Phosphoglycerate kinase (PGK) is found to be controlled by a 25 Mg 2؉ -related magnetic isotope effect. Mg 2؉ nuclear spin selectivity manifests itself in PGK-directed ADP phosphorylation, which has been clearly proven by comparison of ATP synthesis rates estimated in reaction mixtures with different Mg isotopy parameters. Both pure 25 Mg 2؉ (nuclear spin 5͞2, magnetic moment ؉0.85) and 24 Mg 2؉ (spinless, nonmagnetic nucleus) species as well as their mixtures were used in experiments. In the presence of 25 Mg 2؉ , ATP production is 2.6 times higher compared with the yield of ATP reached in 24 Mg 2؉ -containing PGK-based catalytic systems. The chemical mechanism of this phenomenon is discussed. A key element of the mechanism proposed is a nonradical pair formation in which 25 Mg ؉ radical cation and phosphate oxyradical are involved. 25 Mg In one of their brilliant papers, Weber and Senior (1) pointed out that, despite great progress in our knowledge on the structure and our understanding of the molecular dynamics and functioning of ATP-synthesizing enzymes, the chemical mechanism of phosphorylation remains enigmatic: ''Our understanding of ATP synthesis remains rudimentary in molecular terms.'' Thus, our current understanding of ATP-synthesizing enzymes is still rudimentary at the molecular level and the key reaction responsible for the formation of the energy-carrying chemical bond POOOP remains obscure. Indeed, the structures of the protein parts and catalytic sites of phosphorylating enzymes, such as ATP synthase, creatine kinase, and phosphoglycerate kinase (PGK) (2-10), are well known. There is an understanding of their functioning as molecular rotary motors (referring to ATP synthase) or molecular pumps (creatine kinase, for instance); however, within the area of enzymatic reaction chemistry, all ideas are limited to speculations circulating mostly around nucleophilic mechanisms.An insight into the chemical mechanism follows from a recently discovered and remarkable phenomenon: a dependence of the phosphorylating activity of enzymes on Mg isotopy (11,12). This unusual effect was found for creatine kinase and ATP synthase (13). The rate of ATP production by enzymes in which the Mg 2ϩ ion has magnetic nucleus 25 Mg (nuclear spin, 5͞2; magnetic moment, Ϫ0.855 Bohr magneton) was shown to be two to three times higher than that induced by the same enzymes carrying spinless, nonmagnetic nuclei 24 Mg and 26 Mg. The discovery of this attention-catching isotope effect convincingly and unequivocally demonstrates that enzymatic phosphorylation is an ion-radical, electron-spin-selective process in which Mg ion Mg 2ϩ manifests itself as a reagent.The present study deals with our search for other examples of nuclear spin selectivity in biological processes. PGK (EC 2.7.11.08), a typical two-domain enzyme catalyzing the transfer of a phosphate group from ␣-phosphoglycerate to ADP, which leads to ATP production in eukaryotic cells (14), has been chosen for this purpose. Like creatine kinase, PGK contains Mg 2ϩ in its nu...
We have experimentally demonstrated a material-independent mirror for atomic waves that uses the Fresnel diffraction at an array of parallel ridges. He* (2 3 S 1 ) and Ne* (1s 3 ) atomic waves were reflected coherently on a silicon plate with a microfabricated grating structure, consisting of narrow wall-like ridges. We measured the reflectivity at grazing incidence as a function of the incident velocity and angle. Our data show that the reflectivity on this type of mirror depends only on the distance between the ridges, the wavelength, and the incident angle, but is insensitive to the material of the grating structure. The reflectivity is observed to increase by 2 orders of magnitude, compared to that of a flat polished silicon surface, where the reflection is caused by the attractive surface potential. For He* atoms, the measured reflectivity exceeds 10% for normal incident velocities below about 25 cm=s.
The inhibition of enzymatic activity of the ATP-synthesizing creatine kinase by CH 3 HgCl is shown to strongly depend on the mercury isotope substitution. The efficiency of inhibition does not depend on the nuclear mass of the mercury isotopes; however, it is different for magnetic ( 199 Hg, 201 Hg) and nonmagnetic ( 200 Hg, 202 Hg) nuclei. When mercury isotopes in CH 3 HgCl are presented in natural abundance, the reaction of creatine kinase with CH 3 HgCl fractionates magnetic and nonmagnetic mercury isotopes. These observations demonstrate that the reaction between creatine kinase and CH 3 HgCl is nuclear spin selective; that is, isotope fractionation is induced by the magnetic isotope effect. According to the suggested reaction scheme, it operates in the ion-radical pairs generated in enzyme active site from CH 3 HgCl and cysteine residue as the reaction partners.
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