Metal sequestration of the host-defense protein human calprotectin

Abstract

Transition metals are essential nutrients for all organisms. Microbial pathogens must acquire first-row transition metals such as manganese (Mn), iron (Fe), and zinc (Zn) to colonize the mammalian host and to cause disease. As part of the first line of defense against pathogenic infection, the host innate immune response deploys a number of metal-chelating host-defense factors that restrict the access of nutrient metals from these microbes. One of these host-defense factors is the abundant neutrophil protein calprotectin (CP, Si 00A8/S 1 00A9, MRP-8/MRP- 14 oligomer, calgranulins A and B). This calcium (Ca)-binding EF-hand domain protein exhibits two transition-metal-binding sites at the Si 00A8/S 1 00A9 heterodimer interface, and Ca(II) coordination enhances the transition-metal-binding affinities of CP. At the start of this dissertation research, the accepted model stated that CP exhibits antimicrobial activity by withholding bioavailable Mn(II) and Zn(II). In our work, we investigate the coordination chemistry and antimicrobial activity of human CP. We discovered that CP also has the capacity to scavenge Fe(II) from microbes by chelating this metal ion with high affinity at its unusual hexahistidine (His 6) coordination motif, and we show that this site influences the redox speciation of iron in solution. Subsequent studies establish that the His 6 site also contributes to nickel (Ni) withholding, and we evaluate the structural basis for the Ca(II)-modulated Ni(II)- binding properties of CP. Finally, we demonstrate that the functionally versatile His6 motif of CP also contributes to Zn(II) sequestration. In total, our investigations into the bio-inorganic chemistry of CP provide molecular-level insights into host-mediated metal sequestration.