Quaternary Protein Structure - Online Tutor, Practice Problems & Exam Prep

The quaternary structure of proteins represents their highest complexity, formed by interactions between multiple subunits, each a polypeptide chain with a tertiary structure. Multimeric proteins can be dimers (2 subunits), trimers (3), or tetramers (4). As proteins progress from primary to quaternary structure, they exhibit increasing complexity, involving hydrophobic and hydrophilic interactions. Additionally, prosthetic groups, like heme in hemoglobin, enhance functionality by contributing non-amino acid components essential for processes such as iron transport in metabolism.

Quaternary Protein Structure Concept 1

Now, the quaternary structure of a protein is its highest level of complexity. We're going to say it results from the interaction between our side chains of two or more subunits. Now, what exactly is a subunit? Well, a subunit is an individual polypeptide chain possessing a tertiary structure. So that means that it comes with all the types of interactions that are normal for a tertiary structure, such as hydrophobic, hydrophilic, hydrogen bonding.

All these different types of interactions that we see within a tertiary structure would carry over to a quaternary structure. Now, here we also have what's called a multimeric protein. This is a fully functional protein that possesses multiple subunits. Now, with a multimeric protein, we can have terms such as dimer, trimer, and tetramer. Dimer means that we have two subunits together, trimer means we have three subunits, and tetramer means we have four subunits.

Here, we've done a huge journey from our primary structure all the way now to our quaternary structure. Remember, our primary structure has a sequence of amino acids connected to each other through peptide bonds. It then translates into two repeating patterns which create our secondary structures. So, we have our alpha helix and our beta pleated sheets. These happen on the same chain.

So different portions of the chain can create alpha helices or beta pleated sheets. From there, we get to tertiary structures where hydrophobic interactions kind of cause the chain to fold in on itself, creating even further interactions. So here, we'd have our folded peptide. Now, if enough of these start coming together, we can have dimers, trimers, and tetramers. In this example here, we have one, two, three, four subunits interacting with one another coming together to help give us our quaternary structure.

In this case, we'd say that this is a functional protein. Now, you might also notice that within this image, we have these little spheres which we did not have in our tertiary structure. With quaternary structures, sometimes we also have the addition of what's called a prosthetic group. This is basically a non-amino acid component that forms a part of the quaternary structure of a protein. Right?

So, here in this image, we have our four subunits and in addition to this, we have four prosthetic groups. Together, this helps to make a functional protein. Just for reference, this is an example of hemoglobin, a very popular quaterary structure for a protein. And those prosthetic groups are heme groups, which help to carry iron. Remember, this helps us carry iron within our cells in order to help us basically maintain our metabolism and keep us going as human beings.

Alright. So remember, we talk about going from primary all the way to quaternary. It becomes more and more complex. And this is what we're seeing when it comes to this image.

Quaternary Protein Structure Example 1

Which of the following is true for the protein structure of an E. Coli sample? All right. So if we take a look here, we can see that in this E. Coli sample, we have what we have 3 tertiary structures.

Right? So, we have 3 different peptide chains all coalescing, all coming together to form this structure. Now, because we have 2 or more subunits, each one of them representing a subunit, we know that we have a quaternary structure. All of them are saying that, but there are 3 of them, so that would mean that it is a trimeric structure. If we look at our options, it's not dimeric, so that's out.

Not tetrameric because it's not 4, it's 3. Not monomeric, it's not 1. It would be because here, it's a quaternary structure which is trimeric because it's 3 tertiary structure chains or 3 subunits, and they're held together by non-covalent bonds. That is true. In this case, they're not covalently bonded to each other.

They're connected to each other through non-covalent means. So here, option would be the best choice. It's quaternary, yes, and it's the only one that says trimeric.

Hemoglobin represents a commonly discussed tetramer that contains an even number of α and β subunits. Which of the following statements is true?

• A. Hemoglobin must contain with 4 α subunits and 4 β subunits.    
• B. Hemoglobin must contain with 2 α subunits and 2 β subunits. 
• C. Hemoglobin contains R groups that only covalently bind to produce a quaternary structure. 
• D. Hemoglobin represents a multimeric protein with identical subunits.

Which of the following could be classified as a prosthetic group? 

• A. I only 
• B. I, III, IV
• C. II only 
• D. II, IV 
• E. None of the above

Which of the following correctly orders the protein structural terms from lowest to highest complexity?

• A. Peptide Bond < Primary structure < 2 subunits < Secondary structure < tetramer < Tertiary structure.
• B. Primary structure < Peptide Bond < Secondary structure < 2 subunits < Tertiary Structure < tetramer.
• C. Peptide Bond < Primary structure < Secondary structure < Tertiary structure < 2 subunits < tetramer.
• D. Peptide Bond < Primary Structure < Secondary structure < 2 subunits < Tertiary structure < tetramer.