Protein Structure
Before enzymes can catalyze anything, they have to fold correctly. Every protein's function starts with the same four levels of structure, building from a flat sequence up to a real 3D machine — worth understanding before Enzyme Activity, since denaturation there is really a story about these levels breaking down.
Primary structure is simply the sequence of amino acids in a polypeptide chain, linked one after another by peptide bonds. There are 20 different amino acids that can appear in any order and any number of times, so even a short chain has an enormous number of possible sequences. This sequence is determined directly by the gene that codes for it — it’s the one level of structure written down explicitly in DNA, with every higher level built on top of it.