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:
Common causes of hypoxemia include: Anemia; ARDS (Acute respiratory distress syndrome) Asthma; Congenital heart defects in children; Congenital heart disease in adults; COPD (chronic obstructive pulmonary disease) Emphysema; Interstitial lung disease; Medications, such as certain narcotics and anesthetics, that depress breathing; Pneumonia; Pneumothorax (collapsed lung)