What is a peptide? From amino acids to medicines

Every amino acid carries two reactive groups: an acid (carboxylic) group and a basic (amino) group. When the amino group of one amino acid reacts with the acid group of another, the bond between them makes a dipeptide.¹ Chemists call this joining a condensation, because a molecule of water is given off. Digestion runs the same reaction backwards: enzymes in the gut cut the peptide bond by adding water back, which is called hydrolysis.^5,6

A chain therefore has two different ends: the amino end (N-terminus) and the acid end (C-terminus). A sequence is written as a row of three-letter codes that follow international naming rules, and regulators ask makers to mark both ends and any changes to them.⁴ BPC-157, for example, is written Gly-Glu-Pro-Pro-Pro-Gly-Lys-Pro-Ala-Asp-Asp-Ala-Gly-Leu-Val: fifteen amino acids in a row.⁷

There are 20 different amino acids in proteins, and in a natural protein their order is written in a gene.³ Order is everything. It has long been accepted that the sequence of amino acids is what gives each protein its own chemical and physiological properties.⁸ Two chains with the same amino acids in a different order are different molecules.

In the United States the line has legal weight. A federal rule defines a protein as an amino acid chain of more than 40 amino acids, and counts chains that belong together as one.⁹ Insulin, with 51 amino acids in two chains, therefore moved from the drug rules to the rules for biological products on 23 March 2020.¹⁰ The molecule did not change; only the paperwork did.

Your body does not use either word. How well a molecule binds its target depends on its chemistry and its shape, whatever it is called.⁶ In the same way, “peptide” is not a quality mark. It covers tested medicines such as semaglutide, experimental molecules such as BPC-157, and deadly natural toxins (see the last section of this page).

• Insulin is two chains held together. The Nobel committee described in 1958 how Frederick Sanger worked out the order of its 51 amino acids and showed that it has two different chains, joined by two bridges of sulfur atoms.⁸
• Dulaglutide (Trulicity) is built from two identical chains linked by disulfide bonds. Each chain carries a GLP-1-like peptide joined to part of an antibody.¹³
• Ring-shaped peptides. Chemists can close a chain into a ring. A ring helps a short peptide hold a helix or a turn that it could not keep on its own.¹¹

Getting through barriers

Many pills are small molecules, and small size helps a medicine pass through cell membranes.¹⁷ Natural peptides are typically large, water-loving and poorly able to cross membranes, and they are cleared quickly.¹⁸ That is why most peptides work on receptors on the outside of cells (see how signalling works) and why most are injected (see why not a pill).

The kidney filter

The kidney’s filter (the glomerulus) holds molecules back by size and electrical charge. It lets only small amounts of albumin, a large blood protein, through.¹⁹ Peptides are generally small enough to be filtered freely.¹² Fast clearance by the kidneys, together with breakdown by enzymes, is a common reason why peptides have short half-lives.²⁰ Chemists use this: a peptide that clings to albumin, or that is fused to a large protein, becomes too big to pass the filter easily (see pharmacokinetics).¹¹

The immune system

Medicines can provoke antibodies against themselves. Small peptides usually provoke fewer antibodies than larger protein medicines, but antibodies against the drug have been reported for some peptide medicines.^11,12 Clumps of protein (aggregates) can make an immune response more likely.²¹ Makers design against this: dulaglutide was engineered to reduce its immune potential, and its EU label credits its close likeness to the body’s own molecules.¹³

Left- and right-handed amino acids

Except for glycine, every natural amino acid exists in two mirror-image forms, like a left and a right hand (the L-form and the D-form). Natural amino acids have the L-form.⁴ The body’s protein-cutting enzymes are shaped to fit L-chains, so putting a D-form at the spot where an enzyme cuts is an effective way to extend a peptide’s life in the blood.¹¹

Octreotide: four tricks in eight amino acids

The natural hormone somatostatin has a half-life of only a few minutes in the blood.²⁰ Octreotide is a shortened copy of eight amino acids. It carries two D-amino acids, is closed into a ring by a sulfur bridge, and ends in an altered amino acid.^23,24 In laboratory tests it resisted pure enzymes and tissue extracts Cells(Evidence level: Shown in cells or tissue in the laboratory), and in rats and rhesus monkeys it acted much longer than somatostatin Animals(Evidence level: Shown in animal studies).²³ A review gives its plasma half-life as about 1.5 hours, against a few minutes for somatostatin.²⁰

Semaglutide: three changes to a natural hormone

Natural GLP-1 has a half-life of about 1.5 to 2 minutes in the blood.¹³ Semaglutide changes two amino acids (one to block the enzyme DPP-4) and adds a fatty-acid side chain that binds albumin; in people its half-life is about one week.^25,26 How half-life shapes everything else is explained on the pharmacokinetics page.

Why long chains are hard to make

Solid-phase synthesis was developed by Bruce Merrifield, who received the 1984 Nobel Prize in Chemistry for it. Fixing the growing chain to a solid lets chemists wash away by-products after each step instead of purifying every intermediate.¹ The arithmetic in the key number explains the main weakness: a step that does not finish leaves some chains with an amino acid missing.⁴ Making peptides of fewer than about 50 amino acids is now routine; longer chains remain hard to make on a large scale.¹¹

Impurities from chemical synthesis

EU guidance lists the impurities chemists must control:⁴

• Deletion sequences: chains with one or more amino acids missing, from steps that did not finish.
• Truncated and insertion sequences: chains whose growth was stopped early, or with an amino acid added twice.
• Mirror-image errors (racemisation or epimerisation): an amino acid that flipped to its D-form during the reaction.
• Process chemicals: leftover reagents, solvents and metals.

A review of impurities in peptide medicines adds unwanted counter-ions (salt partners) such as trifluoroacetate and, in poorly made products, contamination with other, unrelated peptides.²⁹ Peptides also break down over time, for example by oxidation or by reacting with water (hydrolysis).^4,32

Impurities from recombinant production

Microbes build the chain from its gene, but the raw product contains material from the cells themselves, such as enzymes, DNA and RNA fragments and unrelated proteins.¹¹ When bacteria are used, endotoxin is a particular concern: it is part of the outer wall of some bacteria and a major cause of fever reactions to contaminated medicines.³⁰

Recombinant production made human insulin the first commercial health-care product of this technology, in the early 1980s.³¹ The EU and US labels for semaglutide state that its peptide is produced in yeast (Saccharomyces cerevisiae) by recombinant DNA technology.^26,33

• Amount. Insulin is the body’s own hormone, yet too much causes low blood sugar, with shaking, confusion and, if severe, unconsciousness.³⁴ A Dutch case report describes two people who injected semaglutide obtained without a prescription and came to hospital in a coma with very low blood sugar: the product contained insulin.⁴⁰
• Pattern. In a study in rhesus monkeys whose own hormone signal had been switched off, hourly pulses of GnRH restored the pituitary’s hormone release, while a constant infusion of the same hormone did not; switching from pulses to a constant infusion could desensitise the pituitary again Animals(Evidence level: Shown in animal studies).³⁹ Leuprolide, a synthetic, modified nine-amino-acid version of GnRH, works this way in people: it first raises sex hormones and then, given continuously, lowers testosterone to castrate levels within two to four weeks. Medicine uses this on purpose, in prostate cancer, for example.²² The same natural signal, delivered in a different pattern, has the opposite effect. The signalling page explains why.
• Nature makes poisons too. The death cap mushroom is involved in most fatal mushroom poisonings worldwide, and alpha-amanitin, a double ring of eight amino acids, is mainly responsible.^35,36 It resists heat, acid and enzymes, so cooking and digestion do not destroy it.³⁶

Nature is also a rich source of medicines: exenatide was developed from a peptide found in Gila monster venom.^37,11,41 The point is not that natural peptides are dangerous or good. It is that the origin of a molecule tells you neither.