Proteins IV: Primary, Secondary, Tertiary and Quaternary Structure

Lesson Transcript
Instructor: Meg Desko

Meg has taught college-level science. She holds a Ph.D. in biochemistry.

Proteins can be classified according to their structure as either primary, secondary, tertiary, or quaternary. Explore the classifications of proteins by structure, and discover how structures differ from one another. Updated: 08/14/2021

Higher Order of Protein Structure

In this lesson, we'll be learning about primary, secondary, tertiary and quaternary structure of proteins, but first, let's take a little detour and talk about writing.

What does writing have to do with biochemistry? Well, let's think about this for a second.

Three amino acids can come together to form a tripeptide.

So let's say you need to write a story and your story will be made of several paragraphs. Now when you go to write a paragraph, you don't just think of random letters and have them magically assemble into your finished paragraph, do you? Instead, you take your letters and use them to create words.

There are different types of words, right? So these different types of words can be things like nouns, pronouns, adverbs and verbs, and all of these different things can be put together to form sentences. After you have sentences, these sentences can come together to form a paragraph that makes sense for your story.

Now in the case of proteins, it's not exactly the same, but it's pretty similar. As we know, amino acids can come together to form peptides. These can be very short, like dipeptides that consist of two amino acids, tripeptides, which consist of three amino acids, and polypeptides, which is anything from there up to about 30 amino acids. Finally, amino acids can come together to form proteins.

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  • 1:36 Primary Structure
  • 2:15 Secondary Structure
  • 4:01 Tertiary Structure
  • 5:26 Quaternary Structure
  • 6:56 Denaturation
  • 7:39 Lesson Summary
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Primary Structure

Amino acidsprimary structure

So, at this point, we have the primary structure of the protein well in hand. We know the order of the amino acids, but we don't know how they arrange with respect to one another like the words in a sentence would.

Secondary Structure

A random coil is a type of secondary structure.
Random Coil

This is where the secondary structure comes in. The secondary structure is a repetitive 3-D structure of a protein. These are things that people often come into contact with when they're looking at protein structures. Secondary structures are common among proteins, so different proteins will have the same types of secondary structures. The most common one of these is called the alpha helix. The alpha helix is the secondary structure where the protein coils because there are hydrogen bonds between the backbones of their amino acids.

The alpha helix gets its coil shape from hydrogen bonds.
Alpha helix

So if we look at this example, we can see that there are hydrogen bonds forming between the amine nitrogens and the carbonyl oxygens in different amino acids. Now when we say backbone, what we mean are the peptide bonds in addition to the nitrogen in the amine, the carbon on which the side group sits and the carbonyl carbon. And as you can see, in the case of this alpha helix, it looks like a coil, hence the name 'helix.'

Another common secondary structure is the beta sheet, or the beta pleated sheet. The beta sheet is a secondary structure where the backbones of different strands of the amino acids (meaning not the side chain part) are held together by hydrogen bonds between the amine nitrogen and the carbonyl oxygen. These form a flattened region in the protein.

Finally, there's the random coil. As you might imagine by its name, it's an unstructured peptide chain. So this is what happens when there isn't hydrogen bonding between the backbone of the protein and it's just floating around out in space kind of flexibly. Think of it sort of like a protein piece of spaghetti.

Tertiary Structure

So just like we take words and put them into sentences, we can take the primary and secondary structures of a protein and they can interact to form the tertiary structure. Now the tertiary structure is the complete 3-D structure of a protein, or peptide, including all of its atom arrangements because they tend to stay fairly stationary.

So when we look at hemoglobin, the protein that carries oxygen throughout our bodies, we find that it's made of two alpha protein subunits and two beta protein subunits, so it's made of four different pieces.

The hemoglobin beta subunit is made up of many amino acids.
 Hemoglobin beta subunit

Let's just focus on one of them for right now: the hemoglobin beta subunit. We can see here that the subunit is made up of many amino acids in its primary structure. These amino acids come together to form several alpha helices, which come together to form its tertiary structure.

You may also notice a funky little group sticking in the middle of the hemoglobin that has four sides. This is the heme group and is what carries the oxygen around our bodies. Just a fun fact!

So like we said, hemoglobin has two alpha subunits and two beta subunits. So how do those come together? Just like sentences don't typically stand by themselves when you're writing a story, the proteins don't always stand alone either. Some of them come together to form larger groups of proteins.

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