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:
1 Ischemic hypoxia (orstagnant hypoxia) â Reduced brain oxygen is caused by inadequate blood flow to the brain. 2  Stroke, shock, cardiac arrest and heart attack may cause stagnant hypoxia. 3  Ischemic hypoxia can also be created by pressure on the brain.erebral hypoxia can also be classified by the cause of the reduced brain oxygen: 1  Hypoxic hypoxia â Limited oxygen in the environment causes reduced brain function.