And what are some mechanisms, what are some methods that enzymes use to achieve this decrease in activation energy? So four of these methods are listed on the board and we have many more, but these are the four most important ones. And enzymes can use one of these methods or they can use a variety of these different methods. So let's begin by focusing on the first one we call Covalent catalysis. So in some enzymes, such as, for example, trypsin chymatrypsin, other digestive enzymes, as well as the enzyme we're going to focus in this lecture, glycopeptide transpeptidase. In some enzymes inside the active side, we have catalytic residues, these amino acids, part of the active side of the enzyme that are responsible for actually forming a temporary Covalent bond.