Question

A research technician who is working with organophosphate compounds is required to have a weekly blood test for acetylcholine esterase activity. Typically, esterase activity remains relatively constant for some time and then abruptly drops to zero. If this happens, the technician must immediately stop working with the organophosphate compounds. The organophosphate compounds form stable esters with a critical serine hydroxyl group in the esterase. In the esterase, serine transfers a proton to a histidine residue. Which of the following is correct? A. Serine is acting as a general acid. B. Histidine is acting as a general acid. C. Serine and histidine form a covalent intermediate. D. The enzyme would be relatively insensitive to pH changes. E. Serine is acting as a transition stabilization catalyst.

Short answer

Answer: Serine is acting as a general acid.

Step-by-step solution

Option A - Serine as a general acid

In the given information, serine transfers a proton to histidine residue. Thus, serine is acting as an acid (donating a proton). Therefore, option A is correct.

Option B - Histidine as a general acid

Histidine is accepting a proton from serine, which makes it a base in this reaction, not an acid. So, option B is incorrect.

Option C - Covalent intermediate

Although serine forms a covalent bond with the organophosphate compounds, there is no mention of serine and histidine forming a covalent intermediate during proton transfer. So, option C is incorrect.

Option D - Insensitivity to pH changes

Given that serine transfers a proton to histidine, the enzyme must have some sensitivity to pH changes. Hence, option D is incorrect.

Option E - Transition stabilization catalyst

The provided information doesn't mention anything about serine acting as a transition stabilization catalyst. Therefore, option E is incorrect. Based on the analysis, the correct answer is: A. Serine is acting as a general acid.