Synthesis and hydrolysis of ATP and the phosphate-ATP exchange reaction in soluble mitochondrial F1 in the presence of dimethylsulfoxide

In medium containing 40% dimethylsulfoxide, soluble F1 catalyzes the hydrolysis of ATP introduced at concentrations lower than that of the enzyme [Al-Shawi, M.K. & Senior, A.E. (1992), Biochemistry 31, 886-891]. At this concentration of dimethylsulfoxide, soluble F1 also catalyzes the spontaneous synthesis of a tightly bound ATP to a level of approximately 0.15 mol per mol F1 [Gómez-Puyou, A., Tuena de Gómez-Puyou, M. & de Meis, L. (1986) Eur. J. Biochem. 159, 133-140]. The mechanisms that allow soluble F1 to carry out these apparently opposing reactions were studied. The rate of hydrolysis of ATP bound to F1 under uni-site conditions and that of synthesis of ATP were markedly similar, indicating that the two ATP molecules lie in equivalent high affinity catalytic sites. The number of enzyme molecules that have ATP at the high affinity catalytic site under conditions of synthesis or uni-site hydrolysis is less than the total number of enzyme molecules. Therefore, it was hypothesized that when the enzyme was treated with dimethylsulfoxide, a fraction of the F1 population carried out synthesis and another hydrolysis. Indeed, measurements of the two reactions under identical conditions showed that different fractions of the F1 population carried out simultaneously synthesis and hydrolysis of ATP. The reactions continued until an equilibrium level between F1.ADP + Pi <--> F1.ATP was established. At equilibrium, about 15% of the enzyme population was in the form F1.ATP. The DeltaG degrees of the reaction with 0.54 microM F1, 2 mM Pi and 10 mM Mg2+ at pH 6.8 was -2.7 kcal.mol-1 in favor of F1.ATP. The DeltaG degrees of the reaction did not exhibit important variations with Pi concentration; thus, the reaction was in thermodynamic equilibrium. In contrast, DeltaG degrees became significantly less negative as the concentration of dimethylsulfoxide was decreased. In water, the reaction was far to the left. The equilibrium constant of the reaction diminished linearly with an increase in water activity. The effect of solvent is fully reversible. In comparison to other enzymes, F1 seems unique in that solvent controls the equilibrium that exists within an enzyme population. This results from the effect of solvent on the partition of Pi between the catalytic site and the medium, and the large energetic barrier that prevents release of ATP from the catalytic site. In the presence of dimethylsulfoxide and Pi, ATP is continuously hydrolyzed and synthesized with formation and uptake of Pi from the medium. This process is essentially an exchange reaction analogous to the phosphate-ATP exchange reaction that is catalyzed by the ATP synthase in coupled energy transducing membranes.

Nucleotide binding drives conformational changes in the isolated alpha and beta subunits of the F(1)-ATPase from Escherichia coli

The modeling of the rotatory mechanism performed by the F(1)-ATPase complex during ATP synthesis shows that the beta, but not the alpha subunit, undergoes large conformational changes that depend on the occupancy of the catalytic site. Here we determined by fluorescence spectroscopy the changes in tertiary structure and hydrophobic exposed area of the isolated alpha and beta subunits of the F(1)-ATPase complex from Escherichia coli upon adenine nucleotide binding. The results show that in the absence of intersubunit contacts, the two subunits exhibit markedly similar conformational movements.

Digital tomosynthesis improves chest radiograph accuracy and reduces microbiological false negatives in COVID-19 diagnosis

PURPOSE

Diagnosing pneumonia by radiograph is improvable. We aimed (a) to compare radiograph and digital thoracic tomosynthesis (DTT) performances and agreement for COVID-19 pneumonia diagnosis, and (b) to assess the DTT ability for COVID-19 diagnosis when polymerase chain reaction (PCR) and radiograph are negative.

METHODS

Two emergency radiologists with 11 (ER1) and 14 experience-years (ER2) retrospectively evaluated radiograph and DTT images acquired simultaneously in consecutively clinically suspected COVID-19 pneumonia patients in March 2020-January 2021. Considering PCR and/or serology as reference standard, DTT and radiograph diagnostic performance and interobserver agreement, and DTT contributions in unequivocal, equivocal, and absent radiograph opacities were analysed by the area under the curve (AUC), Cohen's Kappa, Mc-Nemar's and Wilcoxon tests.

RESULTS

We recruited 480 patients (49 ± 15 years, 277 female). DTT increased ER1 (from 0.76, CI95% 0.7-0.8 to 0.79, CI95% 0.7-0.8; P=.04) and ER2 (from 0.77 CI95% 0.7-0.8 to 0.80 CI95% 0.8-0.8, P=.02) radiograph-AUCs, sensitivity, specificity, predictive values, and positive likelihood ratio. In false negative microbiological cases, DTT suggested COVID-19 pneumonia in 13% (4/30; P=.052, ER1) and 20% (6/30; P=.020, ER2) more than radiograph. DTT showed new or larger opacities in 33-47% of cases with unequivocal opacities in radiograph, new opacities in 2-6% of normal radiographs and reduced equivocal opacities by 13-16%. Kappa increased from 0.64 (CI95% 0.6-0.8) to 0.7 (CI95% 0.7-0.8) for COVID-19 pneumonia probability, and from 0.69 (CI95% 0.6-0.7) to 0.76 (CI95% 0.7-0.8) for pneumonic extension.

CONCLUSION

DTT improves radiograph performance and agreement for COVID-19 pneumonia diagnosis and reduces PCR false negatives.