Novel Structures of Type 1 Glyceraldehyde-3-phosphate Dehydrogenase from Escherichia coli Provide New Insights into the Mechanism of Generation of 1,3-Bisphosphoglyceric Acid

Glyceraldehyde-3-phosphate dehydrogenase (GAPDH) is a highly conserved enzyme involved in the ubiquitous process of glycolysis and presents a loop (residues 208–215 of Escherichia coli GAPDH) in two alternative conformations (I and II). It is uncertain what triggers this loop rearrangement, as well as which is the precise site from which phosphate attacks the thioacyl intermediate precursor of 1,3-bisphosphoglycerate (BPG). To clarify these uncertainties, we determined the crystal structures of complexes of wild-type GAPDH (WT) with NAD and phosphate or G3P, and of essentially inactive GAPDH mutants (C150S, H177A), trapping crystal structures for the thioacyl intermediate or for ternary complexes with NAD and either phosphate, BPG, or G3P. Analysis of these structures reported here lead us to propose that phosphate is located in the “new Pi site” attacks the thioester bond of the thioacyl intermediate to generate 1,3-bisphosphoglyceric acid (BPG). In the structure of the thioacyl intermediate, the mobile loop is in conformation II in subunits O, P, and R, while both conformations coexist in subunit Q. Moreover, only the Q subunit hosts bound NADH. In the R subunit, only the pyrophosphate part of NADH is well defined, and NADH is totally absent from the O and P subunits. Thus, the change in loop conformation appears to occur after NADH is produced, before NADH is released. In addition, two new D-glyceraldehyde-3-phosphate (G3P) binding forms are observed in WT.NAD.G3P and C150A+H177A.NAD.G3P. In summary, this paper improves our understanding of the GAPDH catalytic mechanism, particularly regarding BPG formation.

Severe adverse events following benzathine penicillin G injection for rheumatic heart disease prophylaxis: cardiac compromise more likely than anaphylaxis

Objective

Secondary prophylaxis through long-term antibiotic administration is essential to prevent the progression of acute rheumatic fever to rheumatic heart disease (RHD). Benzathine penicillin G (BPG) has been shown to be the most efficacious antibiotic for this purpose; however, adverse events associated with BPG administration have been anecdotally reported. This study therefore aimed to collate case reports of adverse events associated with BPG administration for RHD prophylaxis.

Study design

A literature review was used to explore reported adverse reactions to BPG and inform development of a case report questionnaire. This questionnaire was circulated through professional networks to solicit retrospective reports of adverse events from treating physicians. Returned surveys were tabulated and thematically analysed. Reactions were assessed using the Brighton Collaboration case definition to identity potential anaphylaxis.

Results

We obtained 10 case reports from various locations, with patients ranging in age from early-teens to adults. All patients had clinical or echocardiogram-obtained evidence of valvular disease. The majority of patients (80%) had received BPG prior to the event with no previous adverse reaction. In eight cases, the reaction was fatal; in one case resuscitation was successful and in one case treatment was not required. Only three cases met Level 1 Brighton criteria consistent with anaphylaxis.

Conclusion

These results indicate that anaphylaxis is not a major cause of adverse reactions to BPG. An alternative mechanism for sudden death following BPG administration in people with severe RHD is proposed.

How does hemoglobin generate such diverse functionality of physiological relevance?

The absolute values of the O2-affinities (P50, Klow, and Khigh) of hemoglobin (Hb) are regulated neither by changes in the static T-/R-quaternary and associated tertiary structures nor the ligation states. They are pre-determined and regulated by the extrinsic environmental factors such as pH, buffers, and heterotropic effectors. The effect and role of O2 on Hb are reversibly to drive the structural allosteric equilibrium between the T(deoxy)- and R(oxy)-Hb toward R(oxy)-Hb (the structural allostery). R(oxy)-Hb has a higher O2-affinity (Khigh) relative to that (Klow) of the T(deoxy)-Hb (Khigh>Klow) under any fixed environmental conditions. The apparent O2-affinity of Hb is high, as the globin matrix interferes with the dissociation process of O2, forcing the dissociated O2 geminately to re-bind to the heme Fe. This artificially increases [oxy-Hb] and concomitantly decreases [deoxy-Hb], leading to the apparent increases of the O2-affinity of Hb. The effector-linked high-frequency thermal fluctuations of the globin matrix act as a gating mechanism to modulate such physical, energetic, and kinetic barriers to enhance the dissociation process of O2, resulted in increases in [deoxy-Hb] and concomitant decrease in [oxy-Hb], leading to apparent reductions of the O2-affinity of Hb (the entropic allostery). The heme in Hb is simply a low-affinity O2-trap, the coordination structure of which is not altered by static T-/R-quaternary and associated tertiary structural changes of Hb. Thus, heterotrophic effectors are the signal molecule, which acts as a functional link between these two allosteries and generates the diverse functionality of Hb of physiological relevance. This article is part of a Special Issue entitled: Oxygen Binding and Sensing Proteins.