Calgranulin C performs antimicrobial activity in the human immune response by sequestering Zn(II). This biological function is afforded with the aid of two structurally distinct Ca(II)-binding EF hand motifs, wherein one of which bears an unusual amino acid sequence. Here, we utilize solution state NMR relaxation measurements to investigate the mechanism of Ca(II)-modulated enhancement of Zn(II) sequestration by calgranulin C. Using C13 /N15 CPMG dispersion experiments we have measured pH-dependent major and minor state populations exchanging on micro-to-millisecond timescale. This conformational exchange takes place exclusively in the Ca(II)-bound state and can be mapped to residues located in the EF-I loop and the linker between the tandem EF hands. Molecular dynamics (MD) simulations spanning nano-to-microsecond timescale offer insights into the role of pH-dependent electrostatic interactions in EF-hand dynamics. Our results suggest a pH-regulated dynamic equilibrium of conformations that explore a range of "closed" and partially "open" sidechain configurations within the Zn(II) binding site. We propose a novel mechanism by which Ca(II) binding to a non-canonical EF loop regulates its flexibility and tunes the antimicrobial activity of calgranulin C.
Keywords: Ca(II) binding proteins; NMR spectroscopy; molecular dynamics simulations; zinc sequestration.
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