The phosphotransferase activity of ITPK1. The graphic illustrates the proposed enzymatic reactions by which the 1-phosphate on Ins(1,3,4,5,6)P5 (coloured red) is transferred to Ins(1,3,4)P3. The evidence for this reaction pathway came from HPLC analysis of the reaction products following the metabolism of [1-32P]-Ins(1,3,4,5,6)P5 by ITPK1 (Chamberlain et al., 2007). It has not yet been established whether or not a phosphoryl-enzyme (E–P) intermediate is involved, but this is a likely possibility.