What is the Difference Between Protein Subunit and Domain

The key difference between protein subunit and domain is that a protein subunit is a separate polypeptide chain of a protein that assembles with other polypeptide chains to form a protein complex while, a protein domain is a contiguous region of the polypeptide chain of a protein that frequently folds independently into a compact, local, and semi-independent unit.

Protein subunit and domain are very important parts of a multimeric protein. Proteins are polymers made from polypeptides. Each polypeptide chain builds up from a monomer known as an amino acid. A complex protein contains distinct structural units such as subunits, domains, motif, and fold. These structural units of a complex protein are extremely important for its structure and ultimately for its function.

CONTENTS

1. Overview and Key Difference
2. What is a Protein Subunit 
3. What is a Protein Domain
4. Similarities – Protein Subunit and Domain
5. Protein Subunit vs Domain in Tabular Form
6. Summary – Protein Subunit vs Domain

What is a Protein Subunit?

A protein subunit is a separate polypeptide chain of a protein that assembles with other polypeptide chains to form a protein complex. In structural biology, a protein subunit is a single protein molecule that co-assembles with other protein molecules to form a protein complex. Naturally occurring proteins have a relatively small number of subunits, such as haemoglobin and DNA polymerase. Therefore, they are called oligomeric. Other proteins consist of a large number of subunits, so they are described as multimeric. For example, microtubules and other cytoskeleton proteins are multimeric. The subunits of multimeric protein may be identical (homologous) or totally dissimilar (heterologous).

In some multimeric proteins, one subunit may be a catalytic subunit. On the other hand, the other one is a regulatory subunit. The function of the catalytic subunit is catalysing the enzymatic reaction whereas, the function of the regulatory subunit is facilitating or inhibiting its activity. An enzyme that consists of both catalytic and regulatory subunits when assembled is usually referred to as a holoenzyme. For example, the enzyme class I phosphoinositide 3-kinase has a p110 catalytic subunit and a p85 regulatory subunit. Moreover, a protein must have one gene for each subunit. This is because a subunit is made up of a separate polypeptide chain that has one coding gene.

What is a Protein Domain?

A protein domain is a contiguous region of the polypeptide chain of a protein that frequently folds independently into a compact, local, and semi-independent units. The domain of a protein is also known as a region of the polypeptide chain of protein that is self-stabilising and folds independently from the rest. They are a distinct structural and functional unit in a protein. Normally, domains are responsible for a specific function or an interaction that contributes to the overall role of the protein. For example, the SH3 domain is around 50 amino acids residues. They occur in a diverse range of proteins, including adaptor proteins, phophatidylinositol3-kinases, phospholipases, and myosins. These SH3 domains are involved in protein-protein interactions.

The domains are varying in length from 50 amino acids to 250 amino acids. The shortest domains, such as zinc finger, are stabilised by metal ions and disulphide bridges. A domain often produces functional units such as the calcium-binding EF-hand domain of calmodulin. Furthermore, domains can be swapped through genetic engineering between one protein to another to make chimeric proteins.

What are the Similarities Between Protein Subunit and Domain?

1. Protein subunit and domain are structural units of a multimeric protein.
2. Both are made up of amino acids.
3. They are connected to the term “polypeptide chain”.
4. Moreover, they are both extremely important for the overall function of the protein.

What is the Difference Between Protein Subunit and Domain?

A protein subunit is a separate polypeptide chain of a protein that assembles with other polypeptide chains to form a protein complex. On the other hand, a protein domain is a contiguous region of the polypeptide chain of a protein that frequently folds independently into a compact, local, and semi-independent units. So, this is the key difference between protein subunit and domain. Moreover, a protein subunit is larger in size than a protein domain.

The following table summarises the difference between protein subunit and domain in tabular form.

Summary – Protein Subunit vs Domain

The building blocks of proteins are amino acids. Proteins are formed by the condensation of amino acids. Protein structures range in size from tens to several thousand amino acids. A protein structure is stabilised by non-covalent interactions such as hydrogen bonds, ionic bonds, Van der Waals forces, hydrophobic bonds, and covalent interactions such as disulphide bonds. A complex protein contains distinct structural units such as subunits, domains, motif, and fold. A protein subunit is a separate polypeptide chain of a protein that assembles with other polypeptide chains to form a protein complex. On the other hand, the protein domain is a region of the polypeptide chain of protein that is self-stabilising and folds independently from the rest. Thus, this is the summary of the difference between protein subunit and domain.

Reference:

1.“Protein Subunit.” ScienceDirect Topics.
2. “Protein Domains.” ScienceDirect Topics.

Image Courtesy:

1. “Atpsynthase” (CC BY-SA 2.5) via Commons Wikimedia
2. “Solenoid-domain-examples” By Bubus12 – Own work (CC BY 3.0) via Commons Wikimedia