Final answer:
The hydroxyl group of tyrosine is protonated at a pH below its pKa due to excess protons in the environment, and the hydroxyl group can later be involved in enzymatic phosphorylation. The option (A) is correct.
Step-by-step explanation:
At a pH below its pKa, the hydroxyl group of tyrosine is protonated. This is because at a pH lower than the pKa, protonated species prevail, as the environment has an excess of protons (H+ ions). As the pH increases and surpasses the pKa, the tyrosine side chain's hydroxyl group will lose a proton and become deprotonated. Tyrosine, like other amino acids, can act as a buffer and has specific regions that can either gain or lose protons depending on the pH. The gain or loss of protons is an equilibrium process that is affected by the pH of the surrounding medium.
The hydroxyl groups of certain amino acids like serine, threonine, and tyrosine can be enzymatically phosphorylated, which requires that these groups contain a hydroxyl group that can interact with ATP to bind a phosphate group. Therefore, option (A) is correct.