NADH dehydrogenase, iron-sulfur clusters

Iron-sulfur clusters are found in flavoproteins such as NADH dehydrogenase (Chapter 18) and trimethylamine dehydrogenase (Fig. 15-9) and in the siroheme-containing sulfite reductases and nitrite reductases.312 These two reductases are found both in bacteria and in green plants. 

NAD+/NADH or NADP+/NADPH also participate in a number of other enzyme-catalyzed redox reactions, which do not involve dehydrogenases in the classical sense as described earlier. These redox enzymes contain bound cofactors such as flavins, heme, and iron-sulfur clusters. One group of these enzymes is usually denoted diaphorase (DI) when used in analytical systems and includes both flavin-containing lipoamide and Kpoyl dehydrogenases (EC 1.8.1.4). 

The determination of the exchange integral in binuclear iron-sulfur clusters of proteins of varying complexity has been carried out (Salerno et al, 1977). The well-established model for binuclear iron-sulfur clusters involves two acid-labile sulfur atoms bridging two antiferromagnetically coupled high-spin iron atoms. The values of J obtained are center SI in succinate dehydrogenase, 90 cm Rieske s iron-sulfur center, 65 cm" adrenodoxin, 270 cm The behavior of iron-sulfur center Nla in NADH UQ reductase was also examined Its similarity to that of center SI indicates that center Nla is also binuclear iron-sulfur center with J = 90 cm" The... 

NADH dehydrogenase and succinate dehydrogenase also contain Fe atoms that are bound by the S atoms of cysteine residues of the protein, in association with additional, inorganic sulfide atoms. Structures of these complexes are shown in figure 10.19. Succinate dehydrogenase has three iron-sulfur centers, one with a [2Fe-2S] cluster, one with [4Fe-4S], and one with a cluster containing 3 Fe atoms and 3 (or possibly 4) sulfides. Iron-sulfur centers undergo one-electron oxidation-reduction reactions. 

Abbreviations FAD, flavin adenine dinucleotide Fe-S, iron-sulfur proteins that can he identified in separate clusters by electron paramagnetic resonance analysis (the s-1, s-2 subscripts identify these iron-sulfur proteins as part of the succinate dehydrogenase complex) His, the histidine linkage between FAD and the large (70,000 daltons) protein moiety of the enzyme FMN, flavin mononucleotide N-la, N-2 subscripts identify these iron-sulfur proteins as part of the NADH-dehydro-genase complex UQ, ubiquinone Cyt bf and Cyt b, cytochrome b-566 and b-563, respectively. 

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