Holo- And Apo- Structures of Bacterial Periplasmic Heme Binding Proteins

• Published in: The Journal of biological chemistry Vol. 282
• Main Authors: Ho, W.W., Li, H., Eakanunkul, S., Tong, Y., Wilks, A., Guo, M., Poulos, T.L.
• Format: Journal Article
• Language: English
• Published: United States 01.06.2009
• Subjects: BACTERIA; COORDINATES; CRYSTAL STRUCTURE; HEME; IRON; LIGANDS; MATERIALS SCIENCE; MEMBRANES; ORIENTATION; Other,BIO, CHEM; PATHOGENS; PEPTIDES; PROTEINS; PSEUDOMONAS; RABBIT TUBES; SHIGELLA; SPACE; TRANSPORT; VITAMIN B-12; VITAMINS
• ISSN: 0021-9258; 1083-351X

An essential component of heme transport in Gram-negative bacterial pathogens is the periplasmic protein that shuttles heme between outer and inner membranes. We have solved the first crystal structures of two such proteins, ShuT from Shigella dysenteriae and PhuT from Pseudomonas aeruginosa. Both share a common architecture typical of Class III periplasmic binding proteins. The heme binds in a narrow cleft between the N- and C-terminal binding domains and is coordinated by a Tyr residue. A comparison of the heme-free (apo) and -bound (holo) structures indicates little change in structure other than minor alterations in the heme pocket and movement of the Tyr heme ligand from an 'in' position where it can coordinate the heme iron to an 'out' orientation where it points away from the heme pocket. The detailed architecture of the heme pocket is quite different in ShuT and PhuT. Although Arg{sup 228} in PhuT H-bonds with a heme propionate, in ShuT a peptide loop partially takes up the space occupied by Arg{sup 228}, and there is no Lys or Arg H-bonding with the heme propionates. A comparison of PhuT/ShuT with the vitamin B{sub 12}-binding protein BtuF and the hydroxamic-type siderophore-binding protein FhuD, the only two other structurally characterized Class III periplasmic binding proteins, demonstrates that PhuT/ShuT more closely resembles BtuF, which reflects the closer similarity in ligands, heme and B{sub 12}, compared with ligands for FhuD, a peptide siderophore.