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 is a decreased level of oxygen in all or part of your body, such as your brain. Some conditions can cause hypoxia to occur suddenly. Other conditions may cause hypoxia to occur over time. Hypoxia may be caused by any of the following: 1  Travel to a high altitude. 2  Near drowning or choking. 3  Carbon monoxide poisoning.  Exposure to cold for a long period of 1  time. Severe anemia.  Chronic lung disease, such as 1  emphysema. Congestive heart failure.