Both mutations in G6PD A- are necessary to produce the G6PD deficient phenotype.

The high prevalence of glucose 6-phosphate dehydrogenase (G6PD) deficiency in African populations is due almost entirely to the enzyme variant A-, which differs from the wild-type G6PD B by two amino acid replacements, 68 Val-- > Met and 126 Asn-- > Asp.
The non-deficient polymorphic variant G6PD A contains only the mutation 126 Asn-- > Asp.
The frequencies of the G6PD A and of the G6PD A- genes in parts of Africa are both about 0. 2. The 68 Val-- > Met mutation has not been found in a B background.
This could be because the 68 Val-- > Met mutation happened to arise in an A gene in the first instance, or because the 68 Val-- > Met mutation alone is not sufficient to cause G6PD deficiency.
We have approached this question by producing G6PD B, A, A-, and G6PD 68 Val-- > Met in a bacterial expression system and analysing their biochemical properties.
With each single mutation we found a slight decrease in both the specific activity and the yield of enzyme when compared to G6PD B. When both mutations were introduced together, there was a roughly additive effect on specific activity, but a much more drastic effect on enzyme yield (4% of normal). This synergistic effect was also demonstrated on thermal stability, especially at low NADP concentrations.
Comparable results were produced when the replacement 119 Gln-- > Glu was studied instead of 126 Asn-- > Asp.
We infer that the coexistence of the two mutations is responsible for enzyme deficiency in G6PD A- because they act synergistically in causing instability of the enzyme.

