Buchachenko Anatoly L, Kouznetsov Dmitry A, Breslavskaya Natalia N, Orlova Marina A
N. N. Semenov Institute of Chemical Physics, Russian Academy of Sciences, 119991 Moscow, Russia.
J Phys Chem B. 2008 Feb 28;112(8):2548-56. doi: 10.1021/jp710989d. Epub 2008 Feb 5.
Recent discovery of magnesium isotope effect in the rate of enzymatic synthesis of adenosine triphosphate (ATP) offers a new insight into the mechanochemistry of enzymes as the molecular machines. The activity of phosphorylating enzymes (ATP-synthase, phosphocreatine, and phosphoglycerate kinases) in which Mg(2+) ion has a magnetic isotopic nucleus 25Mg was found to be 2-3 times higher than that of enzymes in which Mg(2+) ion has spinless, nonmagnetic isotopic nuclei 24Mg or 26Mg. This isotope effect demonstrates unambiguously that the ATP synthesis is a spin-dependent ion-radical process. The reaction schemes, suggested to explain the effect, imply a reversible electron transfer from the terminal phosphate anion of ADP to Mg(2+) ion as a first step, generating ion-radical pair with singlet and triplet spin states. The yields of ATP along the singlet and triplet channels are controlled by hyperfine coupling of unpaired electron in 25Mg+ ion with magnetic nucleus 25Mg. There is no difference in the ATP yield for enzymes with 24Mg and 26Mg; it gives evidence that in this reaction magnetic isotope effect (MIE) operates rather than classical, mass-dependent one. Similar effects have been also found for the pyruvate kinase. Magnetic field dependence of enzymatic phosphorylation is in agreement with suggested ion-radical mechanism.
最近在三磷酸腺苷（ATP）酶促合成速率中发现镁同位素效应，为作为分子机器的酶的机械化学提供了新的见解。发现Mg(2+)离子具有磁性同位素核25Mg的磷酸化酶（ATP合酶、磷酸肌酸激酶和磷酸甘油酸激酶）的活性比Mg(2+)离子具有无自旋、非磁性同位素核24Mg或26Mg的酶的活性高2至3倍。这种同位素效应明确表明ATP合成是一个自旋依赖性离子自由基过程。为解释该效应而提出的反应方案表明，第一步是从ADP的末端磷酸阴离子向Mg(2+)离子进行可逆电子转移，产生具有单重态和三重态自旋状态的离子自由基对。沿着单重态和三重态通道的ATP产率由25Mg+离子中未配对电子与磁性核25Mg的超精细耦合控制。具有24Mg和26Mg的酶的ATP产率没有差异；这证明在该反应中起作用的是磁同位素效应（MIE），而不是经典的质量依赖性效应。丙酮酸激酶也发现了类似的效应。酶促磷酸化的磁场依赖性与所提出的离子自由基机制一致。
