Calprotectin is a complex of the mammalian proteins S100A8 and S100A9.Other names for calprotectin include MRP8-MRP14, calgranulin A and B, cystic fibrosis antigen, L1, 60BB antigen, and 27E10 antigen. In the presence of calcium, calprotectin is capable of sequestering the transition metals iron,manganese and zinc via chelation. This metal sequestration affords the complex antimicrobial properties. Calprotectin is the only known antimicrobial manganese sequestration protein complex. Calprotectin comprises as much as 60% of the soluble protein content of the cytosol of a neutrophil, and it is secreted by an unknown mechanism during inflammation.Faecal calprotectin has been used to detect intestinal inflammation (colitis or enteritis) and can serve as a biomarker for inflammatory bowel diseases. Blood based calprotectin (in serum and plasma) is used in diagnostics of multiple inflammatory diseases, including autoimmune diseases, like arthritis, and severe infections including sepsis.
The human homologue of calprotectin is a 24 kDadimer, and is formed by the protein monomers S100A8 (10,835 Da) and S100A9 (13,242 Da). The primary structure of calprotectin can vary between species. For instance, the mouse homologue of S100A8 is 10,295 Da, while the S100A9 homologue is 13,049 Da. Early size exclusion chromatography experiments incorrectly indicated that calprotectin had a molecular mass of 36.5 kDa; occasionally this value is used in contemporary literature. Calprotectin S100A8-S100A9 dimers can non-covalently pair with one another to form 48 kDa tetramers.
Calprotectin has a high affinity for calcium, zinc, iron, and manganese. Each of S100A8 and S100A9 contain two EF-hand type Ca2+ binding sites, and calprotectin is able to bind a total of four calcium ions per dimer or eight calcium ions per tetramer. Calcium binding induces a conformational change in the complex that improves its affinity for transition metals, and promotes tetramer formation. A maximum of two transition metal ions may bind to each calprotectin S100A8-S100A9 dimer.
A calprotectin dimer can bind only one manganese or iron ion with high affinity, and it can do this only in the presence of calcium. Zinc can bind at two sites within the calprotectin dimer, and this can occur in the absence of calcium. Calcium, however, improves calprotectin’s affinity for zinc. While calprotectin metal binding occurs at the interface of S100A9 and S100A8 monomers, the independent monomers have some capacity for zinc binding, and may contribute to zinc homeostasis within mammals.
The first of the two calprotectin metal binding sites consists of a His3Asp motif, with S100A8 contributing two histidine ligands (His83 and His87), and S100A9 contributing a histidine and an aspartic acid ligand (His20 and Asp30). The second site can coordinate metals through a tetra-histidine (His4) or a hexa-histidine (His6) binding motif. In the case of His4 binding, S100A8 coordinates through both His17 and His27 while S100A9 coordinates through His91 and His95. In hexa-histidine binding two further histidine residues, His103 and His105, are recruited from the C-terminal end of S100A9 to enable octahedral coordination of the transition metal. Manganese or iron are bound by the calprotectin dimer at this His6 site. Zinc can be bound to either of the sites that form at the interface between S100A8 and S100A9 monomers.