This idea of a cascade of phosphorylation events was supported by a study where an IRAK4 knockout in mice showed a more severe phenotype than other IRAK knockout experiments and signalling through Toll/IL-1 receptor (TIR) is virtually eliminated.In 2007 it was found that IRAK4 activity was necessary for activating signal pathways which lead to mitogen-activated protein kinases (MAPK), or Toll-like receptor-mediated immune responses (TLR), but was not essential to T-cell Receptor (TCR) signalling as was originally proposed. Protein structure
IRAK4 is a threonine/serine protein kinase made up of 460 amino acids, which contains both a kinase domain and a death domain. Its kinase domain exhibits the typical bilobed structure of kinases, with the N-terminal lobe consisting of a five-stranded antiparallel beta-sheet and one alpha helix. The C-terminal lobe is composed mainly of a number of alpha helices. Also contained within IRAK4s N-terminal is an extension of twenty amino acids, which is unique to IRAK4 among kinases, even within the IRAK family. Situated where the two lobes meet is an ATP binding site, which is covered by a tyrosine gatekeeper. Tyrosine as a gatekeeper is believed to be unique to the IRAK family of kinases. The protein also contains three auto-phosphorylation sites, each of which when mutated results in a decrease in the kinase activity of IRAK4.A structure of the autophosphorylation of the activation loop has been determined in which the activation loop Thr345 of one monomer is sitting in the active site of another monomer in the crystal (PDB: 4U9A, 4U97).
Interactions
Topical
No interaction studies with the topical form have been performed. Mechanism of action
Description
Eflornithine is a "suicide inhibitor," irreversibly binding to ornithine decarboxylase (ODC) and preventing the natural substrate ornithine from accessing the active site (Figure 1). Within the active site of ODC, eflornithine undergoes decarboxylation with the aid of cofactor pyridoxal 5-phosphate (PLP). Because of its additional difluoromethyl group in comparison to ornithine, eflornithine is able to bind to a neighboring Cys-360 residue, permanently remaining fixated within the active site.During the reaction, eflornithines decarboxylation mechanism is analogous to that of ornithine in the active site, where transamination occurs with PLP followed by decarboxylation. During the event of decarboxylation, the fluoride atoms attached to the additional methyl group pull the resulting negative charge from the release of carbon dioxide, causing a fluoride ion to be released. In the natural substrate of ODC, the ring of PLP accepts the electrons that result from the release of CO2. The remaining fluoride atom that resides attached to the additional methyl group creates an electrophilic carbon that is attacked by the nearby thiol group of Cys-360, allowing eflornithine to remain permanently attached to the enzyme following the release of the second fluoride atom and transimination. Evidence
The reaction mechanism of Trypanosoma bruceis ODC with ornithine was characterized by UV-VIS spectroscopy in order to identify unique intermediates that occurred during the reaction. The specific method of multiwavelength stopped-flow spectroscopy utilized monochromatic light and fluorescence to identify five specific intermediates due to changes in absorbance measurements.