol Pharmacol. 2000;32(1):56-67. PMID 11029269 · DOI 10.1006/rtph.2000.1399
  18. 21 CFR 312.20, Requirement for an IND (current edition). 21 CFR 312.20
  19. 21 CFR 312.21, Phases of an investigation (current edition). 21 CFR 312.21
  20. 21 CFR 314.1, Scope of this part (applications for FDA approval to market a new drug; current edition). 21 CFR 314.1
  21. Lian Z, Wang N, Tian Y, Huang L. Characterization of Synthetic Peptide Therapeutics Using Liquid Chromatography-Mass Spectrometry: Challenges, Solutions, Pitfalls, and Future Perspectives. J Am Soc Mass Spectrom. 2021;32(8):1852-1860. PMID 34110145 · DOI 10.1021/jasms.0c00479