As hydrolysis of G3PC has been proposed to proceed via a cyclic intermediate, we wondered if an analogous pathway might provide a rationale for the selective hydrolysis of G2PXs over G3PXs (Fig. and S26-S31). We suggest that hydrolysis of the phosphodiester occurs by intramolecular attack of the glycerol hydroxyl groups to generate G>P, and that subsequent hydrolysis affords G3P and G2P (Fig. ). For a G2PX, two proximal primary hydroxyl groups are available to attack the phosphorous atom to form G>P, while in the case of G3PX only one proximal secondary hydroxyl group is available. This mechanistic proposal would account for the preferential hydrolysis of G2PE over G3PE, and of a G2PX over a G3PX.