Complete the following vocabulary exercise relating to the level of structure in proteins.
1. Quaternary structure is the result of two or more protein subunits assembling to form a larger, biologically active protein complex.
2. Secondary structure describes the alpha-helices and beta-sheets that are formed by hydrogen bonding between backbone atoms located near each other in the polypeptide chain.
3. Primary structure is the sequence of amino acids in a protein.
4. Tertiary structure is achieved when a protein folds into a compact, three-dimensional shape stabilized by interactions between side-chain R groups of amino acids.