A Solanum tuberosum inositol phosphate kinase (StITPK1) displaying inositol phosphate–inositol phosphate and inositol phosphate–ADP phosphotransferase activities

We describe a multifunctional inositol polyphosphate kinase/phosphotransferase from Solanum tuberosum, StITPKα (GenBank accession number: EF362784), hereafter called StITPK1. StITPK1 displays inositol 3,4,5,6-tetrakisphosphate 1-kinase activity: Km = 27 μM, and Vmax = 19 nmol min−1 mg−1. The enzyme displays inositol 1,3,4,5,6-pentakisphosphate 1-phosphatase activity in the absence of a nucleotide acceptor and inositol 1,3,4,5,6-pentakisphosphate–ADP phosphotransferase activity in the presence of physiological concentrations of ADP. Additionally, StITPK1 shows inositol phosphate-inositol phosphate phosphotransferase activity. Homology modelling provides a structural rationale of the catalytic abilities of StITPK1. Inter-substrate transfer of phosphate groups between inositol phosphates is an evolutionarily conserved function of enzymes of this class.