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. Now, why would we want to form a Covalent bond? Well, one reason is to basically keep that substrate molecule in place inside the active side. So many enzymes contain active sites with catalytic residues that can form temporary Covalent bonds with the substrate molecule and that can be used to basically keep that molecule in place for the time being until that reaction actually takes place.