Human glucose-6-phosphate dehydrogenase: the crystal structure reveals a structural NADP(+) molecule and provides insights into enzyme deficiency.
BACKGROUND
Glucose-6-phosphate dehydrogenase (G6PD) catalyses the first committed step in the pentose phosphate pathway; the generation of NADPH by this enzyme is essential for protection against oxidative stress. The human enzyme is in a dimer<-->tetramer equilibrium and its stability is dependent on NADP(+) concentration. G6PD deficiency results from many different point mutations in the X-linked gene encoding G6PD and is the most common human enzymopathy. Severe deficiency causes chronic non-spherocytic haemolytic anaemia; the usual symptoms are neonatal jaundice, favism and haemolytic anaemia.


RESULTS
We have determined the first crystal structure of a human G6PD (the mutant Canton, Arg459-->Leu) at 3 A resolution. The tetramer is a dimer of dimers. Despite very similar dimer topology, there are two major differences from G6PD of Leuconostoc mesenteroides: a structural NADP(+) molecule, close to the dimer interface but integral to the subunit, is visible in all subunits of the human enzyme; and an intrasubunit disulphide bond tethers the otherwise disordered N-terminal segment. The few dimer-dimer contacts making the tetramer are charge-charge interactions.


CONCLUSIONS
The importance of NADP(+) for stability is explained by the structural NADP(+) site, which is not conserved in prokaryotes. The structure shows that point mutations causing severe deficiency predominate close to the structural NADP(+) and the dimer interface, primarily affecting the stability of the molecule. They also indicate that a stable dimer is essential to retain activity in vivo. As there is an absolute requirement for some G6PD activity, residues essential for coenzyme or substrate binding are rarely modified.
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Laboratory of Molecular Biophysics, Department of Biochemistry, University of Oxford, The University of Hong Kong, Department of Biochemistry, Oxford, OX1 3QU, UK, Hong Kong.
Gover
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Lam
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Adams
M J
MJ
D000595:Amino Acid Sequence; D018360:Crystallography, X-Ray; D004220:Disulfides / Q000737:chemistry; D005954:Glucosephosphate Dehydrogenase / Q000737:chemistry* / Q000235:genetics; D005955:Glucosephosphate Dehydrogenase Deficiency / Q000201:enzymology*; D006801:Humans; D008958:Models, Molecular; D008969:Molecular Sequence Data; D009154:Mutation; D009249:NADP / Q000737:chemistry*; D011487:Protein Conformation; D017386:Sequence Homology, Amino Acid
D004220:Disulfides; D009249:NADP; D005954:Glucosephosphate Dehydrogenase