Sequence analysis identified a conserved region (CR) of approximately 150 amino acids that is duplicated in HtaA and present in a single copy in HtaB . HtaA consists of two homologous CRs in the N- and C-terminal regions. We have determined the crystal structures of the N-, and C-terminal CR of HtaA (HtaA-N and HtaA-C, respectively) and HtaB at the resolution of 2.0, 1.3, and 1.7 Å, respectively. HtaA-N consists of 11 β strands and two short α helices and accommodates one heme molecule with Tyr58 located in the first α helix as the heme axial ligand. Tyr58 forms a hydrogen bond with His111. A heme propionate forms hydrogen bonds with Ser54 and Tyr201. Heme is accommodated in an open pocket formed by hydrophobic amino acid residues.
HtaA-C and HtaB show similar global structures to HtaA-N. The key residues for heme-binding and recognition including the axial ligand of heme and residues involved in the hydrogen bonding interactions with heme are conserved among HtaA-N, HtaA-C, and HtaB.
We also determined the crystal structure of HmuT at the resolution of 1.4 Å . HmuT consists of structurally similar two domains located in the N-terminal and C-terminal regions connected a long α helix. A single heme molecule is bound in the cleft between these domains. Heme iron is ligated by His141 and Tyr240, and Tyr240 forms a hydrogen bond with Arg242. Intriguingly, HmuT binds a heme with two different orientations.
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 N. Muraki, S. Aono. (2016) Chem. Lett. 45, 24-26.