For instance, one will discuss carbonic and hydrates. We'll see that in carbonic and hydrates, inside the active side, we have a zinc metal atom that is used to actually form a strong nucleophile, the hydroxide nucleophile. And finally, this metal atom can actually be used to hold that substrate molecule in place. So in the same way that we can use covalent contalisis to basically orient that substrate and hold it in place, we can also use the positive charge of these metal atoms to actually bring the substrate molecules in the proper orientation and hold them in place inside the active side so that reaction can actually take place at a reasonably high rate. So we see that our enzymes inside our body use a variety of different types of methods and mechanisms to basically carry out the general reaction of decreasing the activation energy of that biological process. So we can have covalent catalysis, we can have catalysis by proximity, we can have acid based catalysis, and we can also have metal ion catalysis.