Final answer:
The nucleophilic attack during a thrombin-catalyzed reaction is performed by serine, which leverages its hydroxyl group as a nucleophile to facilitate the reaction. Therefore, correct option is a.
Step-by-step explanation:
The amino acid that performs the nucleophilic attack during a thrombin-catalyzed reaction is serine. In the mechanism of a thrombin-catalyzed reaction, a serine residue's hydroxyl group acts as a nucleophile.
This is supported by information indicating that in the enzymatic activity involving acyl substitution reactions, the hydroxyl group from a serine residue acts as the attacking nucleophile and facilitates the cleavage of peptide bonds.
These serine residues are known for having a hydroxyl group capable of being enzymatically phosphorylated by ATP, as seen in proteins like casein.
It is also noteworthy that serine, alongside other hydroxyl-containing amino acids like threonine and tyrosine, has the ability to be phosphorylated due to its hydroxyl group. This capability is crucial in many biological processes, including signal transduction and enzyme activity modulation.