Pathways cyclic

Reaction (12-9) shows the photochemical dissodation of NO2. Reaction (12-10) shows the formation of ozone from the combination of O and molecular O2 where M is any third-body molecule (principally N2 and O2 in the atmosphere). Reaction (12-11) shows the oxidation of NO by O3 to form NO2 and molecular oxygen. These three reactions represent a cyclic pathway (Fig. 12-4) driven by photons represented by hv. Throughout the daytime period, the flux of solar radiation changes with the movement of the sun. However, over short time periods (—10 min) the flux may be considered constant, in which case the rate of reaction (12-9) may be expressed as... 

Photolysis of the nitrosimines, such as 35a, gives rise to a variety of products that appear to have come from loss of NO and subsequent radical reactions (Scheme 3.22) [196]. Similar products, indicative of radical reactions, are also observed in the thermolysis of sterically hindered nitrosimines (e.g., 40, with Rj = tert-Bu and R2 = 2-MePh) [197]. Steric constraints were proposed to disfavor the cyclic pathway... 

This is a relatively simple cyclical pathway consisting of only four intermediates. However, to complete the structure of urea ((NH2)2CO) a second NH2 group must be introduced and this is achieved by aspartate acting as the donor. 

Oxidation is a cyclical pathway which removes a C2 (acetyl) unit form the fatty acyl-CoA on each cycle. The designation [3 derives from the traditional system of labelling atoms within fatty acid molecules where the carbon attached to the carboxyl group is a and the methyl carbon is always CO (omega) ... 

Isomerization—wherever the structure of the reactant permits the C5 cyclic pathway—is accelerated by hydrogen (27, 27a). The parallelism between the isomer and C5 cyclic yields (27a, 62), and also the composition of isomers (91-92) indicate a prevailing C5 cyclic pathway of isomerization in the presence of hydrogen. 

Kazansky et al. (5) estimated the role of C5 cyclic intermediates in aromatization to be about 5% over platinum on carbon. Dautzenberg and Platteeuw found about 11% C5 cyclic pathway with nonacidic platinum on alumina (23). 2,2,4-Trimethylpentane is forced to produce aromatics via C5 cyclization because of its structure here the quaternary carbon atom facilitates ring enlargement (5, 23). 

Previous tracer studies (22, 23) have ruled out the pictured analogous cyclic pathway for formation of methyl acetate. 

FIGURE 4-30 Chaperones in protein folding. The cyclic pathway by which chaperones bind and release polypeptides is illustrated for the . coli chaperone proteins DnaK and DnaJ, homologs of the eukaryotic chaperones Hsp70 and Hsp40. The chaperones do not actively promote the folding of the substrate protein, but instead prevent aggregation of unfolded peptides. For a population of polypeptides, some... 

Stage 3 Regeneration of Ribulose 1,5-Bisphosphate from Triose Phosphates The first reaction in the assimilation of C02 into triose phosphates consumes ribulose 1,5-bisphosphate and, for continuous flow of C02 into carbohydrate, ribulose 1,5-bisphosphate must be constantly regenerated. This is accomplished in a series of reactions (Fig. 20-10) that, together with stages 1 and 2, constitute the cyclic pathway shown in Figure 20-4. The product of... 

Thromboxanes, like prostaglandins, contain a ring of five or six atoms the pathway from arachidonate to these two classes of compounds is sometimes called the cyclic pathway, to distinguish it from the linear pathway that leads from arachidonate to the leukotrienes, which are linear compounds (Fig. 21-16). Leukotriene synthesis begins with the action of several lipoxygenases that catalyze the incorporation of molecular oxygen into arachidonate. These enzymes, found in leukocytes and in heart, brain, lung, and spleen, are mixed-function oxidases that use cytochrome P-450 (Box 21-1). The various leukotrienes differ in the position of the peroxide... 

FIGURE 21-15 The "cyclic" pathway from arachidonate to prostaglandins and thromboxanes, (a) After arachidonate is released from phospholipids by the action of phospholipase A2/ the cyclooxygenase and peroxidase activities of COX (also called prostaglandin H2 synthase) catalyze the production of PGH2/ the precursor of other... 

Show how this reaction can be incorporated into an ATP-driven cyclic pathway for generating NADPH from NADH. 

In purple photosynthetic bacteria, electrons return to P870+ from the quinones QA and QB via a cyclic pathway. When QB is reduced with two electrons, it picks up protons from the cytosol and diffuses to the cytochrome bct complex. Here it transfers one electron to an iron-sulfur protein and the other to a 6-type cytochrome and releases protons to the extracellular medium. The electron-transfer steps catalyzed by the cytochrome 6c, complex probably include a Q cycle similar to that catalyzed by complex III of the mitochondrial respiratory chain (see fig. 14.11). The c-type cytochrome that is reduced by the iron-sulfur protein in the cytochrome be, complex diffuses to the reaction center, where it either reduces P870+ directly or provides an electron to a bound cytochrome that reacts with P870+. In the Q cycle, four protons probably are pumped out of the cell for every two electrons that return to P870. This proton translocation creates an electrochemical potential gradient across the membrane. Protons move back into the cell through an ATP-synthase, driving the formation of ATP. 

Anaplerotic reaction. An enzyme-catalyzed reaction that replenishes the intermediates in a cyclic pathway. 

This cyclic pathway was first proposed for Chlorobium sp. that use an anoxygenic photosynthesis for energy supply [3]. It reverses the reactions of the oxidative citric acid cycle (Krebs cycle) and forms acetyl-CoA from two C02 (Figure 3.2). 

Figure 2. Schematic of photoinduced electron transport and phosphorylation reactions considered to occur in chloroplast lamellae [from Moreland and Hilton (2)]. Open arrows indicate light reactions solid arrows indicate dark reactions and the narrow dashed line represents the cyclic pathway. Abbreviations used PS I, photosystem I PS II, photosystem II Y, postulated electron donor for photosystem II Q, unknown primary electron acceptor for photosystem II PQ, plastoquinones cyt b, b-type cytochromes cyt f, cytochrome f PC, plastocyanin P700, reaction center chlorophyll of photosystem I FRS, ferredoxin-reducing substance Fd, ferredoxin Fp, ferredoxin-NADP oxidoreductase FeCy, ferricyanide asc, ascorbate and DPIP, 2,6-dichloropheno-lindophenol. The numbers la, lb, 2, 3, and 4 indicate postulated sites of action by...

This an example of coarctate reaction in which two bonds are simultaneously formed and broken at an atom not following a cyclic pathway.94 The 13C KIEs for fragmentation of 50 at 150°C in diphenyl ether were determined for substrate at 65-84% conversion (Figure 19). 

Nitrogen undergoes a complicated series of cyclic pathways in the ecosystem (Fig. 8.2). The atmospheric form of free nitrogen must be fixed —incorporated into chemical compounds (e.g., NH3) which can be utilized by plants. This nitrogen fixation can be accomplished by bacterial action of both free-living soil bacteria such as azotobacter and chlostridium and symbiotic bacteria such as rhizobium. It can also be... 

The mechanistic aspects of LTA oxidation are more complicated and the results indicate several pathways dependent on the steric environment of the glycols. In cases where geometry is favorable, oxidative scission via a cyclic intermediate (11) proceeds by a two-electron transfer (pad) a, Scheme 7). With trans-diols possessing antiperiplanar hydroxy groups, which for steric reasons cannot form the lead(IV) cyclic intermediate (11), an alternative cyclic pathway consisting of an intramolecular proton transfer in... 

As far as we know, the cyclic electron transfer pathways in Chloroflexus are similar to those found in purple bacteria [13,35]. As shown in Fig. 3, there appear to be cyclic pathways involving b- and c-type cytochromes, Fe-S centers, and qui-nones in both kinds of bacteria grown phototrophically. However, Chloroflexus contains only MQ [32], while any given purple bacterium will always contain ubiquinone (UQ) with or without some MQ. On the electron acceptor side of the RC in Chloroflexus there are apparently two MQ molecules in series (MQ and MQb), which are assumed to feed electrons into an MQ pool [30,36]. Electrons presumably enter a Cyt b c complex from the MQ pool and leave the complex via Cyt c = 0.21 V) and Cyt c-554 j = 0.28 V) [35]. The membrane-bound Cyt c-554 ( m,8 +0.26 V, = 43 000) [37] is the direct electron donor to P-865 in the RC [22,30,36]. It contains two hemes with redox potentials of +0.14 and +0.26 V respectively [36] and is absent from aerobically grown cells [38]. 

The conversion of n-hexane over various Pt-zeolite catalysts responded strongly to changes in hydrogen pressures [13-15]. Increasing the hydrogen/n-hexane ratio promoted metal catalyzed skeletal isomerization, mainly by the C5-cyclic pathway and shifted also the fragment composition towards vmues typical of metal-catalyzed hydrogenolysis. 

Calvin cycle. The cyclic pathway used by plants to fix carbon dioxide and produce triose phosphates. Named after Melvin Calvin, an early worker in the field. 

Examples of photorearrangement arising by a 4tt electro-cyclic pathway have been described. The low quantum efficiences... 

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