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We obtained detailed kinetic characteristics&#x2013;stoichiometry, reaction rates, substrate affinities and equilibrium conditions&#x2013;of human PPIP5K2 (diphosphoinositol pentakisphosphate kinase 2). This enzyme synthesizes &#x2018;high-energy&#x2019; PP-InsPs (diphosphoinositol polyphosphates) by metabolizing InsP6 (inositol hexakisphosphate) and 5-InsP7 (5-diphosphoinositol 1,2,3,4,6-pentakisphosphate) to 1-InsP7 (1-diphosphoinositol 2,3,4,5,6-pentakisphosphate) and InsP8 (1,5-bis-diphosphoinositol 2,3,4,6-tetrakisphosphate), respectively. These data increase our insight into the PPIP5K2 reaction mechanism and clarify the interface between PPIP5K catalytic activities and cellular bioenergetic status. For example, stochiometric analysis uncovered non-productive, substrate-stimulated ATPase activity (thus, approximately 2 and 1.2 ATP molecules are utilized to synthesize each molecule of 1-InsP7 and InsP8, respectively). Impaired ATPase activity of a PPIP5K2-K248A mutant increased atomic-level insight into the enzyme's reaction mechanism. We found PPIP5K2 to be fully reversible as an ATP-synthase in&#x00A0;vitro, but our new data contradict previous perceptions that significant &#x2018;reversibility&#x2019; occurs in&#x00A0;vivo. PPIP5K2 was insensitive to physiological changes in either [AMP] or [ATP]/[ADP] ratios. Those data, together with adenine nucleotide kinetics (ATP Km=20&#x2013;40&#x00A0;&#x03BC;M), reveal how insulated PPIP5K2 is from cellular bioenergetic challenges. Finally, the specificity constants for PPIP5K2 revise upwards by one-to-two orders of magnitude the inherent catalytic activities of this enzyme, and we show its equilibrium point favours 80&#x2013;90% depletion of InsP6/5-InsP7

bioscience reports

The kinetic properties of a human PPIP5K reveal that its kinase activities are protected against the consequences of a deteriorating cellular bioenergetic environment