So the bacterial enzyme glycopeptide transpeptidase, utilizes Covalent catalysis. And as we'll see in just a moment, another enzyme that we can basically label as using Covalent catalysis is Kymatrypsin. And this is an important digestive enzyme that exists inside our digestive system. And we'll discuss much in much more detail what chymatrypsin actually does inside the active side. Now let's move on to method number two catalysis by proximity and catalysis by orientation. So if we recall the collision theory from basic chemistry based on the collision theory, for a reaction to actually take place, what must happen?