When you read about proteins, cells, genetics or biochemistry, you will often come across the term ‘polypeptide’. The word is closely related to the word protein, although the two words do not always mean exactly the same thing.
The best way to think of a polypeptide is to imagine a string of beads. Each bead represents an amino acid, and the order of the beads is important. Change the order and you have a molecule that behaves differently.
How is a polypeptide formed?
There are 20 standard amino acids that cells use to make proteins. These amino acids can be combined in different ways to create many different chains.
If you join two amino acids together, you form a peptide bond. More amino acids can then be added to the chain, making a longer molecule called a polypeptide.
This occurs during the synthesis of proteins. DNA has the genetic information needed to create proteins. That’s the blueprint. Messenger RNA, or mRNA, carries a copy of the relevant information to a ribosome. The ribosome reads the sequence and joins up the amino acids in the right order.
Once the chain has been made, it doesn’t just sit there as a straight string. It starts folding into a certain shape. This folding is a crucial part of how proteins function.
What is the difference between a polypeptide and a protein?
This is where things can become a little confusing.
A polypeptide is an amino acid chain. A protein is a functional molecule made from one or more polypeptide chains that have folded into a particular structure.
Some proteins are made from just one polypeptide chain. Others need two or more chains working together.
So, it is fair to say that polypeptides are the basic chains from which many proteins are made. However, calling every polypeptide a complete protein is not always correct.
You may also notice that scientists sometimes use the words almost interchangeably, depending on what they are discussing. In everyday biology writing, authors often overlook the distinction.
Read the full explanation
Why does the order of amino acids matter?
Suppose you have a set of 20 beads of different colours. You could put them together a million different ways, and each would be different.
The same is true with polypeptides.
The particular sequence of amino acids determines how the chain behaves and how it folds in the end. The chemical properties of each amino acid are different. Some like water. Some don’t like water. Some can bond or attract to other parts of the chain that are close by.
All these interactions contribute to the final shape of the polypeptide.
That shape is important, because proteins need the right shape to do their jobs. Sometimes minor change in the amino acid sequence has a small effect. In others, it can radically alter the protein’s function.”
What do polypeptides do in the body?
There is no single job that all polypeptides do. Their role is determined by the chain of amino acids and the shape they take.
Polypeptides can create proteins that function as enzymes, hormones, receptors, antibodies, and structural components.
For example, enzymes accelerate chemical reactions. Receptors allow cells to detect and react to signals. Antibodies help the body fight off disease.
Other proteins add strength and structure to tissues. Collagen is one example. It has long chains of protein and is an important part of connective tissue such as skin, tendons and cartilage.
So, although the word “polypeptide” refers simply to a chain of amino acids, these chains can eventually be involved in some critical processes in the body.
How does a polypeptide fold?
Once a polypeptide has been produced, it starts to fold into a three-dimensional shape.
Scientists often describe protein structure in four levels.
The primary structure is the amino acid sequence. Thus, it tells us which amino acids are present and in what order.
The secondary structure refers to the smaller areas of the chain that have structures like alpha helices and beta sheets.
The tertiary structure is the complete 3-dimensional structure of a single polypeptide chain. This structure is a result of the interactions of the various parts of the chain.
Quaternary structure is when more than one polypeptide chain comes together to form one functional protein.
Not all proteins have four levels. For example, a protein consisting of one polypeptide has no quaternary structure.
Is a polypeptide the same as a peptide?
Not quite, although the terms are closely related.
Both peptides and polypeptides are made up of amino acids linked through peptide bonds. The main difference is normally the length of the chain. Shorter chains are generally called ‘peptides’, and longer chains are called ‘polypeptides’.
There is not always a clean-cut line, though. The terms may be used slightly differently in different scientific sources, depending on context.
The important point is that both are built from amino acids.
Why are polypeptides important?
Without polypeptides the body would not be able to make the vast array of proteins it needs.
Proteins are involved in nearly every aspect of life. They help cells talk to each other, support tissues, transport substances around the body and control chemical reactions. They also have important functions in the immune system and many other biological processes.
Thus, polypeptides are one of the important links between genetic information and working molecules inside living cells.
DNA has the instructions. The ribosome follows the instructions and strings amino acids together. The polypeptide folds into the shape it needs to be to perform its function.
Polypeptide: the simple definition
Briefly, a polypeptide is a chain of amino acids joined together by peptide bonds.
The sequence of amino acids in the chain matters because it affects how the polypeptide folds and what it can ultimately do. A whole protein can consist of one polypeptide. A protein may be made of several polypeptide chains.
So, when you see the word polypeptide, think of it as one of the basic chains that make up proteins. It’s a tiny thing at the molecular scale, but it’s critically important for how living cells work.
Research Use Only. This glossary explains terminology used in published research. It is not medical advice and does not describe any product for human or animal use.