A Level Biology Practice Exam

What describes the quaternary structure of a protein?

1. The arrangement of amino acids in a polypeptide chain

2. The association of two or more protein subunits

3. The three-dimensional shape formed by a single polypeptide

4. The sequence of nucleotides in a protein-coding gene

The correct answer is: The association of two or more protein subunits

The quaternary structure of a protein refers to the complex formed when two or more polypeptide chains, or protein subunits, come together to form a functional protein. These subunits can be identical or different, and their interactions can involve various types of bonds and forces, including hydrogen bonds, ionic interactions, hydrophobic interactions, and disulfide bridges. This structure is crucial for the function of many proteins because the arrangement of the subunits can affect how the protein interacts with other molecules, its stability, and its overall biological activity. For example, hemoglobin is a classic example of a protein with quaternary structure, as it consists of four polypeptide chains that work together to transport oxygen in the blood. The other options describe different aspects of protein structure—primary structure refers to the sequence of amino acids; secondary structure involves local folding patterns like alpha helices and beta sheets; and tertiary structure pertains to the three-dimensional shape of a single polypeptide chain. The sequence of nucleotides in a protein-coding gene is relevant to the process of translation but does not reflect the structural organization of proteins themselves.