3-Dimensional structure of the human G6PD protein. Both the dimer and the tetramer are enzymatically active. The dimer/tetramer ratio within different cells or under different conditions is still not known, but it affects the pH-dependence of G6P binding.⁹²  It is still not known what proportion of G6PD, within cells, is in dimer form. (A) The G6PD dimer, showing the binding sites of the 2 substrates G6P (yellow) and NADP (purple), and the site of the “structural” NADP (blue) (from Gómez-Manzo et al⁷⁸ ). (B) The G6PD tetramer, showing the positions of a small selection of G6PD variants. Note (on top) the relatively large in-frame deletion of G6PD Nara: this is compatible with residual G6PD activity because the 8 aa involved are in a nonstructured region. Modified from Mason et al⁹³  with permission.