Ion-Radical Mechanism of Enzymatic ATP Synthesis: DFT Calculations and Experimental Control

Abstract

A new, ion-radical mechanism of enzymatic ATP synthesis was recently discovered by using magnesium isotopes. It functions at a high concentration of MgCl(2) and includes electron transfer from the Mg(H(2)O)(m)(2+)(ADP(3-)) complex (m = 0-4) to the Mg(H(2)O)(n)(2+) complex as a primary reaction of ATP synthesis in catalytic sites of ATP synthase and kinases. Here, the structures and electron transfer reaction energies of magnesium complexes related to ATP synthesis are calculated in terms of DFT. ADP is modeled by pyrophosphate anions, protonated (HP(2)O(7)H(2-), HP(2)O(7)CH(3)(2-)) and deprotonated (HP(2)O(7)(3-), CH(3)P(2)O(7)(3-)). The reaction generates an ion-radical pair, composed of Mg(H(2)O)(n)(+) ion and pyrophosphate anion-radical coordinated to Mg(2+) ion. The addition of the latter to the substrate P=O bond results in ATP formation. Populations of the singlet and triplet states and singlet-triplet spin conversion in the pair are controlled by hyperfine coupling of unpaired electrons with magnetic (25)Mg and (31)P nuclei and by Zeeman interaction. Due to these two interactions, the yield of ATP is a function of nuclear magnetic moment and magnetic field; both of these effects were experimentally detected. Electron transfer reaction does not depend on m but strongly depends on n. It is exoergic and energy allowed at 0 < or = n << infinity for the deprotonated pyrophosphate anions and at 0 < or = n < 4 for the protonated ones; for other values of n, the reaction is energy deficient and forbidden. The boundary between exoergic and endoergic regimes corresponds to the trigger magnitude n* (n* = 4 for protonated anions and 6 < n* << infinity for deprotonated ones). These results explain why ATP synthesis occurs only in special devices, molecular enzymatic machines, but not in water (n = infinity). Biomedical consequences of the ion-radical enzymatic ATP synthesis are also discussed.

Reexamination of magnetic isotope and field effects on adenosine triphosphate production by creatine kinase

“…Here ADP 3− in the EAS is coordinated to an Mg 2þ ion forming an MgADP − complex whose oxy-anion can perform an inline nucleophilic attack on the phosphorus of the nearby phosphate-donating substrate (e.g., phosphocreatine or inorganic PO 4 ) leading to formation of ATP (23). BK propose that, when the Mg 2þ concentration rises to 50-100 times the normal intracellular concentration, then two additional Mg 2þ ions enter the EAS, with one binding the PO 4 -donating substrate and the other binding n H 2 O molecules (17). In the EAS where water is "squeezed" out (17), the hydration number n of the free Mg 2þ is low (≤6), and this low hydration number enables its conversion to an Mg ·þ radical ion via electron transfer from the MgADP − complex (17) ( Table S2, In ref.…”

“…BK propose that, when the Mg 2þ concentration rises to 50-100 times the normal intracellular concentration, then two additional Mg 2þ ions enter the EAS, with one binding the PO 4 -donating substrate and the other binding n H 2 O molecules (17). In the EAS where water is "squeezed" out (17), the hydration number n of the free Mg 2þ is low (≤6), and this low hydration number enables its conversion to an Mg ·þ radical ion via electron transfer from the MgADP − complex (17) ( Table S2, In ref. 17, BK performed a series of density functional theory (DFT) energy calculations to justify the electron transfer from MgADP − to Mg 2þ and the formation of the Mg ·þ MgADP · radical pair.…”

“…In a series of papers by Buchachenko and Kuznetsov (BK) together with various co-workers (7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20)(21)(22), a radical pair mechanism has been proposed for enzymatic phosphorylation of adenosine diphosphate (ADP) to adenosine triphosphate (ATP). This mechanism is influenced by the presence of a magnesium isotope with a nuclear spin.…”

“…For instance, with creatine kinase the 2.3-fold increase seen for 25 Mg 2þ without a field rose to a fourfold increase at 80 mT (12,15). According to BK (17), the radical-pair-based mechanism and resulting isotope effect operate only at elevated intracellular Mg concentrations (10.5-24 mM), whereas at normal physiological concentrations (<1 mM) the classical nucleophilic mechanism (23,24) is at work. No effect was found for the reverse reaction (ATP → ADP) (12).…”

“…Indeed, Buchachenko and co-workers have been involved in developing a delivery system on the basis of a porphyrin adduct of C 60 (21), and positive results have been reported in preclinical trials on mammalian heart muscle (22,27). The 10% natural abundance of 25 Mg and the requirement for very high Mg 2þ concentrations should limit the influence of the magnetic isotope effect (MIE) and associated magnetic field effects under normal physiological conditions, but BK assert that effects could be significant because of natural local fluctuations in Mg ion concentrations (17). This assertion implies that there could be ramifications for health and safety if ATP production can really be influenced by modest magnetic fields.…”