The place within an enzyme where a substrate binds is called the ATP site Active site Key hole Transition site If t for two different polypeptides some of their R groups come together to hold both polypeptides together in a certain 3-dimentional shape. Which level of protein structure would this describe? primary structure secondary structure tertiary structure quaternary structure In DNA what nitrogenous base complementanly pairs with T (thymine)? Solution 14. Active site the active site is the region of an enzyme where substrate molecules bind and undergo a chemical reaction. 15. Quaternary structure (quaternary structures arise when a protein is made up of two or more polypeptide chains).