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:
Hypoxemia (low oxygen in your blood) can cause hypoxia (low oxygen in your tissues) when your blood doesn't carry enough oxygen to your tissues to meet your body's needs. The word hypoxia is sometimes used to describe both problems. Symptoms. Although they can vary from person to person, the most common hypoxia symptoms are: