The Dual Nature of Life by Gennadiy Zhegunov

The Dual Nature of Life by Gennadiy Zhegunov

Author:Gennadiy Zhegunov
Language: eng
Format: epub
Publisher: Springer Berlin Heidelberg, Berlin, Heidelberg


14.2 Biological Oxidation

The reactions of biological oxidation provide a gradual and discrete extraction of energy from organic substances. The process of energy extraction in animals can be conventionally divided into three different stages. First, at the preparatory stage, macromolecules of food (proteins, polysaccharides, and lipids) are split into monomers (amino acids, monosaccharides, fatty acids) with the participation of digestive enzymes. Then, within cells, this process continues with the participation of intracellular hydrolases. At the second, or anaerobic stage, monomers are partially oxidized, forming several key intermediate low-molecular weight products, mainly through the action of the acetyl coenzyme A and several carboxylic acids. At the third, or aerobic stage, which occurs in the mitochondria, hydrogen is removed from the above-mentioned organic substances by means of special enzymes, and the hydrocarbon skeleton is eventually oxidized into CO2. The segregated hydrogen (universal fuel) then links with oxygen, forming the end product of oxidation—water. The release of energy carriers in oxidation chains is implemented partially and gradually—hydrogen after hydrogen, electron after electron. Finally, energy is accumulated in small ATP molecules, which are a convenient cellular form of energy storage and utilization.

Oxidative processes may be associated with: (a) dehydration–removal of hydrogen from the oxidized substrate; (b) loss of an electron by the substrate; and (c) attachment of oxygen to the substrate. Oxidative processes are always conjugated with reduction reactions, meaning that one substance is oxidized (i.e., gives away an electron) and the other is reduced (i.e., accepts this electron).

The most widespread type of biological oxidation is enzymatic dehydrogenation—the removal of hydrogen. If oxygen is the acceptor, those reactions are called aerobic oxidation. If the acceptor is a substrate of another nature, then such reactions are called anaerobic oxidation. The oxidation of acetyl-CoA in the Krebs cycle is effectively the process of generating protons and electrons, where the acceptors are the coenzymes known as NAD- and FAD-dependent dehydrogenases. During these reactions, electrons with high energetic potentials are transferred by enzymes from reduced NADH and FADH2 coenzymes to oxygen by means of electron carriers within the mitochondrial membrane. The reduction of oxygen results from bonding of electrons (which came from the respiratory chain) and hydrogen ions. With the attachment of 2 electrons and 2 protons to oxygen, one molecule of water is formed. The process of oxidation of organic substances in cells, which is accompanied by the consumption of oxygen and formation of ATP and water, is called tissue respiration, and the electron transport systems that carry electrons within the mitochondrial membrane constitute the respiratory chain.

The energy gained by the movement of electrons through the respiratory chain from one carrier to the other is used to transport protons from the mitochondrial matrix to the intermembranous space, where a high concentration is produced. This creates a high electrochemical gradient at the inner membrane. The generated potential is used to synthesize ATP by ATP-synthase, a complicated transmembrane complex, during the process of proton flow through this complex (Fig. 14.1). The generated ATP is used in numerous reactions for the purposes of biological creation and implementation of various functions.



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