Name and briefly describe the four levels of protein structure.

Short Answer: The four levels of protein structure are: 1. Primary Structure - the linear sequence of amino acids in a polypeptide chain, connected by peptide bonds. 2. Secondary Structure - the local folding of the polypeptide chain into specific conformations such as alpha-helices and beta-sheets, stabilized by hydrogen bonds between backbone atoms. 3. Tertiary Structure - the three-dimensional arrangement of the polypeptide chain, including all secondary structures and any additional folding, stabilized by various interactions (e.g., hydrogen bonds, ionic interactions, van der Waals forces, and disulfide bridges). 4. Quaternary Structure - the interaction and assembly of multiple polypeptide chains (subunits) to form a functional protein complex.