GO:0000832:   inositol hexakisphosphate 5-kinase activity
Catalysis of the reaction: ATP + 1D-myo-inositol 1,2,3,4,5,6-hexakisphosphate = ADP + 5-diphospho-1D-myo-inositol (1,2,3,4,6)pentakisphosphate.