Textbook - BIOLOGICAL CHEMISTRY - Hubsky Y.I. - 2000

Chapter II. GENERAL PATTERNS OF METABOLISM

CHAPTER 9. BIOENERGETIC PROCESSES: ELECTRON TRANSPORT; OXIDATIVE PHOSPHORYLATION IN MITOCHONDRIA

Bioenergetic processes—Biological Oxidation and coupled Oxidative Phosphorylation—constitute the final phase of molecular Catabolism in living organisms, carried out by complex multienzyme systems located in The inner mitochondrial membrane. The result of these reactions, structured within Biomembranes, is the generation of high-energy bonds in ATP molecules, which serve as the primary energy supplier for all endergonic cellular processes.

9.1. REACTIONS OF BIOLOGICAL OXIDATION

The intramolecular oxidation of biological substrates (biological oxidation) is the principal molecular mechanism that satisfies the energetic requirements of living organisms.

Oxidation is the process wherein an oxidized atom or molecule (the oxidation substrate) loses electrons or hydrogen atoms (protons and electrons).

Reduction is the reverse of oxidation, accompanied by the uptake of electrons or hydrogen atoms by an organic substrate (substrate hydrogenation).

A common example of a redox reaction in biological systems is The conversion of a ferrous ion into a ferric ion:

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Oxidation-reduction processes always involve Two Types of substances: an oxidizing agent (an electron acceptor) and a reducing agent (an electron donor).

The electrons and hydrogen atoms involved in redox reactions are referred to as reducing equivalents.

A redox reaction may involve the interatomic or intermolecular transfer of reducing equivalents from an electron donor (the oxidized compound) to an electron acceptor (the reduced compound) without the direct addition of the acceptor to the donor. There are also reactions that result in The formation of a new chemical bond between a carbon atom of the oxidized biomolecule and a more electronegative heteroatom acting as an oxidizing agent (specifically, an oxygen atom).

The oxidizing and reducing agents together form a redox couple (redox system). The ability of a redox system to donate or accept electrons is characterized by its redox potential, the value of which is calculated using the Nernst equation:

where: R is the gas constant, T is the absolute Temperature, n is the number of electrons participating in the reaction, F is the Faraday constant, [OX] and [RED] are the concentrations of the oxidized and reduced forms of the compound, respectively, and E₀ is the standard Redox Potential of the system (i.e., the redox potential under conditions where the concentrations of the oxidized and reduced forms of the redox couple are equal).

The value of E₀ quantitatively determines a system's capacity to act as an electron donor or acceptor relative to another redox system. According to the recommendations of the International Union of Pure and Applied Chemistry (IUPAC), systems with a greater tendency to donate electrons have more negative redox potentials, whereas systems prone to accepting electrons have more positive potentials.

Standard redox potentials (in volts) are measured relative to the potential of the hydrogen electrode H⁺/H₂, which is set at 0.0 V at pH=0. However, for biological systems, it is more convenient to determine redox potentials at pH=7.0; such a standard redox potential is denoted as E₀'. Under these conditions, the E₀' of the hydrogen electrode is — 0.42 V. The E₀' values for biologically important redox systems are listed below (section 9.3).

Tissue Respiration

Biological oxidation reactions form the Molecular Basis of tissue respiration—the uptake of O₂ by living Tissues, which serves as an integral physiological indicator of the intensity of redox processes within them. The source of oxygen for this process is O₂, which is delivered to tissues through the normal functioning of the external respiration system and the oxygen-transporting function of Blood Hemoglobin, subsequently diffusing across Plasma Membranes into the Cells.

As a result of tissue respiration, which takes place in Cell/35.html">Mitochondria, oxygen atoms are incorporated into Water molecules, while the carbon of the oxidized bioorganic compounds is released as carbon dioxide. Mitochondrial respiration itself serves as the biochemical foundation for the generation and storage of free chemical energy utilized in endergonic processes.

In Liver hepatocytes and cells of certain other specialized tissues, a fraction of the oxygen consumed during tissue respiration is utilized for the biological oxidation of exogenous and endogenous substrates within the membranes of the Endoplasmic reticulum—a process known as microsomal oxidation, which serves as a mechanism for modifying hydrophobic molecules within the Organism. In hepatocytes, this pathway accounts for up to 20% of the total cellular oxygen consumption balance.

Fig. 9.1. Otto Warburg, German biochemist (1883–1970). Made a significant contribution to elucidating the Biochemical Mechanisms of tissue respiration, the Structure and function of respiratory Enzymes, and the Coenzymes NAD and FAD. Nobel Prize laureate (1931).

Types of Biological Oxidation Reactions

All redox reactions occurring in living cells are catalyzed by enzymes belonging to the class of oxidoreductases.

In the processes of biological oxidation taking place in living systems, the following classes of reactions are distinguished:

1. Reactions Involving the transfer of hydrogen (i.e., protons and electrons) from the oxidized substrate (SH₂) to a specific acceptor (A):

Reactions of this type are referred to as dehydrogenation reactions, and the enzymes catalyzing them are called dehydrogenases.

The coenzymes of dehydrogenases that act as direct acceptors of reducing equivalents include the following compounds:

- nicotinamide (pyridine) coenzymes — the NUCLEOTIDES NAD⁺ (nicotinamide adenine dinucleotide) and NADP⁺ (nicotinamide adenine dinucleotide phosphate);

- Flavin Coenzymes — the nucleotides FAD (flavin adenine dinucleotide) and FMN (flavin mononucleotide).

These coenzymes transfer electrons to subsequent biochemical acceptors, forming Electron Transport Chains for reducing equivalents in biological systems.

Depending on the Chemical Nature of the acceptor interacting with dehydrogenases, dehydrogenation reactions are divided into the following classes:

1.1. Dehydrogenation reactions in which the acceptor is a chemical compound (R) other than oxygen:

The enzymes catalyzing these reactions are anaerobic dehydrogenases.

1.2. Dehydrogenation reactions in which oxygen is used as the acceptor:

The enzymes catalyzing these reactions are aerobic dehydrogenases or oxidases, resulting in the formation of hydrogen peroxide.

2. Reactions involving The transfer of one or two electrons from the substrate to the acceptor:

Reactions of this type are catalyzed by Cytochromes of the mitochondrial Respiratory Chain.

3. Reactions consisting of the direct incorporation of one or two oxygen atoms into the oxidized substrate.

Such reactions are known as oxygenase reactions, and the respective enzymes catalyzing them are called oxygenases. Depending on the number of oxygen atoms interacting with the substrate, oxygenase reactions are subdivided into:

- monooxygenase reactions:

- dioxygenase reactions:

Monooxygenase reactions are catalyzed by cytochrome P-450 and form The basis of oxidative hydroxylation of numerous hydrophobic substrates of both exogenous and endogenous origin (microsomal oxidation). Dioxygenase reactions include Lipid Peroxidation, specifically of Unsaturated Fatty acids that are components of naturally occurring Lipids.