What amino acid can be found in two different charge states at physiological of pH
The correct answer and explanation is:
The amino acid that can be found in two different charge states at physiological pH is histidine.
Histidine has an imidazole side chain that has a pKa around 6.0, which is close to the physiological pH of 7.4. This proximity to physiological pH allows histidine to exist in a protonated and deprotonated form depending on the local environment or the exact pH.
At physiological pH (around 7.4), histidine typically exists in a partially protonated form. The imidazole ring can either carry a positive charge (protonated) or be neutral (deprotonated). The balance between these two forms can vary based on the surrounding conditions like pH or the presence of specific ions. This characteristic makes histidine important in enzyme active sites and other areas where subtle changes in charge can affect protein structure or function.
The ability to exist in different charge states also allows histidine to act as a proton donor or acceptor, which is crucial in many biochemical processes like enzyme catalysis, buffering, and metal ion binding. For example, in enzymes such as lysozyme, the histidine residue plays a critical role in catalyzing the breaking of bonds through protonation and deprotonation.
In contrast to other amino acids with side chains that are either acidic or basic at physiological pH (e.g., aspartate or lysine), histidine’s side chain offers flexibility in charge state, which is one reason it is so important in the active sites of enzymes, where the ability to adjust its charge is essential for catalytic activity. This also explains why histidine is often found in environments that require precise control of charge and pH, such as the active sites of enzymes involved in pH-sensitive reactions.