Enantiomers.
This is another source of confusion,where the turtle analogy can also help.A turtle has a plane of symmetry running through its head and tail, so distinguishing a left-flippered from a right-flippered turtle will require using a technique that can discriminate D and L configurations (that is, a chiral analysis or separation method). For example, two prominent InsP4 isomers discussed in this review are inositol-3,4,5,6-tetrakisphosphate (Ins(3,4,5,6)P4) and Ins(1,4,5,6)P4,which both have an unphosphorylated hydroxyl in the 2-position (the head), so they differ only in having vacant either the 1- or the 3- positions (the right versus the left front flipper). They are therefore an enantiomeric pair, because they can be converted one to the other by reflection in the plane of symmetry. The standard separation techniques used in inositol phosphate analyses cannot distinguish between enantiomers, so these two InsP4s co-chromatograph exactly.Quantifying Ins(3,4,5,6)P4 and Ins(1,4,5,6)P4 separately is possible only using enantiomer-specific enzyme-based analyses. Note that a left-flippered (L) turtle would obviously regard its left front flipper as number 1, and so because its head is still number 2, the limbs will be numbered clockwise starting at the front left flipper, which is why the alternative name for Ins(3,4,5,6)P4 is L-Ins(1,4,5,6)P4. In biological journals, and in this review, the D numbering is now used universally, but this is not so in chemical journals,where L numbering of the inositol ring is frequently found. This presents no problem because it is always made clear in chemical journals which numbering is being used.However, there is one aspect of nomenclature that could cause confusion, and this is that in the 1970s the official designations of the D and L numberings of the inositol ring were actually swapped over, and so in older papers, what you think should be called D will actually be called L and vice versa.

Nomenclature, structure and enantiomers

Nomenclature.
Inositol phosphates contain only inositol and phosphate, and are therefore water-soluble. Inositol lipids also contain inositol and phosphate, but in addition they have a hydrophobic component (usually a diacylglycerol moiety — two fatty acids esterified onto glycerol,which is in turn attached through a diester phosphate to the 1-hydroxyl of the inositol ring), and are therefore not water-soluble.

Numbering.
Much of the confusion that surrounds inositol phosphate nomenclature can be circumvented by using Bernie Agranoff’s turtle. a shows myo-inositol as a Haworth projection and b shows a more accurate repesentation of the ‘chair’ structure, probably the most thermodynamically stable conformation of myo-inositol in vivo. The 2-hydroxyl is axial, and the other five hydroxyls are equatorial (that is, they are more-or-less in the plane of the ring).Agranoff noted that this structure, if one ignores the hydrogens, superficially resembles a turtle (c).Modern biochemical nomenclature exclusively uses the D-numbering system, so only two facts have to be remembered: first, the turtle is right-flippered, so its number 1 flipper is the front right flipper; and second, the turtle’s head is the 2-hydroxyl.The anticlockwise numbering of the rest of the appendages (viewed from on top of the turtle) then follows logically (c). Mathematically, there are 63 possible inositol monophosphates, a potential that can be expanded further by attaching pyrophosphate moieties instead of monophosphates (see main text, section on InsP7 and InsP8). FIG. 1 illustrates most of those that have so far been shown to occur in cells,with their known metabolic routes of synthesis and degradation.