Electron chains

In the presence of an excess of non-reducible soft nucleophile (for example PhS- or PhSe -), a new reducing species may be formed which allows in certain cases a propagation cycle (electron chain catalysis). 

Figure 6. Pathways of protons and electrons during mitochondrial oxidations. The diagrams show the pathways of electrons which enter the electron chain at the level ofcomplexi (a)orcomplex II (b). Complexes I, III, and IV usethefreeenergy of electron transport to pump protons out of the matrix. This diagram also distinguishes formally between protons released by dehydrogenation and those which are pumped out of the matrix, although they all enter or leave the same pool.

In monooxygenation, the P-450 hemoprotein receives two electrons from cofactor NADPH or NADH, or both these electrons are received one at a time, usually via reductases (flavoproteins). In most organisms, the electron chain is deeply embedded principally in the endoplasmic reticulum (and to some degree in the inner mitochondrial membrane and nuclear envelope). After sheering of the membrane during homogenization, the endoplasmic reticulum is centrifuged at 100,000 for an hour, and the product is called the "microsomal pellet." The microsomal electron chain contains reductase and P-450. 

The Co(f-butylsalophen) complex entrapped in zeolite Y was shown to be an essential member of electron chains designed for alcohol oxidation11301 and for the acetoxylation of 1,3-dienes with dioxygen.1131 1321 The Co(f-butylsalophen)Y... 

A variety of alkaloids bind to or intercalate with DNA or DNA/RNA processing enzymes and affect either transcription or replication (quinine, harmane alkaloids, melinone, berberine), act at the level of DNA and RNA polymerases (vinblastine, coralyne, avicine), inhibit protein synthesis (sparteine, tubulosine, vincrastine, lupanine), attack electron chains (pseudane, capsaicin, solenopsine), disrupt biomembranes and transport processes (berbamine, ellipticine, tetrandrine), and inhibit ion channels and pumps (nitidine, caffeine, saxitoxin). In addition, these natural products attack a variety of other systems that can result in serious biochemical destabilization... 

For a number of alkaloids the mechanisms underlying the toxic effects have already been elucidated in some detail. We can distinguish molecular targets and processes that are important for all cells, such as synthesis of DNA, RNA, and proteins, replication, transcription, translation, membrane assembly and stability, electron chains, or metabolically important enzymes or proteins including receptors, hormones, and signal compounds (Table IV). In the following we discuss some of these toxic effects. 

Molecular Targets of Alkaloids Proteins, Nucleic Acids, Biomembranes, and Electron Chains... 

Electron chains. The respiratory chain and ATP synthesis in mitochondria demand the controlled flux of electrons. This target seems to be attacked by ellipticine, pseudane, pseudene, alpinigenine, sanguinarine, tetrahydropalmatine, CH3-(CH2)i4-2,6-methyl-piperidines, capsaicin, the hydroxamic acid DIMBOA, and solenopsine. As mentioned before, however, only a few alkaloids have been evaluated in this context (Table V). 

Figure 19.10. Electron Chain in the Photosynthetic Bacterial Reaction Center. The absorption of light by the special pair (P960) results in the rapid transfer of an electron from this site to a bacteriopheophytin (BPh), creating a photoinduced charge separation (steps 1 and 2). (The asterisk on P960 stands for excited state.) The possible return of the electron from the pheophytin to the oxidized special pair is suppressed by the "hole" in the special pair being refilled with an electron from the cytochrome subunit and the electron from the pheophytin being transferred to a quinone (Q ) that is farther away from the special pair (steps 3 and 4). The reduction of a quinone (Qg) on the periplasmic side of the membrane results in the uptake of two protons from the periplasmic space (steps 5 and 6). The reduced quinone can move into the quinone pool in the membrane (step 7).

The plastoquinol (QH2) produced by photosystem II contributes its electrons to continue the electron chain that terminates at photosystem I. These electrons are transferred, one at a time, to plastocyanin (Pc), a copper protein in the thylakoid lumen. 

Uncoupling agents are compounds that block ATP synthesis while allowing the respiration of the electron chain to continue. Which of the following would explain this ... 

Zeolite Y encaged Co and Ru(salophen) were very active catalysts in the oxidation of a-pinene (see scheme). For Ru at 100°C and 3 MPa of air, TOF of over 18,000 have been observed, which were a factor 2 higher than the homogeneous complex [67], The presence of a highvalenl Ruv 0 was proposed for the pincnc epoxide product, while the presence of the allylic oxidation product (D-verbenonc) points to the presence of a radical pathway. The same encapsulated Co(salophen) complex has also been used in the electron chain designed for the oxidation of alcohols [68]. 

Two h-type cytochromes are present in green plant photosynthesis 6559 and buz or bt. The former has traditionally been placed between plastoquinone and cytochrome / in the electron chain, and 69 has been placed between ferredoxin and cytochrome / in cyclic photophosphorylation (291). Both of these assignments have been challenged recently (292-295), and we must simply note the controversy and drop the issue for the purposes of this chapter. 

In all of the systems discussed so far, the donor enters the electron chain to the acceptor at a point prior to cytochrome c that is, cytochrome c has a reduction potential between that of the donor and the acceptor, and hence is capable of acting as a valve for regulating flow. 

The best understood system of dissimilatory nitrate metabolism is that of Pseudomonas denitrificans, with the proposed transport scheme shown in Fig. 39 (406 408). The nitrogen intermediates shown between nitrate and Nz are those which have been isolated and identified with a particular reductase enzyme. There remains the possibility of additional intermediates in the form of transient oxidation states of nitrogen. The single electron chain of Oz respiration here has been replicated into parallel pathways, each with the general form donor 6 —> c — reductase acceptor. On the NzO reductase path, the particulate cytochrome Cb52... 

Fig. 3. Desaturation of stearoyl-CoA by stearoyl-CoA desaturase (SCD). The reaction is dependent on Oj and a short electron chain consisting of NAD(P)H, cytochrome h, reductase and cytochrome by...

B. Incorporation of Carotenoids Having Different jr-Electron Chain Lengths.238... 

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