What is a peptide? Explained in five minutes

Let's start with the letters

Picture the alphabet. With a handful of letters you can write every word in a language. Biology does something very similar with small pieces called amino acids (simple molecules the body uses as building blocks).

There are 20 different amino acids that make up the body's proteins¹. Our bodies make some of them, and others, called essential amino acids, come from food¹¹². That is why you sometimes see the word "amino acids" on labels for foods like eggs or quinoa.

What matters is the order. Just as "listen" and "silent" use the same letters but mean different things, two chains with the same amino acids in a different order are different molecules. That order is called the sequence.

Short words: peptides

When a few amino acids join together, they form a peptide. The most common definition is simple: a short chain of amino acids, usually 2 to 50, linked by peptide bonds (the chemical "glue" that joins one piece to the next)².

If amino acids are letters, a peptide is a short word or a brief phrase. Some have just three amino acids; others reach several dozen. Their small size is exactly what makes them so interesting for the lab: they can be studied, copied and modified with great precision. There is even a method to build them one link at a time on a solid support; its inventor, Bruce Merrifield, received the 1984 Nobel Prize in Chemistry for it¹³.

Whole books: proteins

When the chain keeps growing, from 51 amino acids onward, it is called a polypeptide². A protein is made of one or more of those long chains, folded into complex three-dimensional shapes².

Following the analogy: if a peptide is a word, a protein is a whole book. Muscles, many enzymes (the proteins that speed up chemical reactions) and much of the structure of our tissues are proteins. Peptides are their small cousins, written in the same alphabet.

The body's messengers

So what does the body use these short words for? Largely, to send messages. Many hormones (substances one organ releases to give instructions to another) are peptides. Think of them as text messages: short, specific and addressed to whoever has the right receptor to read them. That specificity is one reason peptides are so interesting as medicines: they tend to be highly selective and generally well tolerated¹¹.

That is why the first peptide drugs were mostly copies of human hormones³¹⁰. Since then, more than 80 peptide medicines have reached the market, in areas as different as diabetes, cancer, osteoporosis and chronic pain³. Today, research on natural and modified peptides covers a very wide range of therapeutic areas, thanks to better ways of producing, modifying and analysing them⁶.

Insulin, the most famous example

If you want just one example to remember, make it insulin, the hormone that helps cells use sugar from the blood. In January 1922, a 14-year-old boy named Leonard Thompson became the first person to receive an insulin injection as a treatment for diabetes⁵¹⁴.

The discovery was so important that the following year Frederick Banting and John Macleod received the Nobel Prize in Physiology or Medicine "for the discovery of insulin"⁴⁷. Later, in 1955, Frederick Sanger worked out how insulin's 51 amino acids are linked together: it was the first time the structure of a protein had been determined⁵⁸⁹.

A fun detail: with 51 amino acids split across two short chains, insulin sits right on the border between peptide and protein²⁵⁸. It shows that these categories are a useful guide, not a wall. And it is also the first member of a family of molecules that now has more than a hundred years of history.