Highlights Allostery/Regulation II
1. Protein kinase A is an enzyme involved in covalent modification of enzymes. Like all kinases, it catalyzes the addition of a phosphate to a molecule. Since it is a protein kinase, the molecules it attaches a phosphate to are protein molecules.
2. Phosphates get attached to hydroxyl side chains in proteins. Protein kinase A attaches phosphate to serine or threonine. Other protein kinases can attach phosphate to tyrosine side chains.
3. Protein kinase A is controlled by allosteric means. It is composed of two regulatory subunits and two catalytic subunits. The overall complex is called R2C2. When the catalytic subunits are bound to the regulatory subunits, they cannot catalyze reactions. However, when the molecule cAMP binds to the regulatory subunits, the catalytic subunits are released and are thus active. When active, they covalently modify proteins by adding phosphates.
4. Zymogens are enzymes that are synthesized in an inactive form whose activation requires covalent modification, usually proteolytic cleavage. Examples include digestive enzymes, such as trypsin, chymotrypsin, elastase, and carboxypeptidase whose enzymatic activity might be harmful to the tissue where they are being made.
5. Trypsin is the primary activator of an entire class of proteolytic enzymes. Improper activation of trypsin in or close to the pancreas can lead to pancreatitis, which arises when the proteases attack proteins in the pancreas.
6. Activation of chymotrypsinogen to chymotrypsin, for example, requires trypsin. Trypsin makes an initial cleavage beween amino acids 15 and 16. A disulfide bond keeps the two pieces from coming completely apart, however. This creates an intermediately active form of chymotrypsin called pi-chymotrypsin. This form cleaves itself to remove two dipeptides and this results in full chymotrypsin activity. The three polypeptide pieces are held together by disulfide bonds. Note that three very minor changes in the chymotrypsinogen zymogen have converted a completely inactive enzyme to a fully active one.
7. Alpha-one-antitrypsin is an inhibitor of many proteases, including trypsin and elastase. Normally alpha-one-antitrypsin binds to elastase and prevents it from damaging tissue. People deficient in the factor have proteins in their alveolar walls of the lung (and other connective tissue proteins) destroyed, ultimately resulting in emphysema. Smoke from cigarettes will react with HEALTHY alpha-one-antitrypsin, causing a methionine residue to be oxidized. This oxidation, in turn, prevents alpha-one-antitrypsin from binding to elastase, allowing elastase to damage alveolar tissue and eventually cause emphysema.
9. Blood clotting is a process that is also tightly controlled by zymogens. The process is a cascade of catalytic events in which one enzyme activates another, which in turn activates hundreds/thousands, which in turn activate millions. Cascades are great for amplifying a signal.