Abstract
Antimicrobial proteins such as S100A12 and S100A8/A9 are highly expressed and secreted by neutrophils during infection and participate in human immune response by sequestering transition metals. At neutral pH, S100A12 sequesters Zn(2+) with nanomolar affinity, which is further enhanced upon calcium binding. We investigated the pH dependence of human S100A12 zinc sequestration by using Co(2+) as a surrogate. Apo-S100A12 exhibits strong Co(2+) binding between pH 7.0 and 10.0 that progressively diminishes as the pH is decreased to 5.3. Ca(2+) -S100A12 can retain nanomolar Co(2+) binding up to pH 5.7. NMR spectroscopic measurements revealed that calcium binding does not alter the side-chain protonation of the Co(2+) /Zn(2+) binding histidine residues. Instead, the calcium-mediated modulation is achieved by restraining pH-dependent conformational changes to EF loop 1, which contains Co(2+) /Zn(2+) binding Asp25. This calcium-induced enhancement of Co(2+) /Zn(2+) binding might assist in the promotion of antimicrobial activities in humans by S100 proteins during neutrophil activation under subneutral pH conditions.