What Are Proteins? Definition, Types & Examples

Where are proteins located?

Proteins are in cells all throughout your body. In fact, any given cell holds several thousands of proteins. Each protein in a cell does a specific job to support your body’s needs.

What are proteins made of?

Proteins are made of amino acids held together by chemical bonds. One protein typically contains hundreds of individual amino acids. You can think of these amino acids like beads on a necklace. The word “polypeptide” refers to a single chain, or necklace, of amino acids. “Poly-” means many, and “-peptide” means a molecule containing at least two amino acids.

But proteins are more than simple chains of amino acids. They’re complex molecules made up of one or more chains that twist and fold together in specific ways to make the protein work as it should.

A protein’s function — what it does in your body — determines its exact structure. But all proteins share the same basic structure, which scientists break down into four levels. Let’s take a closer look.

Protein structure

There are four levels of protein structure, ranging from simple to complex:

• Primary structure of protein: This is the sequence of amino acids in a single polypeptide chain. You can compare the primary structure to the order of beads in a beaded necklace. You might choose pink first, then blue, then yellow. A polypeptide chain’s “beads” are amino acids arranged in a specific order. Chemical bonds hold them together. The order changes based on the type of protein and its function in your body.
• Secondary structure of protein: This is the shape a polypeptide chain takes due to chemical interactions between its “beads” (amino acids). The shape may be coiled (alpha helix) or folded in a zig-zag manner (beta sheet). It’s a bit like adding magnetic beads to the necklace so certain parts of the chain are drawn to one another.
• Tertiary structure of protein: This is the 3D shape that forms when the regions of secondary structure fold together into the final structure of the protein. Water attracts some amino acids and repels others. So, some amino acids stay on the outer surface of the protein, where they can bond with water molecules, while others move toward the interior. To picture this happening, imagine stringing beads onto a metal wire and then bending and shaping the wire to form a 3D shape that you can hold in your hand.
• Quaternary structure of protein: This is the fourth and most complex level. Not all proteins have this level of complexity. But some, like hemoglobin, do. It’s when a protein has more than one polypeptide chain, and those chains interact and are folded within each other. Chemical bonds hold these chains together in their unique 3D shapes.

Why do these structures matter? It’s because any changes to a protein’s expected structure can lead to health problems — ranging from sickle cell disease to Alzheimer’s disease. Your body relies on proteins adopting a certain structure to do their jobs as they should. When something goes wrong with a protein structure — even one “bead” that’s different than it should be — the effects on your body can be significant.

What are the building blocks of protein?

Amino acids are the building blocks that make up a protein. You might also hear amino acids referred to as the monomers of protein. A monomer is a single molecule that connects with other molecules to form larger structures (polymers).

What is protein synthesis?

Protein synthesis is the process of making new proteins to support your body’s needs. This process takes place inside your body’s cells constantly — it’s happening in some of your cells now as you read this article.

This is because proteins don’t last your entire life. Over time, they break down, and you need new ones to keep up with your body’s demands. That’s why you need a constant intake of protein from foods you eat so your body has new building blocks (amino acids) for protein synthesis.

You can think of cells in your body like tiny factories with equipment for building proteins. Each cell contains the same basic equipment:

• Your DNA: This acts as an instruction manual for making proteins. DNA is located in the core of the cell, called the nucleus. Each cell contains your entire DNA sequence, but factory “workers” only use the specific “chapter” of the manual they need to make a specific type of protein.
• RNA molecules: RNA stands for ribonucleic acid. It’s similar to DNA but with some structural differences. RNA molecules are the main factory workers. There are three different types of RNA, and each type does a unique job. Messenger RNA (mRNA) is the first type involved. An mRNA molecule carries a copy of the protein-making instructions out of the cell’s nucleus and to an “assembly line.”
• Ribosomes: You can think of these structures as assembly lines for building proteins. Nothing can get made without the necessary building blocks — and that’s where transfer RNA (tRNA) molecules step in. These workers deliver amino acids to the ribosomes. Ribosomal RNA (rRNA) molecules read the instructions and direct the building process of the new protein, amino acid by amino acid. 

Protein synthesis steps

Protein synthesis — all the work that takes place inside each protein-making factory (cell) — involves two major steps:

• Transcription: This is when instructions in your DNA get transcribed (copied) onto an mRNA molecule. It happens in the cell nucleus.
• Translation: This is when rRNA interprets the instructions and uses them to build the protein. Protein building involves assembling amino acids together into chains (polypeptides) and then twisting and folding those chains into the proper shape. Each protein has a unique shape that’s tailored to the job it’ll do in your body.

Like even the best of factories, your cells can make mistakes. Lots of things can go wrong in the process of building a protein. Sometimes, other workers (molecules) catch the mistakes and fix them. But some go unnoticed, and those errors in protein synthesis can lead to a wide range of health problems.