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:
The typical causes of hypoxic-ischemic brain injury â cardiac arrest , respiratory arrest , near-drowning , near-hanging , and other forms of incomplete suffocation , carbon monoxide and other poisonous gas exposures , and perinatal asphyxia â expose the entire brain to potentially injurious reductions of oxygen ( i.e. , hypoxia ) and / or ...