Fig. 5. Catalytic mechanism of ACSs.
Reactions catalyzed by the dual activities of ACS diverge after the formation of external substrate aldimine. For the Cβ-S lyase activity, the K285 of AtACS7 extracts the Cα proton of l-cystine and transfers it to the Sγ atom, thereby breaking the bond between Cβ and Sγ of l-cystine. A quinonoid intermediate is required in this process. Whereas for the ACS activity, a residue other than K285, probably Y160, is required to break the Cγ-Sδ bond of SAM. Following deprotonation of Cα by K285, a new covalent bond is formed between the Cα and Cγ to form ACC. In this process, the quinonoid intermediate is not essential.