Video Transcript
Which of the following best describes the quaternary structure of a protein? (A) The quaternary structure of a protein is the complex 3D structure formed when multiple polypeptides interact and combine. (B) The quaternary structure of a protein is the 3D structure that forms due to interactions between the R groups of amino acids. (C) The quaternary structure of a protein is the folded structure, 𝛼-helix or 𝛽-sheet, formed by additional bonds formed in the polypeptide chain. Or (D) the quaternary structure of a protein refers to the sequence of amino acids in its polypeptide chain.
This question asks about the quaternary structure of a protein. There are in fact four different protein structures: primary, secondary, tertiary, and quaternary. Let’s review these structures briefly to find the best description of the quaternary structure.
Proteins are made up of a combination of 20 different amino acids. They all have the same basic structure but different side groups called R groups. These amino acids bind together via peptide bonds to form a polypeptide chain. The sequence of amino acids in the polypeptide chain is what we refer to as the primary structure of a protein. So answer choice (D) actually describes the primary structure of a protein, not the quaternary structure.
The polypeptide chain can then take on either of two secondary shapes. It can spiral into what we call an 𝛼-helix, or it can fold into what we call a 𝛽-sheet or 𝛽-pleated sheet. Both the 𝛼-helix and 𝛽-sheet structures are held in place by hydrogen bonds, which are additional bonds formed in the polypeptide chain. Answer choice (C) therefore describes the secondary structure of a protein.
The tertiary structure of the protein is the three-dimensional arrangement that the 𝛼-helices and 𝛽-sheets take on. One polypeptide chain can have different secondary structures in different places. The tertiary structure of a protein is held in place by several different types of bonds, which usually involve the different R groups of the amino acids. These include hydrogen bonds, ionic bonds, and disulfide bonds.
Then, there are the hydrophobic and hydrophilic interactions. The hydrophobic interactions are when the nonpolar R groups of amino acids are repelled by the surrounding aqueous solution. Hydro- means water, and -phobic means hating. The hydrophilic interactions are when the polar R groups are attracted to the water molecules, -philic meaning loving. The polypeptide chain ends up folding to minimize the contact with water for the hydrophobic side groups and maximize the contact for the hydrophilic ones, thereby contributing to the tertiary structure. So answer choice (B) describes the tertiary, not the quaternary, structure of a protein.
This would indicate that answer choice (A) describes the quaternary structure of a protein. Indeed, the quaternary structure of a protein is when two or more polypeptide chains in their 3D tertiary shape are bonded together. They are held together by bonds and interactions between their various parts. The combination of different amino acid sequences, along with different three-dimensional structures, means that the possibilities for different proteins are nearly infinite, which is why this macromolecule is able to carry out so many different functions.
The correct answer is therefore given by answer choice (A). The quaternary structure of a protein is the complex 3D structure formed when multiple polypeptides interact and combine.
