= S100 protein family
{c}
The \b[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>.<S-100 conserved domain record in NCBI CDD>[cd00213, family description and conserved binding features]{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.<Human S100A8 and S100A9 records in UniProt>[P05109 and P06702, sequence and subunit fields]{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.<Nutrient metal sequestration by calprotectin inhibits bacterial superoxide defense, enhancing neutrophil killing of Staphylococcus aureus by Thomas E. Kehl-Fie and others>[Figures2–3, binding-site variants and metal add-back]{ref}
Back to article page
