Cycling reactions

Pyruvate produced by glycolysis is a significant source of acetyl-CoA for the TCA cycle. Because, in eukaryotic ceils, glycolysis occurs in the cytoplasm, whereas the TCA cycle reactions and ail subsequent steps of aerobic metabolism take place in the mitochondria, pyruvate must first enter the mitochondria to enter the TCA cycle. The oxidative decarboxylation of pyruvate to acetyl-CoA,... 

FIGURE 20.28 The glyoxylate cycle. The first two steps are identical to TCA cycle reactions. The third step bypasses the C09-evolving steps of the TCA cycle to produce snc-cinate and glyoxylate. The malate synthase reaction forms malate from glyoxylate and another acetyl-CoA. The result is that one torn of the cycle consumes one oxaloacetate and two acetyl-CoA molecnles bnt produces two molecnles of oxaloacetate. The net for this cycle is one oxaloacetate from two acetyl-CoA molecnles. 

Aconitase catalyzes the citric acid cycle reaction citrate isocitrate... 

Balancing the Calvin Cycle Reactions To Account for Net Hexose Synthesis... 

After revising the TCA cycle reactions in more detail we shall return to the subject of metabolic control by ATP. 

The principal carbon-cycle reaction chemistries are shown in the following equations, together with reactant molecular weights. 

Some phosphate-cycle reactions are shown below, and, although for the sake of simplicity only calcium phosphate is shown as a precipitant, depending on the operational circumstances, the reaction produces either tricalcium phosphate, hydroxyapatite, or a combination of both salts. 

Consider the NO/N02-catalyzed ozone destruction cycle, reactions 5 and 6 in Section 5.4.3. One could perform a calculation to determine which reaction is the rate-limiting step (i.e., the slowest step that determines the rate of the overall reaction) in this cycle. In this case, a theoretical doubling of ks reduces the ozone concentration by about 2%. On the other hand, doubling kf, reduces the ozone concentration by nearly 50%. (a) Which reaction is the rate-limiting step in N0/N02-catalyzed ozone destruction (b) The concentrations of NO and NO2 are [NO] = 2.9 X 10 /cm and [NO2] = 6.1 x 10 /cm. How do these data support or refute your answer to (a) ... 

Enzyme activity (urease, amidase, dehydrogenase, pl-glucosidase, phosphatase, arylsulfatase, fluorescein diacetate hydrolysis) Laboratory incubation Indicates potential microbial activity and nutrient cycling reactions determined in nonstandard laboratory with specialized equipment highly spatially and temporally variable dependent upon organic inputs Dick et al. (1996) Parham et aL (2002)... 

S02 is similarly a known inhibitor that affects flammability limits. These catalytic cycles [reactions (4.69)-(4.70), reactions (4.71)-(4.72), and reactions (4.73)—(4.75)] are equivalent to... 

The rate of reaction (8.165) is known accurately only at room temperature, and extrapolation to stratospheric temperature is uncertain nevertheless, the extrapolated values indicate that the N03 catalytic cycle [reactions (8.165) and (8.166)] destroys ozone faster than the N02 cycle below 22 km and in the region where the temperature is at least 220 K. 

The exploratory studies, as conducted, did not distinguish between effects Imposed on the stromal-associated CO2 fixation (Calvin cycle) reactions or on the light reactions associated with the thylakoids. Consequently, studies were conducted on light-induced electron transport and ATP synthesis associated with isolated spinach thylakold membranes. 

In comparison to the S-I and UT-3 cycles the Cu-Cl cycle is attractive due to the significantly lower reaction temperatures [69,111]. The cycle reactions consists of [66] ... 

Fig. 5. Illustration of the 5 -nucleotidase (TaqMan) assay for allele discrimination. (A) The allele discrimination assay employs two unlabeled PCR primers and two doubly fluorescent labeled PCR probes for visuaUzation of a mutant allele. The target sequence is initially denatured and amplified in the presence of each of the primers and probes. Increasing polymerization in the presence of a thermostable polymerase which contains a 5 proofreading function allows cleavage of one fluorescent indicator from an appropriate probe during the cycling reaction. (B) Probes are designed with a fluorescent reporter and a quencher moiety. AmpUfication reactions are spiked with additional fluorescent quenchers in order to render the reaction initially dark to the photomultipUer mbe or diode. The probes are designed...

The carboxylation reaction shown in reaction (11) is catalyzed by both nickel and palladium phosphine complexes. For example, Ni(dppe)Cl2 (where dppe is l,2-bis(diphenylphosphino)ethane) and Pd(PPh3)2Cl2 both catalyze reaction (11) [84-86]. Mechanistic studies have been carried out on these two systems, and the results indicate that two different mechanisms are involved. In the case of the Ni complex, the first step is the reduction of Ni(dppe)Cl2 to a transient Ni(dppe) species [85]. This process occurs in two one-electron steps (reaction 12). Bromobenzene then oxidatively adds to Ni(dppe) to form Ni(dppe)(Br)(Ph), reaction (13). The resulting Ni(II) aryl species is reduced in a one-electron process to form Ni(dppe)(Ph), which reacts rapidly with CO2 to form a Ni—CO2 intermediate as shown in reaction (14). The rate-determining step for the overall catalytic reaction is the insertion of CO2 into the Ni-aryl bond, reaction (15) step 1. This reaction is followed by a final one-electron reduction to regenerate Ni(dppe), the true catalyst in the cycle (reaction 15, step 2). 

However, at the tropopause the temperature profile changes, increasing with altitude throughout the stratosphere. The reason for this increase is a critical series of photochemical reactions involving ozone and molecular oxygen. The Chapman cycle, reactions (l)-(4), hypothesized in the 1930 s by Sir Sydney Chapman,... 

As described earlier, in the stratosphere, a steady-state concentration of 03 is produced naturally by the Chapman cycle, reactions (l)-(4). Until about 1970, relatively little attention was paid to potential anthropogenic (i.e., man-made) perturbations of the stratosphere. At that time, Crutzen (1970) examined the potential role of NO and N02 formed in the stratosphere from reactions of N20 that was originally generated at the earth s surface. Because N20 is unre-active in the troposphere, it has a sufficiently long lifetime to end up in the stratosphere, where it can be converted into NO (see Chapter 12). Crutzen (1970) proposed that the NO and N02 formed from reactions of N20 can then participate in a chain reaction that destroys 03 ... 

Crutzen and co-workers (Sander and Crutzen, 1996 Vogt et al., 1996) have developed a model for chemistry in the marine boundary layer at midlatitudes, in which autocatalytic cycles involving sea salt particles generate photochemically active gases such as BrCl, Br2, and Cl2. It is likely that such chemistry also occurs in the Arctic as well. In these cycles, reactions (125) and (126) in the condensed phase,... 

Start-of-cycle kinetic lumps in KINPTR are summarized in Table V. A C5-light gas lump is required for mass balance. Thirteen hydrocarbon lumps are defined. The reforming kinetic behavior can be modeled without splitting the lumps into their individual isomers (e.g., isohexane and n-hexane). Also, the component distribution within the C5- lump can be described by simple correlations, as discussed later. The start-of-cycle reaction network that defines the interconversions between the 13 kinetic lumps is shown in Fig. 9. This reaction network results from kinetic studies on pure components and narrow boiling fractions of naphthas. It includes the basic reforming reactions... 

Nitrous oxide is nontoxic—it used as the propellant in whipped-cream spray cans—and so might seem to be an unlikely pollutant. However, as noted earlier, it may contribute significantly to greenhouse warming. Furthermore, on diffusing to the stratosphere, N20 becomes involved in the ozone cycle (reactions 8.2, 8.3, and 8.6) following its conversion to nitric oxide (NO) ... 

A nucleophilic addition to the triple bond in alkynylphosphine derivatives was observed by Laguna and Bardaji and although there was no evidence for a gold catalyzed cycle, reaction conditions were extremely milder than in the classic Reppe vinylation [94] (Scheme 8.12). 

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