Heme Iron Coordination Structure of Heme Transport Protein HutB from Vibrio Cholerae

HutB is a putative heme transport protein located in the periplasmic space in Vibrio cholerae. Here, we purified HutB and characterized its heme binding properties. An analysis of the Soret band showed that there are two types of heme binding geometries depending on the heme concentration: 404-nm species are dominant at lower concentrations of heme, and 394-nm species dominate at higher concentrations. Moreover, a mutational study revealed that either Tyr65 or Tyr198 binds heme with the help of histidine, a property shared with another V. cholerae heme transport protein, HutX, despite the absence of sequence similarity, indicating that HutB acts as a heme transport protein in the periplasm.