BIOCHEMISTRY - Textbook - Ostapchenko L. I. - 2012

Chapter 16. ENERGY METABOLISM

16.1. Biological Oxidation

16.1.3. Enzymes and Coenzymes Involved in Redox Reactions

The transfer of electrons from oxidized substrates to oxygen occurs in several stages. It involves A large number of intermediate carriers, each capable of accepting electrons from the preceding component and passing them on to the next. This creates a chain of oxidation-reduction reactions, resulting in the reduction of O₂ and the synthesis of H₂O. A large number of carriers are part of the mitochondrial Respiratory Chain (Fig. 16.4).

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Fig. 16.4. Mitochondrial Electron Transport chain:

Complex I contains FMN and at least five iron-sulfur Proteins (FeS).

Complex III includes two different forms of cytochrome b (with absorption maxima at 562 and 566 nm) and one FeS protein.

Complex IV contains Cytochromes a₁ and a₃ and two copper ions. Complex V is ATP synthase.

Cytochrome c is a peripheral membrane protein and is not part of the complexes.

Complex II is not shown in the figure

With the exception of ubiquinone (coenzyme Q (CoQ)), all Components of the Electron Transport Chain (ETC) are proteins. Their composition includes various non-protein components: FMN, Fe as part of iron-sulfur proteins and porphyrin rings, and Cu ions.

Primary hydrogen acceptors in redox reactions belong to Two Types of dehydrogenases: nicotinamide-dependent, which contain nicotinic acid derivatives as Coenzymes, and flavin-dependent, which comprise riboflavin derivatives.

Nicotinamide-dependent dehydrogenases contain NAD⁺ or NADP⁺ as coenzymes. These coenzymes are part of the active centers of dehydrogenases, but they can reversibly dissociate from the apoenzyme complex and associate with the enzyme during the reaction. Substrates of NAD^- and NADP-dependent dehydrogenases are located in the mitochondrial matrix and Cytosol. Nicotinamide serves as the functional moiety of nicotinamide coenzymes (Fig. 16.5).

Fig. 16.5. Structural formulas of the functional part of NAD⁺ and NADP⁺ coenzymes:

in their oxidized form, nicotinamide coenzymes are designated as NAD⁺ and NADP⁺,

since they carry a positive charge on the nitrogen atom of the pyridine ring.

During dehydrogenation reactions, out of the two hydrogen atoms cleaved from the oxidized substrate, the nicotinamide ring accepts a hydrogen ion and two electrons in the form of a hydride ion (:H^-). The second ion is released into the medium. In the reversible reaction, NADH (NADPH) acts as a donor of electrons and protons.

Most dehydrogenases supplying electrons to the ETC contain NAD⁺. They catalyze Reactions of the type

Thus, by accepting protons and electrons from various substrates, NAD⁺ serves as the primary collector of energy from oxidized substances and the main source of high-energy electrons for the ETC.

NADPH is not a direct electron donor in the ETC; instead, it is utilized in reductive biosyntheses. However, electrons from NADPH can potentially enter the ETC through the action of pyridine nucleotide transhydrogenase, which catalyzes the reaction

Flavin dehydrogenases contain FAD or FMN as coenzymes, which are synthesized in The Human Body from vitamin B₂. Flavin Coenzymes are tightly bound to apoenzymes. The functional part of FAD and FMN is an isoalloxazine conjugated ring system (Fig. 16.6).

FAD serves as an electron acceptor from many substrates in reactions of the type

where E is the protein moiety of the enzyme.

Fig. 16.6. Structural formulas of the functional part of FAD and FMN coenzymes:

during the reaction, FAD and FMN accept two electrons and, unlike NAD⁺, two protons released by the substrate

Most FAD-dependent dehydrogenases are soluble proteins localized in the mitochondrial matrix. An exception is succinate dehydrogenase, which is found in The inner mitochondrial membrane. FMN-containing Enzymes include NADH dehydrogenase, also localized in the inner mitochondrial membrane; it oxidizes NADH generated in the mitochondrial matrix.

The electron transport chain from NADH and FADH2 to oxygen. The transfer of electrons from NADH to O₂ involves a series of carriers localized in the inner mitochondrial membrane. With the exception of ubiquinone and cytochrome c, these are complex Structure/178.html">Protein Complexes.

NADH dehydrogenase (NADH-Q reductase, complex I) consists of several polypeptide chains. FMN acts as the prosthetic group. The sole substrate of the enzyme is NADH, from which two electrons and a proton are transferred to FMN to form FMNH2. The second proton is taken up from the matrix. The reaction proceeds According to the equation

From FMNH₂, electrons are then transferred to a series of iron-sulfur proteins (FeS), which act as a second prosthetic group in the NADH dehydrogenase molecule. The iron atoms in these proteins (non-heme iron) are grouped into clusters known as iron-sulfur centers. FeS centers form complexes with many proteins (Flavoproteins, cytochromes) involved in oxidation-reduction reactions. Three types of FeS centers are known (Fe, Fe₂S₂, Fe₄S₄), in which the iron atom is bonded to the sulfur atoms of Cysteine residues or inorganic sulfur.

NADH dehydrogenase contains several Fe₂S₂ and Fe₄S₄ centers. The iron atoms in such centers can accept and donate electrons alternately, shifting between ferrous (Fe²⁺) and ferric (Fe³⁺) states. From the iron-sulfur centers, electrons are transferred to coenzyme Q (ubiquinone) (Fig. 16.7).

Fig. 16.7. Structure of ubiquinone (coenzyme Q): n is the number of isoprenoid units (n = 6–10).

Ubiquinone can accept one electron to form semiquinone or two electrons to be fully reduced to hydroquinone (ubiquinol)

The designation of this fat-soluble quinone comes from the first letter of the English word "quinone," while the name ubiquinone reflects its widespread occurrence in nature (ubiquitous). Depending on the source from which they are isolated, ubiquinone molecules differ in the length of their hydrocarbon chain, which in mammals contains ten isoprenoid units and is designated as Q10. During Electron transfer from NADH dehydrogenase via FeS to ubiquinone, it is reversibly converted into hydroquinone. Ubiquinone acts as a collector, accepting electrons from NADH dehydrogenase and other flavin-dependent dehydrogenases, notably succinate dehydrogenase. Ubiquinone participates in reactions of the type

Cytochromes, or Hemoproteins, are present in all types of organisms. In Eukaryotic Cells, they are localized in the mitochondrial membranes and The Endoplasmic reticulum. About 30 different cytochromes are known. All of them contain a heme group as a prosthetic group. Their diversity is due to differences in:

✵ side chains in the heme structure;

✵ The structure of The polypeptide chains;

✵ the way the polypeptide chains are linked to the heme.

Depending on their ability to Light absorption in a specific region

of the spectrum, all cytochromes are divided into groups a, b, and c. Within each group, individual species with unique spectral properties are designated by numeric indices (b₁, b₂, etc.).

Structural Features of Different types of cytochromes determine differences in their redox potentials. Six types of cytochromes (a, a₃, b, b₅, c, c₁) participate in the electron transport chain. With the exception of cytochrome c, all of them are embedded in the inner mitochondrial membrane as complex protein complexes (Table 16.3).

QH₂ dehydrogenase (coenzyme Q-cytochrome c reductase, complex III) consists of two types of cytochromes (b₁ and b₂) and cytochrome c₁. QH₂ dehydrogenase transfers electrons from ubiquinol to cytochrome c. Within complex III, electrons are passed from cytochromes b to FeS centers, then to cytochrome c₁, and subsequently to cytochrome c. Similar to FeS centers, heme groups carry only one electron at a time. Thus, two electrons are transferred from a QH₂ molecule to two molecules of cytochrome b. The formation of a semiquinone free radical is possible as an intermediate in these Electron transfer reactions. In type b cytochromes, the heme is not covalently bound to the protein, whereas in Cytochromes c₁ and c, it is attached to the protein via thioether bonds. These bonds are formed by The addition of two cysteine residues to the vinyl groups of the heme.

Table 16.3

Components of the mitochondrial electron transport chain

Cytochrome c is a peripheral, Water-soluble membrane protein with a Molecular Weight of 12,500 Da, featuring a single polypeptide chain of nearly 100 amino acid residues and a heme molecule covalently linked to the polypeptide.

Cytochrome c oxidase (complex IV) consists of two type a cytochromes (a and a₃), each possessing an oxygen-binding site. They contain a characteristic iron-porphyrin prosthetic group called heme a, which differs from the heme of cytochromes b, c, and c₁ by containing a formyl group instead of one of the methyl groups and a hydrocarbon tail instead of one of the vinyl groups.

Another feature of the a-a₃ complex is the presence of copper ions bound to the protein moiety within specific centers. Electron transfer by the a-a₃ complex involves reactions

The cytochrome a-a₃ complex reacts directly with molecular oxygen. Some CHARACTERISTICS OF THE ETC components are listed in Table 16.3.