And now this became a very strong nucleophile. And this can participate in forming covalent, a temporary covalent bond. So as we'll see in the next several lectures, chimetrypsin, which this basically describes, uses not only acid based catalysis but it also uses covalent catalysis in decreasing the activation energy of that particular chemical reaction. That is, what it participates in is breaking different types of peptide bonds breaking different types of proteins that we ingest into our body. And finally, the final mechanism by which our enzymes can decrease the activation energy and therefore increase the rates of reactions is called metal ion catalysis. So, what's so special about metal ions or metal atoms?