Is It a Peptide? How GLP-1, Tirzepatide, NAD+, BPC-157 and Other Research Compounds Are Classified

Introduction

A peptide is a molecule of two to roughly 50 amino acids joined end to end by peptide bonds, the amide linkages formed when one amino acid's carboxyl group condenses with the next one's amino group. Whether a research compound is a peptide is therefore a question of structure: GLP-1, semaglutide, tirzepatide, BPC-157 and glutathione are peptides; NAD+, steroids and melatonin are not.

This guide classifies the compounds most often searched with the question "is it a peptide," one by one, with a citation behind each answer. It is a chemistry reference and says nothing about the effects of any compound; background on amino-acid chemistry is in what peptides are.

Key facts

• A peptide bond is a planar amide linkage with partial double-bond character, as reported by Pauling, Corey and Branson in 1951 [2].
• Glucagon-like peptide-1 circulates as GLP-1(7-36)amide (30 residues) and GLP-1(7-37) (31 residues), both cleaved from proglucagon [3].
• Semaglutide is a 31-residue GLP-1 analogue carrying a C18 fatty diacid on lysine 26 [4].
• Tirzepatide is a 39-residue dual GIP and GLP-1 receptor agonist carrying a C20 fatty diacid [6].
• NAD+ has the molecular formula C21H27N7O14P2 and contains no amino acids [10].
• Glutathione is the tripeptide gamma-L-glutamyl-L-cysteinylglycine, joined through a non-standard gamma-carboxyl linkage [11].
• Insulin contains 51 amino acids in two chains linked by disulfide bonds, as established by Sanger [15].
• Steroids share a four-ring carbon skeleton derived from cholesterol and contain no peptide bonds [17].

What is a peptide?

A peptide is a chain of amino acids, conventionally two to about 50 residues long, linked by peptide bonds [1]. Each amino acid contributes an amino group, a carboxyl group and a side chain; the sequence of side chains is the peptide's primary structure. Chains of more than roughly 50 residues that fold into stable shapes are called proteins, but the boundary is a convention of size rather than a change in bonding [1].

Several related terms describe what a peptide is not. A protein is a longer polypeptide with defined folding. A hormone is a signaling molecule defined by function, and hormones may be peptides (insulin), steroids (cortisol) or amines (melatonin). A steroid is a lipid built on a four-ring skeleton. A coenzyme is a non-protein organic molecule that assists an enzyme, such as NAD+.

A small molecule is a low-molecular-weight organic compound outside the peptide, protein and nucleic-acid classes. Whether a research compound counts as a peptide depends only on whether it is a chain of amino acids joined by peptide bonds, the same criterion applied to research peptides sold as laboratory reagents.

What is a peptide bond?

A peptide bond is the covalent amide bond formed between the carboxyl carbon of one amino acid and the amino nitrogen of the next, with loss of a water molecule [1]. Pauling, Corey and Branson reported in 1951 that resonance between the carbonyl oxygen and the amide nitrogen gives the bond partial double-bond character, so that the six atoms around it lie in a plane and rotation about the carbon-nitrogen bond is restricted [2].

That planarity underlies the alpha-helix and beta-sheet conformations described in the same paper [2]. Every chain has an amino (N) terminus and a carboxyl (C) terminus, and the bond is identical whether formed on a ribosome or by solid-phase chemical synthesis.

Is GLP-1 a peptide?

Yes. Glucagon-like peptide-1 (GLP-1) is an endogenous peptide hormone produced by cleavage of the proglucagon precursor in intestinal L-cells, as reviewed by Holst in Physiological Reviews in 2007 [3]. The two active circulating forms are GLP-1(7-36)amide, 30 amino acids long, and GLP-1(7-37), 31 amino acids long; the numbering refers to positions within the 37-residue GLP-1(1-37) segment [3].

GLP-1 is classified as an incretin, a gut hormone released after nutrient ingestion, and is the reference sequence for the GLP-1 receptor agonist class [3]. The comparison of GLP-1, GLP-2 and GLP-3 analogues sets out how the synthetic analogues below differ from the native hormone.

Is semaglutide a peptide?

Yes. Semaglutide, the active ingredient marketed under brand names including Ozempic, is a synthetic lipidated analogue of GLP-1(7-37) with 31 amino-acid residues [4, 5]. Lau and colleagues reported its design in the Journal of Medicinal Chemistry in 2015: aminoisobutyric acid replaces alanine at position 8, arginine replaces lysine at position 34, and the remaining lysine at position 26 carries a C18 fatty diacid attached through a gamma-glutamate and two 8-amino-3,6-dioxaoctanoic acid spacer units [4].

The fatty diacid is a modification of a peptide, not a departure from the class: semaglutide is a peptide by structure and a GLP-1 receptor agonist by pharmacological class [4]. The FDA approved it under new drug application 209637 in December 2017 [5].

Is tirzepatide a peptide?

Yes. Tirzepatide is a synthetic 39-amino-acid peptide that acts as an agonist at both the glucose-dependent insulinotropic polypeptide (GIP) receptor and the GLP-1 receptor [6]. Coskun and colleagues described it in Molecular Metabolism in 2018 as a single-chain peptide based on the GIP sequence, with aminoisobutyric acid at two positions and a C20 fatty diacid attached to a lysine side chain through a linker [6]. It was approved by the FDA under new drug application 215866 in 2022 as Mounjaro [7].

Retatrutide belongs to the same family: Coskun and colleagues reported in Cell Metabolism in 2022 that LY3437943, later named retatrutide, is a single synthetic peptide carrying a C20 fatty diacid that acts at the GIP, GLP-1 and glucagon receptors [8]. It is investigational and a peptide by structure.

Is NAD+ a peptide?

No. Nicotinamide adenine dinucleotide (NAD+) is a dinucleotide coenzyme. It consists of two nucleotides, one carrying nicotinamide and the other adenine, joined through their phosphate groups; PubChem records the molecular formula C21H27N7O14P2 for the oxidized cation [10]. It contains no amino acids and no peptide bonds.

Cantó, Menzies and Auwerx reviewed NAD+ metabolism in Cell Metabolism in 2015, describing it as a redox cofactor that alternates between oxidized (NAD+) and reduced (NADH) forms and as a substrate for enzyme families including sirtuins and poly(ADP-ribose) polymerases [9]. NAD+ is listed alongside peptides in research catalogs only because it is supplied in the same lyophilized form; NAD+ research material is a coenzyme, not a peptide.

Are peptides steroids?

No. Peptides and steroids are different chemical classes. A peptide is a chain of amino acids joined by peptide bonds [1]. A steroid is a lipid built on a four-ring carbon skeleton, the cyclopentanoperhydrophenanthrene nucleus, and every human steroid hormone is synthesized from cholesterol, as reviewed by Miller and Auchus in Endocrine Reviews in 2011 [17]. Testosterone, estradiol, cortisol and aldosterone are steroids; none contains a peptide bond.

The confusion arises because both classes include hormones and both appear in anti-doping rules. They are legally distinct as well: 21 U.S.C. 802(41) defines an anabolic steroid as a substance "chemically and pharmacologically related to testosterone," which no peptide is [18]. Growth hormone secretagogue peptides such as ipamorelin are short amino-acid chains, not steroids; the regulatory consequences are discussed in are research peptides legal.

Which research compounds are peptides and which are not?

The table below classifies each compound by structure alone. "Yes" means a chain of amino acids joined by peptide bonds; "No" means the molecule belongs to another chemical class.

Compound	Peptide?	Chemical class	Structural facts	Source
GLP-1	Yes	Endogenous peptide hormone (incretin)	30 or 31 residues, cleaved from proglucagon	[3]
Semaglutide	Yes	Lipidated GLP-1 analogue	31 residues, C18 diacid on Lys26, Aib8, Arg34	[4]
Tirzepatide	Yes	Dual GIP/GLP-1 receptor agonist	39 residues, C20 diacid, two Aib residues	[6]
Retatrutide	Yes	Triple GIP/GLP-1/glucagon receptor agonist	Single synthetic peptide with a C20 diacid; investigational	[8]
NAD+	No	Dinucleotide coenzyme	C21H27N7O14P2; two nucleotides, no amino acids	[9, 10]
Glutathione	Yes	Tripeptide	gamma-Glu-Cys-Gly, with a non-standard gamma linkage	[11]
BPC-157	Yes	Synthetic pentadecapeptide	15 residues, Gly-Glu-Pro-Pro-Pro-Gly-Lys-Pro-Ala-Asp-Asp-Ala-Gly-Leu-Val	[12]
TB-500	Yes	Acetylated heptapeptide fragment	Ac-LKKTETQ, residues 17 to 23 of thymosin beta-4	[13]
GHK-Cu	Yes	Tripeptide copper(II) complex	Gly-His-Lys bound to one copper(II) ion	[14]
Insulin	Yes (peptide hormone, often called a small protein)	Two-chain polypeptide hormone	51 residues: A chain 21, B chain 30, disulfide-linked	[15]
Collagen peptides	Yes (mixture)	Hydrolyzed collagen fragments, food ingredient	Variable-length peptides from enzymatic hydrolysis; out of scope for research catalogs	[16]
Steroids (testosterone etc.)	No	Cholesterol-derived lipids	Four-ring carbon skeleton, no peptide bonds	[17, 18]
Melatonin	No	Indoleamine	N-acetyl-5-methoxytryptamine, C13H16N2O2	[19]
MOTS-c	Yes	Mitochondrial-derived peptide	16 residues, encoded within the mitochondrial 12S rRNA gene	[20]
Epithalon	Yes	Synthetic tetrapeptide	Ala-Glu-Asp-Gly (AEDG)	[21]

A few entries need a note. Glutathione has an irregular bond: Meister and Anderson described it in 1983 as gamma-L-glutamyl-L-cysteinylglycine, in which glutamate is joined to cysteine through its side-chain gamma-carboxyl rather than its alpha-carboxyl [11]. It is still a tripeptide, and glutathione research material is classified as one.

BPC-157 is a synthetic 15-residue peptide whose sequence Seiwerth, Sikiric and colleagues report in their reviews [12]; BPC-157 research material is a peptide by any definition. TB-500 is the N-terminally acetylated 17-23 fragment of the 43-residue protein thymosin beta-4, as characterized by Esposito and colleagues in 2012 [13]; the parent protein is covered in the thymic peptides overview.

GHK-Cu is the tripeptide glycyl-L-histidyl-L-lysine, first isolated by Pickart and Thaler in 1973, in complex with a copper(II) ion [14]. The metal makes it a metallopeptide, not a non-peptide, as explained in copper peptides explained.

Insulin sits at the peptide-protein boundary. Sanger established that it contains 51 amino acids in two chains joined by disulfide bonds [15], just above the conventional 50-residue line, and the literature calls it both a peptide hormone and a small protein. Collagen peptides are variable-length fragments produced by enzymatic hydrolysis of collagen for use as a food ingredient, as reviewed by León-López and colleagues in 2019 [16]; they are peptides chemically but outside a defined-sequence research catalog.

MOTS-c is a 16-residue peptide that Lee and colleagues reported in 2015 to be encoded within the mitochondrial 12S ribosomal RNA gene [20]. Epithalon (epitalon) is the synthetic tetrapeptide Ala-Glu-Asp-Gly; Khavinson and colleagues reported identifying the AEDG sequence in a pineal polypeptide preparation in 2017 [21]. Melatonin is an indoleamine, N-acetyl-5-methoxytryptamine, C13H16N2O2 in PubChem [19]; it is a hormone but not a peptide.

Summary

A peptide is a chain of two to roughly 50 amino acids joined by planar amide peptide bonds, and classification is a matter of structure alone. GLP-1 (30 or 31 residues), semaglutide (31 residues, C18 diacid), tirzepatide (39 residues, C20 diacid), retatrutide, glutathione, BPC-157, TB-500, GHK-Cu, MOTS-c and epithalon are peptides; insulin, at 51 residues, is a peptide hormone at the protein boundary.

NAD+ is a dinucleotide coenzyme, steroids are cholesterol-derived four-ring lipids, and melatonin is an indoleamine; none contains a peptide bond. Each classification rests on a primary paper, a database entry or an FDA record listed in the references.

References

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5. U.S. Food and Drug Administration. Drugs@FDA: OZEMPIC (semaglutide), NDA 209637, original approval December 5, 2017. Drugs@FDA
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10. National Center for Biotechnology Information. PubChem Compound Summary for CID 5893, NAD+. PubChem
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