Ferrochelatase catalyzes the insertion of ferrous ion into protoporphyrin IX,the terminal step in heme biosynthesis.To address some well-known unsolved issues in its mechanism,high-level QM/MM and freed-energy studies was performed to explore possible reaction pathways and their accurate energy barriers.They proposed that the ferrous ion is bound from the binding site with Met76,while His263 play the role of proton acceptor.The rate-determined step can be the first proton abstraction or the proton transition within the porphyrin ring.Significant distinct from what in solution,the fast deprotonation step by conservative residues rather than porphyrin deformation provides the driving force for chelation.Though it induced a modest distortion for the porphyrin ring,the SAT complex is not a necessary immediate for the catalysis.The distant mechanism between Bacillus subtilis and human ferrochelatases can be inferred by the rotation of the porphyrin ring.