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

The quaternary structure of a protein is its most complex form, resulting from interactions between multiple subunits, each with a tertiary structure. These subunits can form dimers, trimers, or tetramers, contributing to a functional protein. Quaternary structures may also include prosthetic groups, such as heme in hemoglobin, which assist in functions like iron transport. Understanding the progression from primary to quaternary structure highlights the increasing complexity and functionality of proteins, essential for metabolic processes in living organisms.

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 2 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 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 2 subunits together, trimer means we have 3 subunits, and tetramer means we have 4 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 2 repeating patterns which create our secondary structures. So we have our alpha helix and our beta plated sheets.

These happen on the same chain. So different portions of the chain can create alpha helices or beta plated 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 1, 2, 3, 4 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 4 subunits and in addition to this, we have 4 prosthetic groups. Together, this helps to make a functional protein. Just for reference, this is an example of Hemoglobin, a very popular quaternary 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. Right. 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 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 trimer or trimeric. If we look at our options, it's not dimeric, so that's out. Not tetrameric because it is not 4, it's 3. Not monomeric, it's not 1. It'd be d. 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 are not covalently bonded to each other. They are connected to each other through non-covalent means. So here, option d will 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.

Here’s what students ask on this topic:

What is the quaternary structure of a protein?

The quaternary structure of a protein is its most complex form, resulting from the interaction between multiple subunits, each possessing a tertiary structure. These subunits can form dimers (two subunits), trimers (three subunits), or tetramers (four subunits), contributing to a functional protein. Quaternary structures may also include prosthetic groups, which are non-amino acid components that assist in the protein's function. An example is hemoglobin, which has four subunits and heme prosthetic groups that help in iron transport. Understanding the quaternary structure is crucial for comprehending the protein's overall functionality and its role in metabolic processes.

What is a subunit in the context of quaternary protein structure?

A subunit in the context of quaternary protein structure is an individual polypeptide chain that has its own tertiary structure. These subunits interact with each other through various types of bonds and interactions, such as hydrophobic, hydrophilic, and hydrogen bonding, to form the quaternary structure. Each subunit contributes to the overall functionality of the protein. For example, in hemoglobin, each of the four subunits works together to transport oxygen in the blood. The interactions between these subunits are essential for the protein's biological activity.

What are prosthetic groups in quaternary protein structures?

Prosthetic groups in quaternary protein structures are non-amino acid components that are tightly bound to the protein and are essential for its function. These groups can be organic molecules or metal ions. An example is the heme group in hemoglobin, which contains an iron ion that binds to oxygen. Prosthetic groups play a crucial role in the protein's biological activity, often participating directly in the chemical reactions the protein facilitates. They help stabilize the protein structure and enhance its functionality.

How do primary, secondary, tertiary, and quaternary protein structures differ?

Primary structure is the sequence of amino acids in a polypeptide chain, linked by peptide bonds. Secondary structure refers to local folding patterns like alpha helices and beta sheets, stabilized by hydrogen bonds. Tertiary structure is the overall 3D shape of a single polypeptide chain, formed by interactions among side chains, including hydrophobic interactions, hydrogen bonds, and disulfide bridges. Quaternary structure involves the assembly of multiple polypeptide subunits into a functional protein complex, often including prosthetic groups. Each level of structure adds complexity and functionality to the protein.

What is a multimeric protein?

A multimeric protein is a functional protein composed of multiple polypeptide subunits, each with its own tertiary structure. These subunits can be identical or different and are held together by various interactions, such as hydrophobic interactions, hydrogen bonds, and ionic bonds. Multimeric proteins can be classified based on the number of subunits they contain: dimers (two subunits), trimers (three subunits), and tetramers (four subunits). An example of a multimeric protein is hemoglobin, which consists of four subunits and is essential for oxygen transport in the blood.