Hemophores from Serratia marcescens (HasAsm) and Pseudomonas aeruginosa (HasAp) bind hemin between two loops, which harbor the axial ligands H32 and Y75. Hemin binding to the Y75 loop triggers closing of the H32 loop and enables binding of H32. Because Yersinia pestis HasA (HasA yp) presents a Gln at position 32, we determined the structures of apo- and holo-HasAyp. Surprisingly, the Q32 loop in apo-HasA yp is already in the closed conformation, but no residue from the Q32 loop binds hemin in holo-HasAyp. In agreement with the minimal reorganization between the apo- and holo-structures, the hemin on-rate is too fast to detect by conventional stopped-flow measurements.
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