BIOCHEMISTRY - L. Stryer - 1984

VOLUME 2

PART II GENERATION AND STORAGE OF METABOLIC ENERGY

CHAPTER 14. OXIDATIVE PHOSPHORYLATION

14.4. Flavins, Iron-Sulfur Complexes, Quinones, and Heme Groups Transfer Electrons from NADH to O₂

Electrons are transferred from NADH to O₂ through a series of carriers: flavins, iron-sulfur complexes, Quinones, and Hemes (Fig. 14.4). With the exception of quinones, these electron carriers are prosthetic groups of Proteins. The first reaction involves The oxidation of NADH by NADH-Q reductase (also called NADH dehydrogenase), an enzyme consisting of at least sixteen polypeptide chains. Two electrons are transferred from NADH to flavin mononucleotide (FMN), a prosthetic group of the enzyme, yielding its reduced form, FMNH₂

NADH + Н⁺ + FMN → FMNH₂+ NAD⁺.

Class="center">Fig. 14.4. Pathway of electron carriers in the respiratory assembly. Protons are pumped out by the three shaded complexes shown in the figure

From FMNH₂, electrons are subsequently transferred to a series of iron-sulfur complexes (abbreviated as Fe-S), which serve as a second prosthetic group in the NADH-Q reductase molecule. The iron in these compounds is nonheme iron, which is why iron-sulfur proteins are also referred to as nonheme iron proteins. Recent studies have demonstrated that Fe-S complexes play a crucial role in a wide range of oxidation-reduction reactions in biological systems. Three types of Fe-S centers are known (Fig. 14.5). In the simplest case, a single iron atom is tetrahedrally coordinated to The sulfhydryl groups of four Cysteine residues of the protein. The second type of complex (designated as Fe₂-S₂) contains 2 iron atoms and two inorganic sulfides attached to four cysteine residues. The third type of complex (Fe₄-S₄) contains four iron atoms, four inorganic sulfides, and four cysteine residues. The iron atoms in these complexes can exist in either the reduced (Fe²⁺) or oxidized (Fe³⁺) state. NADH-Q reductase contains both the second (Fe₂-S₂) and third (Fe₄-S₄) types of complexes.

From the iron-sulfur centers of NADH-Q reductase, electrons are further transferred to coenzyme Q (abbreviated as Q).

Fig. 14.5. Molecular models of Fe-S complexes: A) Fe-S center; B) Fe₂-S₂ center; C) Fe₄-S₄ center. Iron atoms are colored red, cysteine sulfur atoms yellow, and inorganic sulfur atoms green

Coenzyme Q is a quinone derivative with a long isoprenoid tail. It is also called ubiquenone because of its ubiquitous distribution in biological systems. The number of isoprene units in coenzyme Q depends on the species of living organisms. In mammals, its most common form contains ten isoprenoid units and is designated as Q₁₀.

The isoprenoid tail confers high nonpolarity on Q, which facilitates its rapid diffusion within the hydrocarbon phase of The inner mitochondrial membrane. Coenzyme Q is the only electron carrier in the Respiratory Chain that is not firmly bound to a protein and is not covalently attached to it. Indeed, coenzyme Q Functions as a highly mobile electron carrier between Flavoproteins and Cytochromes in the electron-transfer chain.

Recall that FADH₂ is formed in The Tricarboxylic Acid Cycle during the oxidation of succinate to fumarate by succinate dehydrogenase. Succinate dehydrogenase is one of the two Components of the succinate-Q reductase complex; the second component is an Fe-S protein. Like NADH-Q reductase, this complex is an integral component of the inner mitochondrial membrane. The high-potential electrons of FADH₂ in the succinate dehydrogenase component are then transferred to the Fe-S centers of the complex and further to coenzyme Q for entry into the electron-transfer chain. Similarly, glycerol phosphate dehydrogenase (Section 14.9) and fatty acyl-CoA dehydrogenases (Section 17.8) transfer their high-potential electrons to coenzyme Q, yielding its reduced form, QH₂.

The electron carriers between QH₂ and O₂, apart from a single Fe-S protein, include all the cytochromes. The Central Role of cytochromes in Respiration was discovered in 1925 by David Keilin. A cytochrome is an electron-transfer protein containing a heme prosthetic group. During electron transfer, the iron atom oscillates between the reduced ferrous form (+2) and the oxidized ferric form (+3). Unlike NADH, flavins, and coenzyme Q, which transfer two electrons each, a heme group—much like an Fe-S center—transfers only a single electron. Thus, a molecule of QH₂ (the reduced form of quinone) transfers its two high-potential electrons to two molecules of cytochrome b, the next member of the electron-transfer chain. The semiquinone free radical (designated as QH•) may be formed as an intermediate in these electron-transfer reactions.

Five cytochromes are located in the electron-transfer chain between QH₂ and O₂. Cytochromes b and c₁, along with an Fe-S protein, are components of the QH₂-cytochrome c reductase complex. Cytochrome c transfers electrons from this complex to the cytochrome c oxidase complex, which contains cytochromes a and a₃. The oxidation-reduction potential (electron affinity, or oxidizing capacity) of these cytochromes increases in the order

QH₂ → Cytochrome b → FeS → Cytochrome c₁ → Cytochrome c → Cytochrome a → Cytochrome a₃ → O₂.

These cytochromes differ in Structure and properties. The prosthetic group of cytochromes b, c₁, and c is iron protoporphyrin IX, commonly referred to as heme, which is also the prosthetic group of Myoglobin and Hemoglobin. In cytochrome b, the heme is not covalently bound to the protein, whereas in Cytochromes c and c₁, it is covalently attached to the protein via thioether bonds (Fig. 14.6). These bonds are formed by The addition of the sulfhydryl groups of two cysteine residues to the vinyl groups of the heme.

Fig. 14.6. The heme in cytochromes c and c₁ is covalently attached to two cysteine side chains

Cytochromes a and a₃ possess a different iron-porphyrin prosthetic group, known as heme a. It differs from the heme of cytochromes c and c₁ by the presence of a formyl group instead of one of the methyl groups, and a hydrocarbon chain instead of one of the vinyl groups. Cytochromes a and a₃ represent the terminal components of the respiratory chain. They exist as a complex, sometimes referred to as cytochrome oxidase.

The QH₂-cytochrome c reductase complex transfers electrons from QH₂ to cytochrome c, a Water-soluble peripheral membrane protein (Sections 14.15–14.17).

Next, the reduced cytochrome c transfers its electrons to the cytochrome c oxidase complex. The Role of cytochrome c is analogous to that of coenzyme Q: it acts as a mobile electron carrier between various complexes in the respiratory chain. Electrons are transferred to the cytochrome a component of the complex and then to the copper-containing cytochrome a₃. During electron transport from cytochrome a₃ to molecular oxygen, the copper atom cycles between the oxidized (+2) and reduced (+1) states. Water formation is a four-electron process, whereas heme groups function as single-electron carriers. Just how four electrons converge to reduce an O₂ molecule remains to be fully elucidated:

O₂ + 4Н⁺ + 4е^- → 2Н₂O.