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.
Town
M
M
Department of Haematology, Royal Postgraduate Medical School, Hammersmith Hospital, London, UK.
Bautista
J M
JM
Mason
P J
PJ
Luzzatto
L
L
D000483:Alleles; D000595:Amino Acid Sequence; D001483:Base Sequence; D004247:DNA / Q000235:genetics; D004795:Enzyme Stability; D004926:Escherichia coli / Q000235:genetics; D005954:Glucosephosphate Dehydrogenase / Q000235:genetics*; D005955:Glucosephosphate Dehydrogenase Deficiency / Q000201:enzymology / Q000235:genetics*; D006801:Humans; D008969:Molecular Sequence Data; D016297:Mutagenesis, Site-Directed; D010641:Phenotype; D017354:Point Mutation; D011110:Polymorphism, Genetic
D004247:DNA; D005954:Glucosephosphate Dehydrogenase