What are four levels of protein structure and what is a distinguishing factor of each?

The four levels of protein structure are:

1. Primary structure: This is the linear sequence of amino acids in a polypeptide chain, determined by the genetic code.
2. Secondary structure: This level involves local folding of the polypeptide chain into structures like alpha-helices and beta-sheets, stabilized by hydrogen bonds between the backbone atoms.
3. Tertiary structure: This refers to the overall three-dimensional shape of a single polypeptide, formed by interactions between side chains (R groups), including hydrophobic interactions, ionic bonds, and disulfide bridges.
4. Quaternary structure: This level occurs when two or more polypeptide chains (subunits) come together to form a functional protein complex, with the arrangement determined by various interactions between the subunits.