Which amino acid has a pKa of 4 in its R-group?

The correct choice is Glutamate, which has a side chain (R-group) that contains a carboxylic acid functional group. The pKa of the R-group carboxyl group in glutamate is typically around 4. This characteristic enables glutamate to act as a proton donor at physiological pH, making it an important amino acid involved in various biochemical processes, such as acting as a neurotransmitter and participating in metabolic pathways.

In contrast, aspartate, another amino acid with a similar structure, also has a carboxylic acid in its R-group, but its pKa value is similar to that of glutamate, which might create confusion. Cysteine contains a thiol group in its R-group with a pKa around 8.3, making it less acidic compared to glutamate. Histidine, on the other hand, has an imidazole side chain with a pKa around 6.0, which allows it to participate in buffering at physiological pH but does not fit the criteria of having a pKa of approximately 4. This makes glutamate the amino acid with the R-group pKa of interest in this context.