Biochemistry and Molecular Biology - Belyasova N.A. 2002

Metabolism. Processes Leading to Energy Storage
Respiration
Characteristics of Respiratory Chain Components

The Components of the Respiratory Chain are Carriers of Reducing equivalents, including hydrogen carriers and electron carriers. Their arrangement within the membrane is strictly defined: hydrogen carriers alternate with electron carriers, and the strongest electron Donors always precede the strongest acceptors.

Since Electron Transport and hydrogen transport are coupled and equivalent processes, the respiratory chain can be viewed as an Electron Transport Chain. Its main components are Flavoproteins, iron-sulfur Proteins, Quinones, and Cytochromes.

Flavoproteins. These are Enzymes containing FMN or FAD as prosthetic groups. These carriers of reducing equivalents are described in Chapter 7. It should be emphasized that flavin Cofactors transport hydrogen and are stronger oxidizing agents than NAD⁺.

Iron-sulfur proteins. These redox systems contain iron atoms bonded, on one hand, to the sulfur of Cysteine amino acid residues, and on the other, to inorganic sulfide sulfur. Iron-sulfur centers (Fig. 12.1) can be considered as prosthetic groups of enzymes, which, however, have a Structure different from heme. Therefore, iron-sulfur proteins are also referred to as Non-heme iron proteins.

The number of iron and sulfide sulfur atoms in these proteins can vary, but the most common are proteins containing 2Fe, 2S^2- and 4Fe, 4S^2-. Iron-sulfur proteins transfer only electrons. In particular, 2Fe, 2S^2- centers transport one electron at a time. In this case, the electrons are not localized on atoms of any single type, but interact with both iron and sulfur nuclei, i.e., they are in a delocalized state.

Iron-sulfur proteins participate in the fixation of molecular nitrogen by Nitrogenase systems, in the reduction of sulfites and nitrites, in Photosynthesis, and in The oxidation of alkanes.

Quinones. These are low-molecular-weight hydrogen carriers present in the interior of Cell/29.html">The Lipid Bilayer of mitochondrial membranes in a 10- to 15-fold excess compared to Other components of The electron transport system. Due to the presence of a nonpolar hydrophobic chain, quinone molecules move freely within the lipid bilayer. Several families of quinones are distinguished, among which the most common are ubiquinones (translated from Latin as "ubiquitous quinones"), also known as Coenzymes Q (CoQ). The number of isoprenoid units in a quinone molecule is designated by a subscript (Q_n). Mammalian mitochondrial ubiquinones contain 10 isoprenoid units (Q₁₀), while bacterial ubiquinones contain 6 (Q₆). Fig. 12.1 shows the structure and redox stages of ubiquinones.

Among other groups of quinones, plastoquinones (found in chloroplast membranes, they transport hydrogen in Photosystems), tocopherols (Various Forms of vitamin E with antioxidant Functions), phylloquinones, and menaquinones (a family of K-group Vitamins involved in hydrogen transport in mycobacteria and Blood clotting in mammals) are well known. Additional information on these compounds can be found in Chapter 17.

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Fig. 12.1. Structure and Functional Features of the respiratory chain components. Within the dashed boxes is sulfide sulfur, which can be released as hydrogen sulfide upon acidification

Cytochromes. These redox enzyme systems contain heme as a prosthetic group (Fig. 12.1). The central iron atom in heme participates in electron transfer by changing its valence state:

Cytochromes act as electron carriers, and some of them (such as cytochrome oxidase) are capable of transferring electrons directly to molecular oxygen.

Many cytochromes isolated from various sources have been identified. They are usually designated by a combination of letters and numbers: the letters typically indicate the type of heme (e.g., a, b, c, o), and the numbers represent the wavelength of the alpha-band in the absorption spectrum (e.g., 552; 557.5; 450). For the electron transport system, it is particularly important that different cytochromes are characterized by different Redox Potentials and are arranged in the membrane in a specific sequence relative to one another. Only cytochrome c is believed to exist in a soluble state on the outer surface of The inner mitochondrial membrane.