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 occurs when there is not enough oxygen in the air or when decreasing atmospheric pressure prevents diffusion of oxygen from the lungs to the bloodstream. Aviation personnel are most likely to encounter this type of hypoxia at high altitudes due to the reduction of PO2 in the atmosphere.