1) Primary structure: Single polypeptide chain with amino acids connected by peptide bonds. (No H-bonds between nearby amino acids!)

2) Secondary structure: Single polypeptide chain. H bonding to side groups of amino acids several places away, forms folded or coiled structure.

3) Tertiary structure: Single polypeptide chain. 3-D shape due to H-bonds between amino acids that are nearby as well as distant!

4) Quaternary structure: two or more polypeptide chains linked to form complex 3-D structure.

[Protein Structure #1] ---- [Protein Structure #2]

Denaturation: using extremes of pH or temperature to alter the shape of a protein (breaks some of the H-bonds that hold the polypeptide chain in its original shape).