Quaternary Structure of Protein: Definition, Lesson & Quiz
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.
We also recommend watching Lewis Dot Structures: Polyatomic Ions and Resonance Structures and Crystal Structures and the Unit Cell
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.
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.
Homomultimeric and Heteromultimeric Proteins
It might seem as though the discussion so far suggests that multimeric proteins are always composed of different proteins. It is true that they can be different protein subunits. Multimeric proteins with different subunits are called heteromultimeric. When the subunits are all the same the multimeric protein is said to be homomultimeric.
The majority of proteins in the Protein Data Bank (a library of 3-dimensional protein images submitted by biologists and biochemists) are homomultimeric. In fact, homomultimeric proteins are responsible for the diversity and specificity of many cellular pathways. They can also regulate gene expression, activity of enzymes, ion channels, receptors, and cell adhesion processes.
Proteins have primary, secondary, tertiary, and quaternary structure. Quaternary structure is the only one which involves multiple protein subunits. Multimeric proteins can be heteromultimeric (having different protein subunits) or homomultimeric (having all the same protein subunits). Multimeric proteins can undergo complicated conformational changes. This is due first to the fact that individual protein subunits within the complex can shift conformation. This is due second to the fact that the individual protein subunits can shift position with respect to each other.
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