ISSN 2415-3060 (print), ISSN 2522-4972 (online)
  • 49 of 60
УЖМБС 2019, 4(6): 329–343

Spin–catalysis of Unsaturated Substrates Oxidation by Cofactor–free Mono– and Di–oxygenases. How Triplet Oxygen Can Overcome Spin Prohibition

Minaev B. F., Panchenko A. A.

Oxygen from the air is O2 molecule in the ground triplet state, which can not react because of spin prohibition with organic substances. The late always have even number of electrons with paired spins and are diamagnetic (they have total spin equal to zero and the singlet ground state). That is why reactions of organic substances with oxygen are spin forbidden. Oxidases and oxygenases can activate the triplet O2 molecule which is in the ground state for the controlled chemical synthesis initiated by electron transfer, which leads to oxygen reduction and formation of superoxide ion–radical. As a rule, these enzymes use organic cofactor or paramagnetic metal ion in order to initiate oxygen activation and to overcome spin prohibition for the triplet O2 reactions. A number of mono– and di–oxygenases are found recently which can operate even in the absence of any cofactor. Analysis of spin–orbit coupling in such enzymes assists to unravel some puzzles of reaction mechanisms and to explain why aromatic substrate operates instead cofactor. In the present work we have shown that the common attribute of all such enzymes – superoxide anion radical О2–• – implements a good opportunity to induce the triplet–singlet quantum transition in the active center of oxygenase due to internal magnetic perturbations in the О2–• species and provides a spin flip being a common trick in order to overcome spin prohibition for biochemical oxygen activation. Driving force for such spin flip is spin–orbit coupling inside the superoxide ion radical, which is independent of the organic partner in the radical pair М+•… О2–• (where М – is a cofactor or substrate). At the same time the π–delocalized electronic shell of substrate is able to give up an electron to oxygen and to work instead of cofactor like flavin or pterin. Thus, the substrate with a long π–delocalized conjugation chain can successfully imitate typical cofactor; its main destination is to provide electron transfer to oxygen. We showed that neither cofactor nor substrate contributed into spin–orbit coupling matrix element which governed the triplet–singlet transition rate constant in the radical pair М+•… О2–• of the enzyme active center. In the biochemistry of oxidative enzymes the Vincent Massey’s model for spin conversion was widely used for radical pairs, where M was reduced flavin in solvent. The driving force for the spin conversion has never been discussed in the model of Massey. Our theory of spin–orbit coupling in superoxide ion provides the first well–grounded physical mechanism for spin flip in oxidative enzymes. Understanding of driving force for triplet–singlet transition in oxygen activation by oxygenases and oxidases opens new perspectives in medicine and practical biology.

Keywords: oxygen activation, glucose oxidase, monooxygenases, cofactor–free dioxgenases, electron spin, superoxide anion radical, spin–orbital interaction, biology

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