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Calprotectin is a protein complex composed of the S100A8 and S100A9 protein subunits. It is abundant in neutrophils and can restrict microbial access to manganese and zinc through metal binding. This is an example of nutritional immunity: changing nutrient availability can impair microbial functions without directly cutting apart the microorganism.^[ref]

A primary study compared ordinary calprotectin with a variant defective in its transition-metal-binding sites. Isothermal titration calorimetry tested binding, while growth and enzyme-activity assays tested functional consequences. Loss of metal binding strongly reduced antimicrobial activity; manganese add-back restored an affected bacterial superoxide dismutase activity. These comparisons linked metal availability to the phenotype rather than relying on calprotectin abundance alone.^[ref]

An original one-metal model separates total from free metal:If the stipulated system contains $1.0\mu \mathrm{M}$ metal and $0.8\mu \mathrm{M}$ is protein-bound, $0.2\mu \mathrm{M}$ remains free. The total metal concentration has not changed, although its availability has. Other ligands and competing metals would require additional terms.

Calprotectin therefore illustrates how a host protein can alter a microbe's chemical environment. Its metal-binding behavior depends on conditions and should not be reduced to a universal fixed number of ions bound in every sample; neither the simplified balance nor the presence of the protein alone determines an organism's response.

S100A8 is a calcium-binding protein of the S100 protein family. It is one of the two different protein subunits of calprotectin, together with S100A9. The reviewed human reference sequence contains 93 amino acids; a gene symbol, the encoded protein and an assembled protein complex are related but different objects.^[ref]

The distinction is useful when interpreting an assay. Suppose a stipulated preparation contains $5\mu \mathrm{M}$ S100A8 chains and $2\mu \mathrm{M}$ S100A9 chains. If only one-to-one heterodimers form, their concentration cannot exceed $2\mu \mathrm{M}$. Even at complete use of S100A9, $3\mu \mathrm{M}$ S100A8 would remain outside those heterodimers. Measuring the amount of one subunit therefore does not by itself measure assembled calprotectin.

In a primary metal-binding study, changing binding-site residues contributed by both subunits impaired manganese/zinc binding and antimicrobial activity of the complex. Binding was measured with isothermal titration calorimetry and compared with functional assays. The experiment tests the assembled system; it does not establish that isolated S100A8 has every activity of calprotectin.^[ref]

S100A8 consequently illustrates two levels of biological description: the properties of a particular polypeptide and the properties produced when that polypeptide assembles with a different partner.

The S100 protein family is a family of small proteins characterized by a paired EF hand architecture: an S100-specific N-terminal motif and a more conventional C-terminal motif. These motifs can bind calcium ions, linking molecular structure to calcium-sensitive interactions. S100A8 and S100A9 are family members, and their joint assembly forms calprotectin.^[ref]

A protein family groups related molecular structures and sequences; it is not one interchangeable reagent. The human reference S100A8 and S100A9 sequences contain93 and114 amino acids, respectively, and their complexes can differ from the isolated partners.^[ref]

For an original site-count example, a two-chain assembly with two EF hand motifs per chain contains four such motifs. That count does not establish that four calcium ions are bound in a particular solution: site integrity, calcium ion availability and binding affinity matter. It also does not count all possible binding sites for other metals, which may involve residues at the interface between chains.

An informative experiment therefore compares a specified family member or defined complex under controlled ion conditions. In calprotectin, isothermal titration calorimetry and metal-addition experiments linked transition-metal binding to altered bacterial defense. That result concerns the tested assembly, rather than a universal antimicrobial activity of all S100 proteins.^[ref]