Structural Defects Underlying Protein Dysfunction in Human Glucose-6-phosphate Dehydrogenase A− Deficiency

Abstract

The enzyme variant glucose-6-phosphate dehydrogenase (G6PD) A ؊ , which gives rise to human glucose-6-phosphate dehydrogenase deficiency, is a protein of markedly reduced structural stability. This variant differs from the normal enzyme, G6PD B, in two amino acid substitutions. A further nondeficient variant, G6PD A, bears only one of these two mutations and is structurally stable. In this study, the synergistic structural defect in recombinant G6PD A ؊ was reflected by reduced unfolding enthalpy due to loss of ␤-sheet and ␣-helix interactions where both mutations are found. This was accompanied by changes in inner spatial distances between residues in the coenzyme domain and the partial disruption of tertiary structure with no significant loss of secondary structure. However, the secondary structure of G6PD A ؊ was qualitatively affected by an increase in ␤-sheets substituting ␤-turns related to the lower unfolding enthalpy. The structural changes observed did not affect the active site of the mutant proteins, since its spatial position was unmodified. The final result is a loss of folding determinants leading to a protein with decreased intracellular stability. This is suggested as the cause of the enzyme deficiency in the red blood cell, which is unable to perform de novo protein synthesis.

Updates on Glucose 6-Phosphate Dehydrogenase (G6PD): From Prokaryotes to Human

“…The side chain of Lys-148 is less tightly bound in subunit A than in subunit B as a result of the different conformations of the two proline residues (A149 and B149) in the two subunits. This leads to a tighter binding of phosphate in subunit B since only one phosphate binds in subunit B; although limited knowledge is provided on this [104,105]. A theoretical reason might be that the specificity of the site of phosphate is greater if Pro-149 exist in a Trans in subunit B (Figure 3).…”

“…Some of the clinical manifestations of G6PD include neonatal jaundice, favism, drug-induced haemolytic anaemia, infection-induced haemolytic anaemia and chronic nonspherocytic haemolytic anaemia [104]. Sometimes, heart attack and diabetes could also induce haemolysis in individuals who are G6PD deficient [14].…”

“…[125]. G6PD deficiency creates the formation of different proteins with varying levels of enzyme activity [8], with clinical manifestations such as neonatal Licensed Under Creative Commons Attribution CC BY jaundice (NNJ), acute haemolytic anaemia, chronic haemolysis which leads to chronic nonspherocytic haemolytic anaemia (CNSHA) [104]. These clinical manifestations are usually triggered by external factors such as drugs and fava beans [133].…”

“…Associated with the clinical manifestations are tiredness, anaemia, back ache and jaundice. Other markers include lactate dehydrogenase and unconjugated bilirubin [104].…”

“…The first is the catalytic site and the second is the structural site. However, NADP + which is required to aid folding and monomer hybridization is bound at the structural site and not involved in the catalysis of the enzyme [16,103,104]. The auxiliary cofactor helps to maintain the stability and integrity of the enzyme when it is in its active form [105].…”