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:
Although chronic hypoxia is a recognized cause of pulmonary hypertension, it would rarely lead to severe pulmonary hypertension. Nonetheless, patients with advanced chronic obstructive pulmonary disease (COPD) and interstitial lung disease who develop pulmonary hypertension tend to have a worse outcome.