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:
Hypoxia does not cause discomfort or pain so its onset can be insidious and pass un-noticed by crews who are not fully aware of its dangers. Factors that affect the onset and severity of hypoxia include an individualâs physical fitness, cabin temperature, altitude, rate of ascent and duration at altitude.