BIOCHEMISTRY - Textbook - Ostapchenko L. I. - 2012

Chapter 16. ENERGY METABOLISM

16.1. Biological Oxidation

16.1.2. Oxidation-Reduction Reactions. Redox Potential

Oxidation is defined as the loss of electrons, whereas reduction is the gain of electrons. The oxidation of an electron donor is always coupled with the reduction of an electron acceptor. This fundamental principle of oxidation-reduction processes also applies to biochemical systems. Any redox reaction involves an electron acceptor (oxidizing agent) and an electron donor (reducing agent), for example:

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The overall reaction (1) can be conventionally divided into two half-reactions (2), (3):

Each of these involves the oxidized and reduced forms of a single compound, which are referred to as a conjugate or redox pair. Different redox pairs exhibit varying affinities for electrons. Those with a lower electron affinity transfer electrons to those with a higher affinity. The electron affinity of a redox pair is measured by its oxidation-reduction potential, or redox potential (E₀), the magnitude of which is directly related to The change in Free energy. Eo is expressed in volts; the lower (more negative) the value, the weaker the affinity of the substance for electrons. Conversely, a higher affinity corresponds to a higher reduction potential.

The redox potentials Eo are related to the change in free energy by the Nernst equation:

where n is the number of electrons transferred in the reaction; F is the Faraday constant (95 500 C·mol^-1); ΔЕ₀ is the difference in redox potentials between the electron-donating and electron-accepting pairs (V).

The value Δ Е₀ represents the standard oxidation-reduction potential, determined under standard conditions where the concentrations of all reactants are 1 mol/L, the gas pressure is 1013 hPa (1 atm), and the pH is 7.0 (Table 16.2).

MAIN STAGES OF energy transformation in catabolic processes. Energy is released during the enzymatic oxidation of metabolites by specific dehydrogenases. In dehydrogenation reactions, electrons and protons are transferred from organic

substrates to the Coenzymes of NAD^- and FAD-dependent dehydrogenases. High-energy electrons are transferred from the reduced coenzymes NADH and FADH₂ to oxygen via a chain of carriers localized in The inner mitochondrial membrane. The reduction of the O₂ molecule occurs As a result of The transfer of four electrons. With each addition of two electrons to oxygen—delivered along the carrier chain—two protons are taken up from the mitochondrial matrix, ultimately forming an H₂O molecule.

Table 16.2

Standard oxidation-reduction potentials of some conjugate pairs

The oxidation of organic substances in Cells, accompanied by oxygen consumption and Water synthesis, is referred to as tissue Respiration, and the Electron Transport Chain (ETC) is called the Respiratory Chain. As electrons pass through the ETC from one carrier to the next, they lose free energy. A significant portion of this Energy is stored in the form of ATP, while some is dissipated as heat. Additionally, high-energy electrons generated during the oxidation of various substrates can be utilized in biosynthetic reactions, which require reducing equivalents such as NADPH In addition to ATP.