Fig. 1.

The structures of Ins(3,4,5,6)P₄ and Ins(1,3,4,5,6)P₅. The myo-inositol building block is a ring structure made from six carbon groups, each of which has a free hydroxyl. The hydroxyl attached to the 2-carbon is axial (perpendicular) to the plane of the ring, and the remaining hydroxyls are equatorial (i.e. approximately in the same plane as the ring). The carbons are numbered in an anticlockwise direction when the ring is viewed from above. Substitution of four of the hydroxyls with phosphates at positions 3, 4, 5 and 6 produces inositol 3,4,5,6-tetrakisphosphate. The standard abbreviation for this polyphosphate – Ins(3,4,5,6)P₄ – therefore reflects the recognition by inositol phosphate nomenclature of the number of phosphate groups (denoted by the subscript), as well as their positions around the inositol ring.