THREE-DIMENSIONAL {MAPPING} {OF} {DIFFERENTIAL} {AMINO} {ACIDS} {OF} HUMAN, MURINE, {CANINE} {AND} {EQUINE} TLR4/MD-2 {RECEPTOR} {COMPLEXES} {CONFERRING} {ENDOTOXIC} {ACTIVATION} {BY} {LIPID} A, {ANTAGONISM} {BY} {ERITORAN} {AND} SPECIES-DEPENDENT {ACTIVI

摘要

A literature review concerning the unexpected species differences of the vertebrate innate immune response to lipid {IVA} was published in {CSBJ} prior to the present computational study to address the unpaired activity-sequence correlation of prototypic E. coli -type lipid A and its precursor lipid {IVA} regarding human, murine, equine and canine species. To this end, their sequences and structures of hitherto known Toll-like receptor 4 (TLR4) and myeloid differentiation factor 2 (MD-2) complexes were aligned and their differential side chain patterns studied. If required due to the lack of the corresponding X-ray crystallographic data, three-dimensional models of TLR4/MD-2/ligand complexes were generated using mono and dimeric crystal structures as templates and in silico docking of the prototypic ligands lipid A, lipid {IVA} and Eritoran. All differential amino acids were mapped to pinpoint species dependency on an atomic scale, i.e. the possible concert of mechanistically relevant side chains. In its most and general form the three-dimensional (3D-) models devise a triangular interface or “wedge” where molecular interactions between TLR4, MD-2 and ligand itself take place. This study identifies two areas in the wedge related to either agonism or antagonism reflecting why ligands like lipid {IVA} can possess a species dependent dual activity. Lipid {IVA} represents an imperfect (underacylated and backbone-flipped), low affinity ligand of mammalian TLR4/MD-2 complexes. Its specific but weak antagonistic activity in the human system is in particular due to the loss of phosphate attraction in the wedge-shaped region conferred by nonhomologous residue changes when compared to crystal and modeled structures of the corresponding murine and equine TLR4/MD-2 complexes. The counter-TLR4/MD-2 unit was also taken into account since agonist-mediated dimerization in a defined m-shaped complex composed of two TLR4/MD-2/agonist subunits triggers intracellular signaling during the innate immune response to bacterial endotoxin exposure.