Calprotectin

Calprotectin is a complex of the mammalian proteins S100A8 and S100A9. In the presence of calcium, calprotectin is capable of sequestering the essential nutrients manganese and zinc. 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 neutrophil cytosol, and is secreted by an unknown mechanism during inflammation. Faecal calprotectin has been used to detect intestinal inflammation, and can serve as a marker for inflammatory bowel diseases. The complex has several synonyms, including MRP8-MRP14, calgranulin A and B, cystic fibrosis antigen, 60BB antigen, and 27E10 antigen.

Structure

The human homologue of calprotectin is a 24 kDa dimer, 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.