Variant G6PD Properties

Almost all G6PD variants are caused by point mutations  in the gene for G6PD. Some mutations do not affect enzymic activity. However, other mutations result in decreased catalytic activity, decreased stability, or an alteration of binding affinity for NADP+ or glucose 6-phosphate. [Note: Active G6PD exists as a homodimer or tetramer. Mutations at the interface between subunits can affect stability.] The severity of the disease usually correlates with the amount of residual enzyme activity in the patient’s RBC.
For example, variants can be classified as shown in Figure 1. G6PD A– is the prototype of the moderate (class III) form of the disease. The RBC contain an unstable but kinetically normal G6PD, with most of the enzyme activity present in the reticulocytes and younger RBC (Fig. 2).
Therefore, the oldest RBC have the lowest level of enzyme activity and are preferentially removed in a hemolytic episode. Because hemolysis does not affect younger cells, the episodes are self-limiting. G6PD Mediterranean is the prototype of a more severe (class II) deficiency. Class I mutations (rare) are the most severe and are associated with chronic nonspherocytic hemolytic anemia, which occurs even in the absence of oxidative stress.

Figure 1:  Classification of glucose 6-phosphate dehydrogenase (G6PD) deficiency variants. [Note: Class V variants (not shown) result in overproduction of G6PD.] * = most common.

Figure 2:  Decline of erythrocyte glucose 6-phosphate dehydrogenase (G6PD) activity with cell age for the three most commonly encountered forms of the enzyme.