Cytochrome reduced

Figure 17.9 Microsomal (nonmitochondrial) hydroxylation sequence involving cytochrome 450- RH is the substrate to be hydroxylated (ox, oxidized red, reduced cyt, cytochrome). Reduced cytochrome P450 has Fe2+, oxidized cytochrome P450 has Fe3+, reduced cytochrome oxidase has FADH2/FMNH2, and oxidized cytochrome oxidase has FAD/FMN.

DPNH-Cytochrome Reduc- Pig myocardium Fe 2,700 Mahler and Elowe (1954)... 

Fig. 3. Absorbance change due to quinone reduction in a reaction-center complex isolated from the carotenoidless mutant of Rb. sphaeroides. The sample contained reduced cytochrome and excess ascorbate as the secondary electron-donor system so that photooxidized P does not accumulate. The presence of excess ascorbate kept the oxidized cytochrome reduced, the net quinone reduction spectrum was obtained. Figure source Clayton (1980) Photosynthesis Physical Mechanisms and Chemical Patterns, p 95. Cambridge University Press.

PI", PI Oa", and PI"Qa difference spectra. A second approach to identify absorptions belonging to Qa" involves the selective reduction of the high potential cytochromes (of the tightly bound cytochrome subunit) with ascorbate. In ascorbate-reduced samples containing only Qa/ an electron will be transferred to Qa in approximately 200 ps [17]. Before this charge-separated state can recombine (about 1 ms at pH 8, [18]) the highest potential cytochrome reduces P in 0.32 jis [19], allowing a second electron to be transferred to I (about 3 ps, [20]). Thus steady state illumination of ascorbate-reduced reaction centers initially produces PI Qa [21]. Such samples can be compared to... 

At between pH 7 and 8, cytochrome a is rapidly oxidized by potassium ferricyanide and reduced by sodium dithionite, like other normal cytochromes. Reduced cytochrome c and p-phenylenediamine are also good reductants of cytochrome a, while borohydride and substrates of cytochrome oxidase such as hydroquinone and ascorbic acid hardly reduce it in the absence of a small amount of cytochrome c. Figure 10 shows the... 

Electron Transport Between Photosystem I and Photosystem II Inhibitors. The interaction between PSI and PSII reaction centers (Fig. 1) depends on the thermodynamically favored transfer of electrons from low redox potential carriers to carriers of higher redox potential. This process serves to communicate reducing equivalents between the two photosystem complexes. Photosynthetic and respiratory membranes of both eukaryotes and prokaryotes contain stmctures that serve to oxidize low potential quinols while reducing high potential metaHoproteins (40). In plant thylakoid membranes, this complex is usually referred to as the cytochrome b /f complex, or plastoquinolplastocyanin oxidoreductase, which oxidizes plastoquinol reduced in PSII and reduces plastocyanin oxidized in PSI (25,41). Some diphenyl ethers, eg, 2,4-dinitrophenyl 2 -iodo-3 -methyl-4 -nitro-6 -isopropylphenyl ether [69311-70-2] (DNP-INT), and the quinone analogues,... 

Insects poisoned with rotenone exhibit a steady decline ia oxygen consumption and the iasecticide has been shown to have a specific action ia interfering with the electron transport iavolved ia the oxidation of reduced nicotinamide adenine dinucleotide (NADH) to nicotinamide adenine dinucleotide (NAD) by cytochrome b. Poisoning, therefore, inhibits the mitochondrial oxidation of Krebs-cycle iatermediates which is catalysed by NAD. 

The principal route of macroHde excretion is by way of the Hver. Effects of macrohdes on hepatic metaboHc enzymes, particularly cytochrome P-450, have been studied in order to identify and reduce potential interference with metaboHsm of other dmgs (21—23,444—447). Several macrohdes are initially... 

In the third complex of the electron transport chain, reduced coenzyme Q (UQHg) passes its electrons to cytochrome c via a unique redox pathway known as the Q cycle. UQ cytochrome c reductase (UQ-cyt c reductase), as this complex is known, involves three different cytochromes and an Fe-S protein. In the cytochromes of these and similar complexes, the iron atom at the center of the porphyrin ring cycles between the reduced Fe (ferrous) and oxidized Fe (ferric) states. 

FIGURE 21.9 Typical visible absorption spectra of cytochromes, (a) Cytochrome c, reduced spectrum (b) cytochrome c, oxidized spectrum (c) the difference spectrum (a) minus (b) (d) beef heart mitochondrial particles room temperature difference (reduced minus oxidized) spectrum (e) beef heart submitochondrial particles same as (d) but at 77 K. a- and /3- bauds are labeled, and in (d) and (e) the bauds for cytochromes a, h and c are indicated. 

Cytochrome c, like UQ is a mobile electron carrier. It associates loosely with the inner mitochondrial membrane (in the intermembrane space on the cytosolic side of the inner membrane) to acquire electrons from the Fe-S-cyt C aggregate of Complex 111, and then it migrates along the membrane surface in the reduced state, carrying electrons to cytochrome c oxidase, the fourth complex of the electron transport chain. 

FIGURE 21.17 The electron transfer pathway for cytochrome oxidase. Cytochrome c binds on the cytosolic side, transferring electrons through the copper and heme centers to reduce O9 on the matrix side of the membrane. 

Cytochrome c oxidase contains two heme centers (cytochromes a and %) as well as two copper atoms (Figure 21.17). The copper sites, Cu and Cug, are associated with cytochromes a and respectively. The copper sites participate in electron transfer by cycling between the reduced (cuprous) Cu state and the oxidized (cupric) Cu state. (Remember, the cytochromes and copper sites are one-electron transfer agents.) Reduction of one oxygen molecule requires passage of four electrons through these carriers—one at a time (Figure... 

Write a balanced equation for the reduction of molecular oxygen by reduced cytochrome e as carried out by complex IV (cytochrome oxidase) of the electron transport pathway. 

FIGURE 22.17 The R. viridis reaction center is coupled to the cytochrome h/Cl complex through the quinone pool (Q). Quinone molecules are photore-duced at the reaction center Qb site (2 e [2 hv] per Q reduced) and then diffuse to the cytochrome h/ci complex, where they are reoxidized. Note that e flow from cytochrome h/ci back to the reaction center occurs via the periplasmic protein cytochrome co- Note also that 3 to 4 are translocated into the periplasmic space for each Q molecule oxidized at cytochrome h/ci. The resultant proton-motive force drives ATP synthesis by the bacterial FiFo ATP synthase. (Adapted from Deisenhofer, and Michel, H., 1989. The photosynthetic reaction center from the purple bac-terinm Rhod.opseud.omoaas viridis. Science 245 1463.)... 

This impressive reaction is catalyzed by stearoyl-CoA desaturase, a 53-kD enzyme containing a nonheme iron center. NADH and oxygen (Og) are required, as are two other proteins cytochrome 65 reductase (a 43-kD flavo-protein) and cytochrome 65 (16.7 kD). All three proteins are associated with the endoplasmic reticulum membrane. Cytochrome reductase transfers a pair of electrons from NADH through FAD to cytochrome (Figure 25.14). Oxidation of reduced cytochrome be, is coupled to reduction of nonheme Fe to Fe in the desaturase. The Fe accepts a pair of electrons (one at a time in a cycle) from cytochrome b and creates a cis double bond at the 9,10-posi-tion of the stearoyl-CoA substrate. Og is the terminal electron acceptor in this fatty acyl desaturation cycle. Note that two water molecules are made, which means that four electrons are transferred overall. Two of these come through the reaction sequence from NADH, and two come from the fatty acyl substrate that is being dehydrogenated. 

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