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Beom Seok Park is a researcher credited with structural studies of recognition in innate immunity. He was first author of the 2009 study of the Toll-like receptor 4–MD-2 complex with lipopolysaccharide; the original paper records his Department of Chemistry affiliation at KAIST.^[ref]

The crystal structure connected ligand recognition to assembly. Most of the lipopolysaccharide's lipid chains fit inside a pocket in MD-2, while an exposed chain helps make an interface with another Toll-like receptor 4. The assembly contains two receptor-complex units. Thus the ligand helps organize contacts between proteins, rather than merely occupying an isolated binding pocket.^[ref]

A simple comparison shows why this matters. Imagine two ligands that both bind a receptor-associated pocket, but only one presents an exposed surface in the orientation required to join two receptor units. Equal pocket occupancy would not guarantee equal assembly or signaling. This is a conceptual example, not a numerical affinity measurement from the paper.

Park and collaborators' contribution was to provide a molecular account linking the arrangement of lipopolysaccharide, MD-2 and Toll-like receptor 4 to receptor activation. A crystal structure reveals the contacts in the captured complex; claims about response strength or behavior in an intact organism require further experimental measurements.^[ref]

MD-2, myeloid differentiation factor 2, is a protein that associates with Toll-like receptor 4 in recognition of bacterial lipopolysaccharide. Its ligand-binding pocket and its contacts with the receptor help connect molecular recognition to assembly of a signaling complex. It is distinct from lipopolysaccharide-binding protein and CD14, other participants in recognition of the same microbial material.^[ref]

A 2009 crystal structure revealed two copies of the receptor–MD-2–ligand unit in a symmetric assembly. Five of the examined ligand's six lipid chains occupied the MD-2 pocket; the remaining chain contributed to contact with the neighboring receptor. The location of a chemical group, rather than just the number of ligand molecules present, helped explain the assembly interface.^[ref]

For an original counting example, two such ligand-bound units contain two MD-2 molecules, two Toll-like receptor 4 molecules and two ligand molecules. Six molecules in total do not imply six independent signaling units: their organization into a particular complex is part of the mechanism. The arithmetic counts the modeled assembly; it is not a dose–response law.

A useful experimental comparison would measure ligand binding and a downstream response separately while perturbing the interface. Preserved binding with impaired response would distinguish recognition from productive assembly. This is an experiment-design illustration, not a reported result from the structural study. A crystal structure supplies spatial evidence, while a cellular response assay tests a different level of function.

Toll-like receptor 4, commonly abbreviated TLR4, is a membrane receptor involved in innate immunity. Together with the accessory protein MD-2, it recognizes lipopolysaccharide and helps couple extracellular recognition to intracellular signal transduction. The relevant recognition unit is a receptor complex, rather than TLR4 acting independently of every partner.^[ref]

A motivating experiment determined the complex's structure by X-ray crystallography. Lipopolysaccharide occupied a largely hydrophobic pocket in MD-2 and participated in the interface joining two TLR4–MD-2 units. This supplied a structural explanation for how binding can accompany assembly of a signaling-competent arrangement, instead of merely filling an isolated binding site.^[ref]

The example connects chemical structure with receptor organization: different portions of one ligand can interact with an accessory protein and with the assembled receptor interface. A crystal structure constrains that recognition model; it does not by itself measure the magnitude or duration of every downstream immune response in a living organism.