Short answer: No. Peptides and steroids are different classes of molecule. A peptide is built from amino acids joined by peptide (amide) bonds [2][4]. A steroid is built on a fused four-ring carbon skeleton, cyclopenta[a]phenanthrene, and is grouped with the lipids [1][6]. The U.S. National Library of Medicine's MeSH vocabulary files the two in separate branches: steroids under polycyclic, fused-ring compounds, and peptides under amino acids, peptides, and proteins [3][4].
This article is educational chemistry for laboratory readers. It does not cover legal status, health effects, or use in people or animals.
What makes a molecule a peptide
ChEBI, the EMBL-EBI chemical database, defines a peptide as an amide derived from two or more amino carboxylic acid molecules, formed by a covalent bond from the carbonyl carbon of one to the nitrogen atom of another, with formal loss of water [2]. MeSH describes peptides as compounds made of amino acids joined by peptide bonds between adjacent amino acids, in linear, branched, or cyclical structures [4].
In a peptide chain, the amino group of one residue forms an amide bond with the carboxyl group of the next, and so on down the chain. The repeating –N–CH–CO– atoms form the backbone [9]. The amide bond is planar, and a second kind of covalent link, a disulfide bridge, can form between two cysteine residues [9]. For more on the bond itself, see our guide to the peptide bond.
Size labels vary by source. The National Human Genome Research Institute calls a chain of typically 2 to 50 amino acids a peptide and 51 or more a polypeptide [5]. MeSH puts oligopeptides at approximately 2 to 12 amino acids and polypeptides at approximately 13 or more [4]. Our peptide vs protein article covers those size terms in more depth.
What makes a molecule a steroid
ChEBI defines steroids as naturally occurring compounds and synthetic analogues based on the cyclopenta[a]phenanthrene carbon skeleton, partially or completely hydrogenated. There are usually methyl groups at C-10 and C-13, and often an alkyl group at C-17 [1]. MeSH describes the same core as a fused, reduced 17-carbon ring system, and notes that most steroids also carry two methyl groups and an aliphatic side chain [3].
OpenStax Organic Chemistry describes steroid structures as based on a tetracyclic ring system, with the four rings designated A, B, C, and D. The three six-membered rings (A, B, and C) adopt chair conformations, but their rigid geometry prevents the usual cyclohexane ring-flips [10]. OpenStax Biology 2e puts it simply: all steroids have four linked carbon rings, and several of them, like cholesterol, have a short tail. Many steroids also carry an –OH group, which puts them in the alcohol classification (sterols) [6].
Steroids are grouped with the lipids. They don't resemble fats or phospholipids, but they are also hydrophobic and insoluble in water [6].
Peptides vs steroids, side by side
- Building blocks: Peptides are assembled from amino acids [2][4]. Natural steroids are derived biogenetically from squalene, a triterpene [1]; OpenStax describes them as derived from the triterpenoid lanosterol [10].
- Core structure: A peptide is a chain (linear, branched, or cyclical) of amino acid residues on an –N–CH–CO– backbone [4][9]. A steroid is a fused four-ring carbon skeleton [1][6].
- What defines it: A peptide is defined by its amide (peptide) bonds, each a covalent bond from the carbonyl carbon of one amino acid to the nitrogen atom of another [2]. A steroid is defined by its ring skeleton [1][3].
- Chemical family: Steroids are grouped with the lipids [6]. MeSH places peptides with amino acids and proteins, and steroids with fused-ring polycyclic compounds [3][4].
- Water and lipid solubility: Steroids are hydrophobic and insoluble in water [6]. Polypeptide hormones are described as water-soluble and insoluble in lipids [7].
A look at the formulas
PubChem's records make the difference concrete:
- Testosterone (a steroid): C₁₉H₂₈O₂, molecular weight 288.4. Its systematic IUPAC name is built on "cyclopenta[a]phenanthren-3-one", the steroid ring system [11].
- Cholesterol (a steroid): C₂₇H₄₆O, molecular weight 386.7, also named as a cyclopenta[a]phenanthrene derivative [12].
- Oxytocin (a peptide hormone): C₄₃H₆₆N₁₂O₁₂S₂, molecular weight 1007.2 [13].
Why "hormone" doesn't settle it
Part of the confusion is that both classes include hormones. OpenStax Biology 2e divides hormones into three classes by chemical structure: lipid-derived, amino acid-derived, and peptide (peptide and protein) hormones [7]. "Hormone" describes a job, not a chemical class.
- Steroid hormones are the primary class of lipid hormones in humans. Most lipid hormones are derived from cholesterol and are structurally similar to it; examples include estradiol and testosterone [7]. Cholesterol is the most common steroid [6].
- Peptide hormones have the structure of a polypeptide chain. The class runs from short chains such as oxytocin and antidiuretic hormone to small proteins and large glycoproteins [7].
Insulin is a useful check. It is a peptide hormone [7], made of two chains that total 51 amino acids, linked by two cysteine disulfide bridges [9]. It has amino acid chains and peptide bonds, not a steroid ring system, so it is not a steroid.
Not every steroid is a hormone, and not every peptide is either
The steroid family is broader than hormones. MeSH lists cholesterol, numerous hormones, precursors of certain vitamins, bile acids, sterols, and certain natural drugs and poisons as steroids [3]. Peptides span a wide range too. NobelPrize.org's 1984 press release notes that a number of hormones and other signal substances are peptides that contain considerably fewer amino acid residues than proteins do [14].
How each is made in the lab
The 1984 Nobel Prize in Chemistry went to R. Bruce Merrifield for developing a method of chemical synthesis on a solid matrix for obtaining peptides and proteins. His method binds the first of the many amino acid residues to a polymer, and the Nobel press release notes it is more rapid than earlier methods and gives a considerably greater quantity of final product [14]. Our guide to peptide synthesis walks through the solid-phase method step by step. Steroid chemistry is separate. OpenStax notes that, in addition to the hundreds of steroids isolated from plants and animals, many more have been synthesized in pharmaceutical laboratories [10].
FAQ
Are peptides steroids? No. Peptides are amino acids joined by peptide bonds [2][4]. Steroids are compounds built on the cyclopenta[a]phenanthrene four-ring skeleton [1].
What is the main chemical difference between peptides and steroids? The core structure. A peptide is a chain of amino acid residues linked by amide bonds [2][9]. A steroid is built on a fused four-ring carbon skeleton [1][6], usually with methyl groups at C-10 and C-13 [1].
Is insulin a steroid? No. Insulin is a peptide hormone made of two chains totaling 51 amino acids [7][9].
Is cholesterol a steroid? Yes. Cholesterol is the most common steroid and the precursor to many steroid hormones, such as testosterone and estradiol [6].
Are steroids proteins? No. Steroids are grouped with the lipids [6], while proteins are larger versions of peptides [4].
Are all steroids hormones? No. The steroid class also includes cholesterol, bile acids, sterols, and precursors of certain vitamins [3].
Bottom line
Some members of each class act as hormones [7], but peptides and steroids are built differently. Peptides are amino acid chains held together by peptide bonds [2][4]. Steroids are four-ring molecules grouped with the lipids [6]; natural steroids are derived biogenetically from squalene [1]. For more terminology, see What Are Peptides? and our research peptide glossary.
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