Structural defects underlying protein dysfunction in human glucose-6-phosphate dehydrogenase A(-) deficiency

The enzyme variant glucose-g-phosphate dehydrogenase (G6PD) A(-), which giv es rise to human glucose-6-phosphate dehydrogenase deficiency, is a protein of markedly reduced structural stability. This variant differs from the no rmal enzyme, G6PD B, in two amino acid substitutions, A further nondeficien t variant, G6PD G bears only one of these two mutations and is structurally stable. In this study, the synergistic structural defect in recombinant G6 PD A(-) was reflected by reduced unfolding enthalpy due to loss of beta-she et and alpha-helix interactions where both mutations are found. This was ac companied by changes in inner spatial distances between residues in the coe nzyme domain and the partial disruption of tertiary structure with no signi ficant loss of secondary structure. However, the secondary structure of G6P D A- was qualitatively affected by an increase in beta-sheets substituting beta-turns related to the lower unfolding enthalpy, The structural changes observed did not affect the active site of the mutant proteins, since its s patial position was unmodified. The final result is a loss of folding deter minants leading to a protein with decreased intracellular stability. This i s suggested as the cause of the enzyme deficiency in the red blood cell, wh ich is unable to perform de novo protein synthesis.