it reacted with oxygen in the air. Asan analogy, consider a locked door. There is no easy way for people (say,teenage boys and girls) on opposite sides of the door to get together, evenif they would be happy to do so. If someone has the key, however, then thedoor can be opened and proper introductions can be made. The enzymecatalysts play the role of the key, allowing the hydrogen peroxide and hydroquinone to get togetheron the atomic level so that a reaction can take place.The cascadeThe body commonly stores enzymes (proteins that catalyze a chemical reaction, like the cleavage of fibrinogen) in an inactive form for later use. The inactive forms are called proenzymes. When a signal is received that a certain enzyme is needed, the corresponding proenzyme is activated to give the mature enzyme. As with the conversion of fibrinogen to fibrin, proenzymes are often activated by cutting off a piece of the proenzyme that is blocking a critical area. The strategy is commonly used with digestive enzymes. Large quantities can be stored as inactive proenzymes, then quickly activated when the next good meal comes along.Thrombin initially exists as the inactive form, prothrombin. Because it is inactive, prothrombin can't cleave fibrinogen, and the animal is saved from death by massive, inappropriate clotting. Still, the dilemma of control remains. If the cartoon saw were inactivated, the telephone pole would not fall at the wrong time. If nothing switches on the saw, however, then it would never cut the rope; the pole wouldn't fall even at the right time. If fibrinogen and prothrombin were the only proteins in the blood-clotting pathway, again our animal would be in bad shape. When the animal was cut, prothrombin would just float helplessly by the fibrinogen as the animal bled to death. Because prothrombin cannot cleave fibrinogen to fibrin, something is needed to activate prothrombin. Perhaps the reader can see why the blood-clotting system is called a cascadea system where one component activates another component, which activates a third component, and so on.A protein called Stuart factor cleaves prothrombin, turning it into active thrombin that can then cleave fibrinogen to fibrin to form the blood clot. Unfortunately, as you may have guessed, if Stuart factor, prothrombin,and fibrinogen were the only blood-clotting proteins, then Stuart factor would rapidly trigger the cascade, congealing all the blood of the organism. So Stuart factor also exists in an inactive form that must first be activated.At this point there's a little twist to our developing chicken-and-egg scenario. Even activated Stuart factor can't turn on prothrombin. Stuart factor and prothrombin can be mixed in a test tube for longer than it would take a large animal to bleed to death without any noticeable production of thrombin. It turns out that another protein, called accelerin, is needed to increase the activity of Stuart factor. The dynamic duoaccelerin and activated Stuart factor cleave prothrombin fast enough to do the bleeding animal some good. So in this step we need two separate proteins to activate one proenzyme.Yes, accelerin also initially exists in an inactive form, called