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:
The causes of both environmental and tissue hypoxia often result in the intermediate state of hypoxemia; thus the causes of any type of hypoxia are also potential causes of hypoxemia. Some of the many causes of hypoxia are the following: Chemical or gas poisoning (for example cyanide, carbon monoxide)