Question

Describe how protein domains are different from protein subunits. Give an example for both.

Answer 1

Proteins are large heteropolymers of amino acids. The interaction between amino acids within a single polymer or other molecules attributes towards the structural variability and characteristics. This signature of each protein makes it unique and functional. But when it comes to functionality, many proteins become functional with a combination of similar or different kind of molecules (including proteins). If there is more than one protein molecule involved int his process each molecule(remember, these are large heteropolymers folded into three-dimensional structures) can be called as a subunit (Eg. Alpha and beta subunits in haemoglobin molecule). The subunits attribute towards the quaternary structure of the protein. Since each amino acids can interact with others based on their nature, they can form different three-dimensional shapes in space rather than existing as long linear molecule. The three-dimensional shapes are obtained over time with a sequential folding process where the protein forms it's secondary and tertiary structures. At times the tertiary structures can be repetitive for many types of proteins and thus can be used for the general classification of those structures. That structural classification is called a domain. These structures are conserved for several proteins. Eg. TIM barrel domain (consist of eight alpha-helices), alpha-solenoid, etc.