S100A8 (source code)
= S100A8
{c}
\b[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.<Human S100A8 and S100A9 records in UniProt>[UniProt P05109, ID/DE/GN/OS and sequence fields]{ref}
The distinction is useful when interpreting an assay. Suppose a stipulated preparation contains $5\,\mathrm{\mu M}$ S100A8 chains and $2\,\mathrm{\mu M}$ <S100A9> chains. If only one-to-one <heterodimers> form, their concentration cannot exceed $2\,\mathrm{\mu M}$. Even at complete use of S100A9, $3\,\mathrm{\mu M}$ 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>.<Nutrient metal sequestration by calprotectin inhibits bacterial superoxide defense, enhancing neutrophil killing of Staphylococcus aureus by Thomas E. Kehl-Fie and others>[Figure2 and associated Results, metal-site variant and binding/activity comparisons]{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.
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