Highlights Enzyme Control II
1. Another type of covalent modification of enzymes affecting their activity is that which activates zymogens. Zymogens are enzymes made in an inactive form and then activated by cleavage of one or more peptide bonds in them.
2. Chymotrypsinogen is a zymogen that get activated by peptide cleavage to form chymotrypsin. It is made in the pancreas and normally activated in the digestive system. If it gets activated too soon, pancreatitis results from attack of the enzyme on the proteins in the pancreas.
3. Another important set of zymogens in the body are the proteins involved in the clotting of blood. Like the situation with pancreatitis, activation of these zympogens for the wrong reasons or at the wrong places can have disastrous consequences.
4. Chymotrypsin is an enzyme that catalyzes a reaction in two phases – a slow phase and a fast phase. The fast phase occurs first. In this phase, the peptide bond is broken and the first peptide is released. As a result of this action, the other peptide is covalently linked to the enzyme transiently. The release of the second peptide from the enzyme is the slow phase and involves the action of water.
5. Chymotrypsin is a so-called serine protease, meaning that it uses the side chain of serine to catalyze proteolytic cleavage. The side chain of serine (hydroxyl group) is ionized in the reaction as a result of removal of its proton by a nearby histidine. This occurs when the substrate binds the active site, moving the histidine slightly closer to the serine.
6. In the catalytic mechanism of chymotrypsin, serine becomes covalently attached to one peptide when the other peptide is released. Release of the second peptide from serine requires the action of water.
7. Serine proteases act by creation of a reactive alkoxide ion. This is made by 1) binding of the proper substrate at the active site; 2) slight changes in shape that bring the catalytic triad (aspartic acid, histidine, and serine) closer together; 3) removal of a proton from the hydroxyl group of serine (creating the alkoxide ion).
8. The alkoxide ion is reactive and attacks the peptide bond in the active site, breaking it. One piece from the break is released and the other piece becomes transiently attached to the oxygen of the serine. This is the fast step of the process. Detachment of the attached peptide requires action of water in the active site. This is the slow step of the reaction.
9. Related proteases include cysteine proteases which activate a sulfur of a sulhydryl group during catalysis instead of activating an oxygen of a hydroxyl. The sulfur gets activated by a histidine, just like in serine proteases, and the activated sulfur acts like a nucleophile, just like in serine proteases. A transient intermediate is formed with a covalent bond to one of the peptides.