Quaternary Structure of Protein: Definition & Overview

Instructor: Shannon Compton

Shannon teaches Microbiology and has a Master's and a PhD in Biomedical Science. She also researches cancer and neurodegenerative diseases.

Proteins have four levels of structure: primary, secondary, tertiary, and quaternary. The first three involve only one molecule. However, quaternary structure describes how proteins interact to form complex molecular structures.

What Is Quaternary Protein Structure?

By definition, quaternary structure is the arrangement of more than one protein molecule in a multi-subunit complex. The nomenclature here can get a bit confusing because we call a single polypeptide chain a protein if it can function on its own. However, many proteins are actually comprised of several polypeptide chains. In this case, the individual peptide chains are called protein subunits and they cannot function on their own.

The image below shows a protein that is made of several protein subunits. Each color represents a separate protein. Each protein subunit is produced individually by a ribosome. Then, each protein subunit attains secondary and tertiary structure. Finally, the protein subunits come together to form a fully functional protein.

Image of a protein with quaternary structure
Image of a protein with quaternary structure

Types of Proteins with Quaternary Structure

An example of a protein with quaternary structure is hemoglobin. In hemoglobin, one protein binds to oxygen while another binds carbon dioxide. This is how one protein can serve two functions.

Enzymes can consist of a single protein or multiple protein subunits. Holoenzymes are those with several subunits. Each subunit of a holoenzyme has a different function. For example, one protein subunit may be the regulatory subunit. Enzymes can also be referred to as multimeric. Another protein subunit may be the catalytic subunit. Another name for a protein with quaternary structure is called a multiprotein complex. Microtubules, which act like highways in your cells, are multiprotein complexes.

Functions of Quaternary Structure

As mentioned above, quaternary structure allows a protein to have multiple functions. It also allows for a protein to undergo complicated conformational changes. This has several mechanisms. First, an individual subunit can change shape. In addition, the subunits can change their orientation with regard to one another. A multimeric protein can do either one of these or it can do both. These conformational changes allow for a high degree of regulation. They are also key to a multimeric protein's ability to perform its physiological function.

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