Substrate promiscuity of inositol 1,4,5-trisphosphate kinase driven by structurally-modified ligands and active site plasticity

d - myo -inositol 1,4,5-trisphosphate (InsP 3 ) is a fundamental second messenger in cellular Ca 2+ mobilization. InsP 3 3-kinase, a highly specific enzyme binding InsP 3 in just one mode, phosphorylates InsP 3 specifically at its secondary 3-hydroxyl group to generate a tetrakisphosphate. Using a chemical biology approach with both synthetised and established ligands, combining synthesis, crystallography, computational docking, HPLC and fluorescence polarization binding assays using fluorescently-tagged InsP 3 , we have surveyed the limits of InsP 3 3-kinase ligand specificity and uncovered surprisingly unforeseen biosynthetic capacity. Structurally-modified ligands exploit active site plasticity generating a helix-tilt. These facilitated uncovering of unexpected substrates phosphorylated at a surrogate extended primary hydroxyl at the inositol pseudo 3-position, applicable even to carbohydrate-based substrates. Crystallization experiments designed to allow reactions to proceed in situ facilitated unequivocal characterization of the atypical tetrakisphosphate products. In summary, we define features of InsP 3 3-kinase plasticity and substrate tolerance that may be more widely exploitable. InsP 3 3-kinase phosphorylates 1,4,5-trisphosphate (InsP 3 ) specifically at its secondary 3-hydroxyl group to generate a tetrakisphosphate. Here, the authors used a combination of methods to survey InsP 3 3-kinase ligand specificity and determined that IP3K specificity surpasses that of its natural substrate, allowing it to bind diverse ligands with a primary hydroxyl in the reactive position and based on a carbohydrate moiety.