Final answer:
Serine is acylated by penicillin at the side chain OH group, taking advantage of the strained ß-lactam ring in penicillin which is highly reactive in nucleophilic acyl substitution reactions.
Step-by-step explanation:
Serine is most likely to be acylated by penicillin at the side chain OH group. This is because the acyl substitution reaction involves nucleophilic attack by the hydroxyl group of the serine residue in the transpeptidase enzyme. Penicillin is a strong acylating agent because of the presence of the strained ß-lactam ring, making it adept at undergoing nucleophilic acyl substitution reactions with serine's hydroxyl group.
In this reaction, the hydroxyl group of serine acts as the nucleophile attacking the carbonyl carbon of penicillin, resulting in the formation of a covalent bond between serine and penicillin.