Amino Acid Protonation at Physiological pH

Researchers are investigating a novel enzyme that exhibits a highly specific catalytic activity towards certain amino acids. The enzyme's active site exhibits an electrostatic environment conducive to facilitating the conversion of a substrate that is an amino acid into its corresponding alpha-keto acid. During the reaction, the enzyme stabilizes the transition state through a series of interactions involving charged amino acid side chains.

During their studies, they note that the enzyme displays a remarkable specificity constant (k_cat/K_m) only for certain amino acids, particularly those containing positively charged side chains at physiological pH. The researchers hypothesize that the charged nature of these amino acids likely plays a crucial role in the enzyme's mechanism. They are specifically focused on understanding the acid-base properties and the pKa values associated with the side chains of the amino acids involved.

Which of the following amino acids would likely exhibit the highest protonation state at physiological pH (approximately 7.4), thereby enhancing binding and reactivity with the enzyme?