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 can also occur when there is poor ventilation of the lungs such as occurs in emphysema and in cardiovascular disease. The production of erythropoietin decreases in kidney failure and various chronic diseases such as AIDS, certain cancers and chronic inflammatory diseases like rheumatoid arthritis.