Covalent Bond Formation in Serine Proteases

The structural and functional properties of proteins are largely determined by the unique sequences of amino acids that compose them. Amino acids can be classified into various categories, including essential and non-essential amino acids, based on the body's ability to synthesize them. Furthermore, certain amino acids have distinct side chains that can lead to various chemical properties such as hydrophobicity, charge, and reactivity. Understanding these properties is crucial for predicting protein folding, function, and interaction with other biological molecules. In a study examining enzyme catalysis, the association between the side chain properties of amino acids at the active site was analyzed, particularly focusing on their roles in substrate specificity and reaction mechanisms.

Which of the following amino acids is uniquely positioned to facilitate the formation of a covalent bond during the catalytic cycle of a serine protease?