So what exactly is reversible covalent modification? Well, this is the process by which we essentially modified our enzymes in a reversible fashion. We basically add some type of group and then we can remove that same type of group. Now, a significant number of enzymes are regulated by adding or removing some type of group. And one very common type of group is our phosphoryl group. So basically a group or a class of proteins that are responsible for phosphorylating adding these phosphoryl groups onto enzymes are known as protein kinases.