Song 2011 Free Radical Biol Med

Song Y, Buettner GR (2011) Thermodynamic and kinetic considerations for the reaction of semiquinone radicals to form superoxide and hydrogen peroxide. Free Radical Biol Med 919-62.

» PMID: 20493944 Open Access

Song Yang, Buettner Garry R (2011) Free Radical Biol Med

Abstract: The quinone/semiquinone/hydroquinone triad (Q/SQ^•−/H₂Q) represents a class of compounds that has great importance in a wide range of biological processes. The half-cell reduction potentials of these redox couples in aqueous solutions at neutral pH, E°′, provide a window to understanding the thermodynamic and kinetic characteristics of this triad and their associated chemistry and biochemistry in vivo. Substituents on the quinone ring can significantly influence the electron density “on the ring” and thus modify E°′ dramatically. E°′ of the quinone governs the reaction of semiquinone with dioxygen to form superoxide. At near-neutral pH the pK_a's of the hydroquinone are outstanding indicators of the electron density in the aromatic ring of the members of these triads (electrophilicity) and thus are excellent tools to predict half-cell reduction potentials for both the one-electron and two-electron couples, which in turn allow estimates of rate constants for the reactions of these triads. For example, the higher the pK_a's of H₂Q, the lower the reduction potentials and the higher the rate constants for the reaction of SQ^•− with dioxygen to form superoxide. However, hydroquinone autoxidation is controlled by the concentration of di-ionized hydroquinone; thus, the lower the pK_a's the less stable H₂Q to autoxidation. Catalysts, e.g., metals and quinone, can accelerate oxidation processes; by removing superoxide and increasing the rate of formation of quinone, superoxide dismutase can accelerate oxidation of hydroquinones and thereby increase the flux of hydrogen peroxide. The principal reactions of quinones are with nucleophiles via Michael addition, for example, with thiols and amines. The rate constants for these addition reactions are also related to E°′. Thus, pK_a's of a hydroquinone and E°′ are central to the chemistry of these triads. • Keywords: Semiquinone, Free radical, Reduction potentials, Thermodynamics • Bioblast editor: Komlodi T

Cited by

• Komlódi T, Cardoso LHD, Doerrier C, Moore AL, Rich PR, Gnaiger E (2021) Coupling and pathway control of coenzyme Q redox state and respiration in isolated mitochondria. Bioenerg Commun 2021.3. https://doi.org/10.26124/bec:2021-0003

Labels:

Stress:Oxidative stress;RONS 

Regulation: Q-junction effect 

HRR: Theory 

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