Final answer:
The phenolic group of tyrosine is deprotonated at a pH above its pKa. The ability of amino acids like serine, threonine, and tyrosine to be phosphorylated is due to their hydroxyl group. Hydroxyl groups are polar, contributing to hydrophilicity.
Step-by-step explanation:
At a pH above its pKa, the phenolic group of tyrosine is deprotonated. The pKa is a measure of the acid strength of a compound, representing the pH at which half the compound is protonated (has an extra hydrogen atom) and half is deprotonated (missing a hydrogen atom). When the pH is higher than the pKa, it indicates that the environment is more basic than the acid's strength, leading to the loss of a proton from the acid group. In the case of tyrosine, the phenolic hydroxyl group will lose a proton and become deprotonated.
The property that enables the residues of the amino acids serine, threonine, and tyrosine to be phosphorylated is that they contain a hydroxyl group. The hydroxyl group is a reactive site where phosphorylation, the addition of a phosphate group, can occur, modulating the activity of many enzymes and receptors.
Among functional groups, hydroxyl group is polar, contributing to the hydrophilicity or ability to interact with water. This is in contrast to hydrophobic groups, which tend to repel water or fail to interact with it.