The extracellular heme-binding protein HbpS from the soil bacterium Streptomyces reticuli is an aquo-cobalamin binder [Protein Structure and Folding]

October 23rd, 2014 by Ortiz de Orue Lucana, D., Fedosov, S. N., Wedderhoff, I., Che, E. N., Torda, A. E.

The extracellular protein HbpS from Streptomyces reticule interacts with iron ions and heme. It also acts in concert with the two component sensing system SenS/SenR in response to oxidative stress. Sequence comparisons suggested that the protein may bind a cobalamin. UV/Vis spectroscopy confirmed binding (Kd = 34 μM) to aquocobalamin (H2OCbl+), but not to other cobalamins. Competition experiments with the H2OCbl+ coordinating ligand CN− and comparison of mutants identified a histidine residue (His156) that coordinates the cobalt ion of H2OCbl+ and substitutes for water. HbpS·Cobalamin lacks the Asp-X-His-x-x-Gly motif seen in some cobalamin binding enzymes. Preliminary tests showed that a related HbpS protein from a different species also binds H2OCbl+. Furthermore, analyses of HbpS-heme binding kinetics are consistent with the role of HbpS as a heme-sensor and suggested a role in heme transport. Given the high occurrence of HbpS like sequences amongst Gram positive and Gram negative bacteria, our findings suggest a great functional versatility among these proteins.
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