LAUSR

A homology model of ferrochelatase (HemH), the heme biosynthesis terminal step enzyme from Salmonella Typhi was generated to understand the mechanism of metal insertion into protoporphyrin IX for heme biosynthesis. The overall fold of membrane associated ferrochelatase (StFc) from S. Typhi is similar to human and Yeast ferrochelatase than Bacillus subtilis, and Bacillus anthracis. An insertion of 16 amino acid residues in helical switch having hydrophobic patch proposed to interact with membrane lipids and in opening and closing of heme binding cleft. The sequence analysis and the docking study revealed that the protoporphyrin binding site in StFc has a crucial replacement of Tyr/Met to Leu13 unique in comparison to other known structures, where Tyr13 observed in B. subtilis/B. anthracis while Met76 in human/yeast play important role in holding protoporphyrin in optimal orientation for metalation. A sitting-a-top (SAT) complex mechanism for metalation is proposed with His194 and Glu264 lie at the bottom and Leu13 on the top of the porphyrin ring. In addition, an entry and exit mechanism is also proposed for protoporphyrin binding into cavity by opening and closing of helical switch using molecular dynamics simulation studies of Apo and heme complexed model structure of S. Typhi HemH.