Question
Which of the following amino acids primarily acts as a buffer in blood due to its ability to accept and donate protons at physiological pH?
| A. |
Histidine
|
| B. |
Arginine
|
| C. |
Tryptophan
|
| D. |
Tyrosine
|
Show Answer
|
Correct Answer � A
Explanation
|
|
Histidine
-
Histidine is the amino acid that primarily acts as a buffer in the blood because of the unique property of its imidazole side chain, which has a pKa around 6.8.
-
This is close to the physiological pH (7.35–7.45), allowing histidine to accept or donate protons (H⁺) as needed, making it an excellent buffering agent in the blood.
-
Hemoglobin, a major protein buffer in blood, contains histidine residues that help in buffering H⁺ ions generated from CO₂ transport.
Why Other Options Are Incorrect:
B. Arginine
-
Has a guanidino group with a high pKa (~12.5), meaning it is fully protonated and not suitable as a buffer at physiological pH.
-
Functions more in nitric oxide production and urea cycle, not as a primary buffer.
C. Tryptophan
-
Contains an indole group, not ionizable at physiological pH.
-
Plays roles in serotonin and melatonin synthesis, but not in buffering.
D. Tyrosine
-
Has a phenolic hydroxyl group with a pKa ~10.1, too high to act effectively as a buffer at physiological pH.
-
Functions in signal transduction via phosphorylation.
