NADH:Q oxidoreductase

For a long time Fe/S clusters in the enzyme complexes of the respiratory chain of oxidative phosphorylation have been suggested to be directly involved in energy transduction, e.g., in the generation of a proton-motive force. A specific example is the putative cubane, center N2, in NADH Q oxidoreductase [6], One could formally write the process as a catalysis of the reaction H+in -> H+out. 

Oxidative phosphorylation is the culmination of a series of energy transformations that are called cellular respiration or simply respiration in their entirety. First, carbon fuels are oxidized in the citric acid cycle to yield electrons with high transfer potential. Then, this electron-motive force is converted into a proton-motive force and, finally, the proton-motive force is converted into phosphoryl transfer potential. The conversion of electron-motive force into proton-motive force is carried out by three electron-driven proton pumps—NADH-Q oxidoreductase, Q-cytochrome c oxidoreductase, and... 

Figure 18.11. Structure of NADH-Q Oxidoreductase (Complex I). The structure, determined by electron microscopy at 22-A resolution, consists of a membrane-spanning part and a long arm that extends into the matrix. NADH is oxidized in the arm, and the electrons are transferred to reduce Q in the membrane. [After N. Grigorieff, J. Mol. Biol. 277 (1998) 1033-1048.]...

Oxidative phosphorylation is susceptible to inhibition at all stages of the process. Specific inhibitors of electron transport were invaluable in revealing the sequence of electron carriers in the respiratory chain. For example, rotenone and amytal block electron transfer in NADH-Q oxidoreductase and thereby prevent the utilization of NADH as a substrate (Figure 18.43). In contrast, electron flow resulting from the oxidation of succinate is unimpaired, because these electrons enter through QH2, beyond the block. AntimycinA interferes with electron flow from cytochrome h Q-cytochrome c... 

The electron carriers in the respiratory assembly of the inner mitochondrial membrane are quinones, flavins, iron-sulfur complexes, heme groups of cytochromes, and copper ions. Electrons from NADH are transferred to the FMN prosthetic group of NADH-Q oxidoreductase (Complex I), the first of four complexes. This oxidoreductase also contains Fe-S centers. The electrons emerge in QH2, the reduced form of ubiquinone (Q). The citric acid cycle enzyme succinate dehydrogenase is a component of the succinate-Q reductase complex (Complex II), which donates electrons from FADH2 to Q to form QH2.This highly mobile hydrophobic carrier transfers its electrons to Q-cytochrome c oxidoreductase (Complex III), a complex that contains cytochromes h and c j and an Fe-S center. This complex reduces cytochrome c, a water-soluble peripheral membrane protein. Cytochrome c, like Q, is a mobile carrier of electrons, which it then transfers to cytochrome c oxidase (Complex IV). This complex contains cytochromes a and a 3 and three copper ions. A heme iron ion and a copper ion in this oxidase transfer electrons to O2, the ultimate acceptor, to form H2O. 

The flow of two electrons through NADH-Q oxidoreductase, Q-cytochrome c oxidoreductase, and cytochrome c oxidase generates a gradient sufficient to synthesize 1, 0.5, and 1 molecule of ATP, respectively. Hence, 2.5 molecules of ATP are formed per molecule of NADH oxidized in the mitochondrial matrix, whereas only 1.5 molecules of ATP are made per molecule of FADH2 oxidized because its electrons enter the chain at QH2, after the first proton-pumping site. 

Energisation and ATP synthesis can be coupled to NADH-Q oxidoreductase activity after reconstitution of Complex I (and ATP synthase) into liposomes. 

Special electron carriers ferry the electrons from one complex to the next. Electrons are carried from NADH-Q oxidoreductase to Q-cytochrome c oxidoreductase, the second complex ot the chain, by the reduced form of coeti2 ymt2 Q (Q), also known as ubiquinone because it is a ubiquitous quinone in biological systems. Ubiquinone is a hydrophobic quinone that diffuses rapidly within the inner mitochondrial membrane. Cytochrome c. a small soluble protein, shuttles electrons from Q-cytochrome c oxidoreductase to cytochrome c oxidase, the final component in the chain and the one that catalyses the reduction of Oi. Electrons from the FADH generated bv... 

The High Potential Electrons of NADH Enter the Respiratory Chain at NADH-Q Oxidoreductase... 

The electrons of NADH enter the chain at NADH-Q oxidoreductase (also called Complex I and NADH dehydrogenase), an enormous enzyme (>900 kd) consisting of approximately 46 polypeptide chains. This proton pump, like that of the other two in the respiratory chain, is encoded by genes residing in both the mitochondria and the nucleus. N AD FI-Q oxidoreductase is 1.-shaped, with a horizontal arm lying in the membrane and a vertical arm that projects into the matrix. 

Figure 18.10 Coupled electron-proton transfer reactions through NADH-Q oxidoreductase. Electrons flow in Complex I from NADH through FMN and a series of iron-sulfur cluster to ubiquinone (Q). The electron flow results in the pumping of four protons and the uptake of two protons from the mitochondria matrix. [Based on U, Brandt et al, FEB5 Letters 54S(2003) 9-17, Figure 2.]...

We can now estimate how many molecules of ATP are formed when glucose is completely oxidized to GO2. The number of ATP (or GTP) molecules formed in glycolysis and the citric acid cycle is unequivocally known because it is determined by the stoichiometries of chemical reactions. In contrast, the AT P yield of oxidative phosphorylation is less certain because the stoi chiometries of proton pumping, ATP synthesis, and metabolite-transport processes need not be integer numbers or even have fixed values. As stated earlier, the best current estimates for the number of protons pumped out of the matrix by NADH-Q oxidoreductase, Q-cytochrome c oxidoreductase, and cytochrome c oxidase per electron pair are four, two, and four, respectively. The synthesis of a molecule of ATP is driven by the flow of about three protons through ATP synthase. An additional proton is consumed in transporting ATP from the matrix to the cytoplasm. Hence, about 2.5 molecules of cytoplasmic ATP are generated as a result of the flow of a pair of electrons from NADH to O2. For electrons that enter at the level of Q-cytochrome c oxidoreductase, such as those from the oxidation of succinate or cytoplasmic NADH, the yield is about 1.5 molecules of ATP per electron pair, Hence, as tallied in Table 18.4, about 30 molecules of ATP are formed... 

Elbehti A, Brasseur G, Lemesle-Meunier D (2000) First evidence for existence of an uphill electron transfer through the bcY and NADH-Q oxidoreductase complexes of the acidophilic obligate chemolithotrophic ferrous iron-oxidizing bacterium Thiobacillus ferrooxidans. J Bacterid 182 3602-3606... 

Describe the entry of electrons from NADH into NADH-Q oxidoreductase (Complex 1) and trace their path through this proton pump. State the roles of flavin mononucleotide (FMN), iron-sulfur clusters, and coenzyme Q. 

Discuss the role of coenzyme Q as a mobile electron carrier between NADH-Q oxidoreductase and cytochrome reductase (Complex 111). 

The respiratory chain begins in complex I (NADH-Q oxidoreductase), where NADH is oxidized by coenzyme Q (Q, Atlas M5) in a two-electron reaction ... 

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