English: Chemcial structure of phytate, also known as inositol hexakisphosphate (IP6).

Protonation

The pKa value of the second deprotonation of single-phosphorylated carbohydrate(-like) molecules is somewhere around 6 (see pKa data compiled by Williams). In this case, I'd expect the pKa values of the second deprotonation to increase for each phosphate group, as electrostatic interactions may make double-deprotonations of all phosphates less favorable. At a cellular pH of slightly above 7, one would expect some phosphates to be double-deprotonated and some to be single-deprotonated. To make things less complicated, I decided to show all phosphates in their single-deprotonated form, although that's probably not accurate at cellular pH.