Affinity of S100A1 protein for calcium increases dramatically upon glutathionylation

S100A1 is a typical representative of a group of EF‐hand calcium‐binding proteins known as the S100 family. The protein is composed of two α subunits, each containing two calcium‐binding loops (N and C). At physiological pH (7.2) and NaCl concentration (100 mm), we determined the microscopic binding constants of calcium to S100A1 by analysing the Ca2+‐titration curves of Trp90 fluorescence for both the native protein and its Glu32 → Gln mutant with an inactive N‐loop. Using a chelator method, we also determined the calcium‐binding constant for the S100A1 Glu73 → Gln mutant with an inactive C‐loop. The protein binds four calcium ions in a noncooperative way with binding constants of K1 =4 ± 2 × 103 m−1 (C‐loops) and K2≈ 102 m−1 (N‐loops). Only when both loops are saturated with calcium does the protein change its global conformation, exposing to the solvent hydrophobic patches, which can be detected by 2‐p‐toluidinylnaphthalene‐6‐sulfonic acid – a fluorescent probe of protein‐surface hydrophobicity. S‐Glutathionylation of the single cysteine residue (85) of the α subunits leads to a 10‐fold increase in the affinity of the protein C‐loops for calcium and an enormous – four orders of magnitude – increase in the calcium‐binding constants of its N‐loops, owing to a cooperativity effect corresponding to ΔΔG = −6 ± 1 kcal·mol−1. A similar effect is observed upon formation of the mixed disulfide with cysteine and 2‐mercaptoethanol. The glutathionylated protein binds TRTK‐12 peptide in a calcium‐dependent manner. S100A1 protein can act, therefore, as a linker between the calcium and redox signalling pathways.