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Unformatted text preview: Sun 26.10.2008 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. - NADH-Q oxidoreductase is L-shaped, with a horizontal arm lying in the membrane and a vertical arm that projects into the matrix.- The reaction catalyzed by this enzyme appears to be 1- The initial step is the binding of NADH and the transfer of its two high- potential electrons to the flavin mononucleotide (FMN) prosthetic group a non-amino acid compound attached to a protein, often in a reversible fashion, that confers new properties of this complex to give the reduced form, FMNH2 (Figure 18.8). The electron acceptor of FMN, the isoalloxazine ring , is identical with that of FAD.--------------------------------- Figure 18.8 Oxidation states of flavins.--------------------------------- 2- Electrons are then transferred from FMNH 2 to a series of ironsulfur clusters, the second type of prosthetic group in NADH-Q oxidoreductase. Fe-S clusters in ironsulfur proteins (also called nonheme iron proteins) play a critical role in a wide range of reduction reactions in biological systems . Several types of Fe-S clusters are known (Figure 18,9). In the simplest kind, 1- a single iron ion is tetrahedrally coordinated to the sulfhydryl groups of four cysteine residues of the protein. - A second kind , denoted by 2Fe-25, contains two iron ions, two inorganic sulfides, and usually four cysteine residues. - A third type, designated 4Fe-45, contains four iron ions, four inorganic sulfides, and four cysteine residues. NADH-Q oxidoreductase contains both 2Fe-25 and 4Fe-45 clusters. Iron ions in these Fe-S complexes cycle between Fe (reduced) and Fe (oxidized) states. Unlike quinones and flavins, ironsulfur clusters generally undergo oxidationreduction reactions without releasing or binding protons.---------------------- Figure 18.9 Ironsulfur clusters. (A) A single iron ion bound by four cysteine residues. (B) 2Fe-2S cluster with iron ions bridged by sulfide ions. (C) 4Fe-4S cluster. Each of these clusters can undergo oxidationreduction reactions....
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