Refer to the Biochemistry in Focus section of your text for this chapter to answer this question. Sarin inhibits acetylcholinesterase by covalently modifying the enzyme active site. Pyridostigmine is a competitive inhibitor of acetylcholinesterase and can serve as an antidote for sarin poisoning. How does pyridostigmine prevent sarin from inhibiting acetylcholinesterase?

a. Pyridostigmine binds to sarin and prevents it from binding to and modifying the enzyme active site.
b. Pyridostigmine increases the affinity of acetylcholinesterase for acetylcholine, thus preventing sarin from interacting with the enzyme active site
c. Pyridostigmine binds to the acetylcholinesterase active site and prevents sarin from binding to and modifying the enzyme active site.
d. Pyridostigmine reverses the covalent modification caused by sarin in the acetylcholinesterase active site.

c. Pyridostigmine binds to the acetylcholinesterase active site and prevents sarin from binding to and modifying the enzyme active site.

Explanation:

Sarin is a toxic compound which inhibits acetylcholinesterase enzyme. This enzyme degrades the neurotransmitter acetylcholine after the required muscle cell stimulation. In absence of its activity, acetylcholine is not degraded and the muscle cells are stimulated uncontrollably. The muscles are not able to relax which ultimately proves fatal for the victim.

Pyridostigmine is also an acetylcholinesterase inhibitor like Sarin. However its period of inhibition is short compared to organophosphorus inhibitors like sarin. It competes with sarin and replaces it in the active site of acetylcholinesterase. The enzyme is still inhibited but for a shorter time as compared to inhibition by Sarin. Hence after sometime, it becomes functional again and resumes its normal activity. Acetycholine starts getting degraded relaxing the muscles.