Proteins are made up of amino acids, which are the building blocks of proteins. There are 20 different amino acids that can be combined in various ways to create a vast array of proteins.

While you didn't ask about it, there are four levels of protein structure:

1. Primary structure: The linear sequence of amino acids.

2. Secondary structure: The local folding of the polypeptide chain, forming alpha-helices and beta-sheets.

3. Tertiary structure: The overall three-dimensional shape of a single polypeptide chain.

4. Quaternary structure: The arrangement of multiple polypeptide chains (subunits) in a protein complex.