Lock and Key Model. Each type of enzyme has a unique 3-D shape, including a surface groove called an active site, which fits its target substrate much like a key fits in a lock. Other substances that donât fit can't enter the active site and no reaction occurs.
Some of the many causes of hypoxia are the following: 1  Chemical or gas poisoning (for example cyanide, carbon monoxide) 2  Low or absent concentration of oxygen (for example, high altitudes reached without supplemental oxygen as seen in mountain climbing, and aviation, drowning or fires)  Lung problems, for example:
Hypoxic hypoxia (hypoxemic hypoxia) â- In this type of hypoxia, the tissues do not have enough oxygen because there is a lack of oxygen in the blood flowing to the tissues. Hypoxic hypoxia can be caused by inadequate breathing as well as other causes. Anemic hypoxia -- In the setting of anemia, low hemoglobin levels result in a reduced ability of the blood to carry oxygen that is breathed in, and hence, a diminished supply of oxygen available to the tissues.