Synthesis of 5PP-InsP5. To synthesize 5PP-InsP5 chemoenzymatically, we recombinantly expressed IP6KA, an InsP6kinase from the parasitic amoeba E. histolytica.28,29 Theadvantage of this enzyme, compared to the commonly usedhuman IP6K1, is its fast reaction kinetics, good stability, andhigh expression yield (∼100 mg of protein/L of Escherichia coliculture). The InsP6 substrate can be obtained commercially(Calbiochem) or prepared from myo-inositol using astraightforward two-step literature procedure.30,31 Next, thereaction conditions (pH, time, and enzyme and Mg2+concentrations) were optimized using 13C-labeled InsP6([13C6]InsP6) as a substrate to monitor PP-InsP kinase activityin vitro by nuclear magnetic resonance (NMR) spectroscopy.32Full conversion to the 5PP-InsP5 product was achieved within30 min, applying 0.3 μM IP6KA in combination with an ATPrecycling system (pCr and CrK) (Figure 2a and Figure S1).32Determining the right balance among phosphate-containingspecies, Mg2+ ions, and pH proved to be an integralconsideration to prevent precipitation of InsP6 during thereaction.With the optimized conditions (0.3 μM IP6KA, 2 mM ATP,7 mM MgCl2, and 250 μM InsP6), we scaled up the reaction to350 mg of InsP6 starting material and a total reaction volumeof 2 L. To accurately control the reaction time, it was crucial topreincubate the reaction mixture at 37 °C before the additionof IP6KA and to quench the solution by cooling to 4 °C within3−5 min using a −78 °C dry ice bath. For large scalepurification, the proteins were removed by passing the solutionFigure 1. Current synthetic strategies for the preparation of distinct PP-InsPs from commercially available starting materials. Organic multistepsyntheses (left) produce PP-InsPs in high purity from myo-inositol with low overall yields due to several challenging steps. Enzymatic syntheses(right) convert InsP6 in one or two steps into PP-InsPs. To chemoenzymatically obtain 5PP-InsP5 and 1PP-InsP5, IP6K1 and Vip1 have beenapplied, respectively.Figure 2. Biochemical synthesis and purification of 5PP-InsP5. (a)Optimized conditions for the conversion of InsP6 by IP6KA.Conditions: 250 μM InsP6, 2 mM ATP, 7 mM MgCl2, 5 mMcreatine phosphate (pCr), 1 U/mL creatine kinase (CrK), 0.3 μMIP6KA, 50 mM NaCl, and 20 mM MES (pH 6.4). (b) Separation of5PP-InsP5 from the reaction components. Enzymes were removed bya C18 plug, and the product precipitated as a PP-InsP−Mg complexby addition of excess MgCl2. Mg2+ ions were subsequently exchangedby solid phase chelation in an ammonium carbonate buffer, andlyophilization afforded the product as the ammonium salt. (c) 31PNMR spectrum of purified 5PP-InsP5