HGM2002 Poster Abstracts: 8. Disease Mechanisms
POSTER NO: 428
Post-genomic Glucose-6-phosphate dehydrogenase (G6PD): different mechanisms for enzyme deficiency from structural and kinetic analyses
1Veronica M.S. Lam, 1Xiao Tao Wang, 1Yu Xiang Huang, 1Colin J. Kwok, 2Paul C. Engel
Hereditary deficiency in human glucose-6-phosphate dehydrogenase (G6PD) affects about 400 million people world-wide, including about 5% of males in Hong Kong and South China. The most common clinical manifestations of G6PD deficiency are neonatal jaundice and acute hemolytic anaemia.
Steady state kinetic analysis of some Class II mutants i.e., G6PD Canton (Arg 459 to Leu) Union (Arg 454 to Cys) and Andalus (Arg 454 to His) showed that mutations which disrupt hydrogen bonding between helices (3D details) result in an increase in the affinity for both substrates, NADP+ and G6P. However, all three mutant enzymes have lower enzyme turnovers (kcat) i.e., a decrease in catalytic efficiency.
Results from the use of chaperons and antibodies suggest that in G6PD Plymouth (Gly 163 to Asp) and G6PD Mahidol (Gly 163 to Ser) the protein cannot fold properly during synthesis. The extent of steric hindrance caused by the substituted amino acid residues in Plymouth and Mahidol correlates inversely with the amount of properly folded G6PD obtained, which in turn correlates with the degree of enzyme deficiency. There is no significant difference in the kinetic parameters of these mutant enzymes, when folded properly, compared with the wild type enzyme.
The first crystal structure of a human G6PD (the mutant Canton, Arg 459 to Leu) showed that a molecule of bound NADP+ and the integrity of the dimer interface are important for enzyme stability. The 3D structure provides insights into the mechanisms of enzyme deficiency, particularly those of the most severe, Class I mutants associated with chronic non-spherocytic hemolytic anemia (CNSHA) e.g., G6PD Volendam (Pro172 to Ser), Shinagawa (Gly 410 to Asp), Campinas (Gly 488 to Val) and others. A web-accessible relational database of human G6PD mutations ( http://www.bioinf.org.uk/g6pd/ ) has been created with a recently developed computational procedure to identify mutations which distort secondary structure, destroy hydrogen bonding, electrostatic interaction or simply cause bad clashes. This database facilitates understanding of the structure and function relationship of this enzyme and provides a means to summit new/additional mutations.
Using a combination of biochemical and computational approaches we have shown that different mechanisms could be the underlying cause of G6PD deficiency.
Other abstracts in same session