Iron-sulfur centers cytochrome

Studies (see, e.g., (101)) indicate that photosynthesis originated after the development of respiratory electron transfer pathways (99, 143). The photosynthetic reaction center, in this scenario, would have been created in order to enhance the efficiency of the already existing electron transport chains, that is, by adding a light-driven cycle around the cytochrome be complex. The Rieske protein as the key subunit in cytochrome be complexes would in this picture have contributed the first iron-sulfur center involved in photosynthetic mechanisms (since on the basis of the present data, it seems likely to us that the first photosynthetic RC resembled RCII, i.e., was devoid of iron—sulfur clusters). 

The reaction-center proteins for Photosystems I and II are labeled I and II, respectively. Key Z, the watersplitting enzyme which contains Mn P680 and Qu the primary donor and acceptor species in the reaction-center protein of Photosystem II Qi and Qt, probably plastoquinone molecules PQ, 6-8 plastoquinone molecules that mediate electron and proton transfer across the membrane from outside to inside Fe-S (an iron-sulfur protein), cytochrome f, and PC (plastocyanin), electron carrier proteins between Photosystems II and I P700 and Au the primary donor and acceptor species of the Photosystem I reaction-center protein At, Fe-S a and FeSB, membrane-bound secondary acceptors which are probably Fe-S centers Fd, soluble ferredoxin Fe-S protein and fp, is the flavoprotein that functions as the enzyme that carries out the reduction of NADP+ to NADPH. 

Pyridine nucleotide-dependent flavoenzyme catalyzed reactions are known for the external monooxygenase and the disulfide oxidoreductases However, no evidence for the direct participation of the flavin semiquinone as an intermediate in catalysis has been found in these systems. In contrast, flavin semiquinones are necessary intermediates in those pyridine nucleotide-dependent enzymes in which electron transfer from the flavin involves an obligate 1-electron acceptor such as a heme or an iron-sulfur center. Examples of such enzymes include NADPH-cytochrome P4S0 reductase, NADH-cytochrome bs reductase, ferredoxin — NADP reductase, adrenodoxin reductase as well as more complex enzymes such as the mitochondrial NADH dehydrogenase and xanthine dehydrogenase. 

With the exception of coenzyme Q, all members of this chain are pro teins. These may function as enzymes as is the case with the dehy drogenases, they may contain iron as part of an iron-sulfur center, they may be coordinated with a porphyrin ring as in the cytochromes, or they may contain copper, as does the cytochrome a + a3 complex. 

During the 1940s, when it had become clear that formation of ATP in mitochondria was coupled to electron transport, the first attempts to pick the system apart and understand the molecular mechanism began. This effort led to the identification and at least partial characterization of several flavoproteins, iron-sulfur centers, ubiquinones, and cytochromes, most of which have been described in Chapters 15 and 16. It also led to the picture of mitochondrial electron transport shown in Fig. 10-5 and which has been drawn in a modem form in Fig. 18-5. 

Electron transfer to 02 occurs stepwise, through a series of flavoproteins, cytochromes (heme-proteins), iron-sulfur proteins, and a quinone. Most of the electron carriers are collected in four large complexes, which communicate via two mobile carriers— ubiquinone (UQ) and cytochrome c. Complex I transfers electrons from NADH to UQ, and complex II transfers electrons from succinate to UQ. Both of these complexes contain flavins and numerous iron-sulfur centers. Complex III, which contains three cyto-... 

The Z scheme. [(Mn)4 = a complex of four Mn atoms bound to the reaction center of photosystem II Yz = tyrosine side chain Phe a = pheophytin a QA and Qb = two molecules of plastoquinone Cyt b/f= cytochrome hf,f complex PC = plastocyanin Chi a = chlorophyll a Q = phylloquinone (vitamin K,) Fe-Sx, Fe-SA, and Fe-SB = iron-sulfur centers in the reaction center of photosystem I FD = ferredoxin FP = flavoprotein (ferredoxin-NADP oxidoreductase).] The sequence of electron transfer through Fe-SA and Fe-SB is not yet clear. 

Xiao K, Engstrom G, Rajagukguk R, et al. Effect of famoxadone on photoinduced electron transfer between the iron-sulfur center and cytochrome Ci in the cytochrome... 

Succinate dehydrogenase contains iron-sulfur centers and covalently bound FAD (both on the 70,000-dalton subunit). Iron-sulfur centers are also present on the 27,000-dalton subunit. The 15,500-dalton subunit of complex II is cytochrome bw Electrons from FADH2 are channeled to UQ via cytochrome b. ... 

The mechanism of fatty acid unsaturation (e.g., stearic acid — oleic acid) utilizes a system similar to that involving cytochrome P-450 (Chapter 17) it is micro-some bound, and it includes a heme-containing protein (cytochrome b5), an FAD-containing reductase, and an iron-sulfur center-containing "desaturase." The electron source is NADH or NADPH. Equation (19.14) summarizes this process ... 

Heme proteins (cytochromes) Iron-sulfur centers Other non-heme iron0... 

Figure 1 The mitochondrial respiratory chain. Electron transfer (brown arrows) between the three major membrane-bound complexes (I, III, and IV) is mediated by ubiquinone (Q/QH2) and the peripheral protein c)dochrome c (c). Transfer of protons hnked to the redox chemistry is shown by blue arrows red arrows denote proton translocation. NAD+ nicotinamide adenine dinucleotide, FMN flavin mononucleotide, Fe/S iron-sulfur center bH,bi, and c are the heme centers in the cytochrome bc complex (Complex III). Note the bifurcation of the electron transfer path on oxidation of QH2 by the heme bL - Fe/S center. Complex IV is the subject of this review. N and P denote the negatively and positively charged sides of the membrane, respectively...

B. how the electrons are passed through the electron transport chain by coenzymes, iron-sulfur centers, and cytochromes... 

Figure 4-13. The electron transport chain and oxidative phosphorylation. Heavy arrows indicate the flow of electrons. Fe-S = iron-sulfur centers FMN = flavin mononucleotide CoQ = coenzyme Q (ubiquinone) Cyt = cytochrome. nH+ indicates that an undetermined number of protons are pumped from the matrix to the cytosolic side. The numbers at the top of the figure correspond to the three major stages of electron transfer described in the text in V B.

Orientation of the components of the electron transport complexes within the inner mitochondrial membrane. Fe-S = Iron-sulfur center b, c, c, a, and 03 = cytochromes Cu = copper ion. 

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