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You are watching: Which electron carrier(s) function in the citric acid cycle?

Berg JM, Tymoczko JL, Stryer L. Biochemistry. Fifth edition. Brand-new York: W H Freeman; 2002.

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Roundabouts, or traffic circles, duty as hubs come facilitate web traffic flow. The citric mountain cycle is the biochemical hub the the cell, oxidizing carbon fuels, generally in the type of acetyl CoA, and also serving as a source of precursors for biosynthesis. (more...)

From chapter 16, we recognize that glucose deserve to be metabolized to pyruvate anaerobically come synthesize ATP with the glycolytic pathway. Glycolysis, however, harvests but a fraction of the ATP easily accessible from glucose. We now start an exploration of the aerobic handling of glucose, i m sorry is the source of most of the ATP created in metabolism. The aerobic handling of glucose starts with the finish oxidation the glucose derivatives come carbon dioxide. This oxidation takes location in the citric acid cycle, a series of reactions also known together the tricarboxylic mountain (TCA) bicycle or the Krebs cycle. The citric mountain cycle is the final typical pathway for the oxidation the fuel molecules—amino acids, fat acids, and also carbohydrates. Many fuel molecules get in the cycle together acetyl coenzyme A.


Under aerobic conditions, the pyruvate created from glucose is oxidatively decarboxylated to form acetyl CoA. In eukaryotes, the reactions of the citric mountain cycle take location inside mitochondria, in comparison with those that glycolysis, i beg your pardon take ar in the cytosol (Figure 17.1).


Figure 17.1

Mitochondrion. The twin membrane of the mitochondrion is evident in this electron micrograph. The countless invaginations that the inside mitochondrial membrane are dubbed cristae. The oxidative decarboxylation that pyruvate and also the succession of reaction (more...)

17.0.1. Review of the Citric acid Cycle:

The citric mountain cycle is the central metabolic hub the the cell. It is the gateway come the aerobic line of any kind of molecule that can be transformed right into an acetyl team or dicarboxylic acid. The bike is also an important source that precursors, not only for the storage forms of fuels, but also for the structure blocks of many other molecule such as amino acids, nucleotide bases, cholesterol, and also porphyrin (the organic ingredient of heme).

What is the role of the citric mountain cycle in transforming fuel molecules into ATP? Recall the fuel molecules are carbon compounds that are capable of being oxidized—of shedding electrons (Chapter 14). The citric acid cycle contains a collection of oxidation-reduction reactions that result in the oxidation of one acetyl group to 2 molecules that carbon dioxide.

The overall pattern the the citric acid cycle is shown in number 17.2. A four- carbon link (oxaloacetate) condenses with a two-carbon acetyl unit to productivity a six-carbon tricarboxylic mountain (citrate). An isomer that citrate is then oxidatively decarboxylated. The result five-carbon link (α-ketoglutarate) also is oxidatively decarboxylated to productivity a four-carbon compound (succinate). Oxaloacetate is then regenerated from succinate. 2 carbon atoms get in the cycle together an acetyl unit and two carbon atoms leave the cycle in the type of 2 molecules of carbon dioxide. Three hydride ions (hence, six electrons) space transferred to three molecules of nicotinamide adenine di nucleotide (NAD+), conversely, one pair the hydrogen atom (hence, two electrons) is moved to one molecule the flavin adenine dinucleotide (FAD). The duty of the citric acid cycle is the harvesting of high-energy electrons from carbon fuels. Keep in mind that the citric mountain cycle chin neither generates a big amount of ATP nor includes oxygen together a reactant (Figure 17.3). Instead, the citric acid cycle removes electrons native acetyl CoA and also uses these electrons to kind NADH and also FADH2. In oxidative phosphorylation (Chapter 18), electrons exit in the reoxidation that NADH and also FADH2 flow through a collection of membrane proteins (referred to together the electron-transport chain) to generate a proton gradient across the membrane. This protons then circulation through ATP synthase to create ATP indigenous ADP and inorganic phosphate. Oxygen is compelled for the citric mountain cycle indirect inasmuch together it is the electron acceptor at the end of the electron-transport chain, vital to regenerate NAD+ and FAD.


Figure 17.2

Overview of the Citric acid Cycle. The citric mountain cycle oxidizes two-carbon units, creating two molecule of CO2, one molecule of GTP, and high-energy electrons in the type of NADH and FADH2.

Figure 17.3

Cellular Respiration. The citric acid cycle constitutes the an initial stage in to move respiration, the removal of high-energy electrons from carbon fuel (left). This electrons mitigate O2 to create a proton gradient (middle), which is used to synthesize (more...)

The citric mountain cycle, in conjunction with oxidative phosphorylation, provides the vast majority of energy used through aerobic cells—in human beings, greater than 95%. It is extremely efficient since a restricted number of molecules deserve to generate huge amounts the NADH and also FADH2. Keep in mind in figure 17.2 the the four-carbon molecule, oxaloacetate, that initiates the an initial step in the citric mountain cycle is regenerated in ~ the finish of one passage through the cycle. The oxaloacetate acts catalytically: that participates in the oxidation that the acetyl group yet is itself regenerated. Thus, one molecule that oxaloacetate is capable of participating in the oxidation of plenty of acetyl molecules.

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17.1. The Citric acid Cycle Oxidizes Two-Carbon Units17.2. Entry come the Citric mountain Cycle and also Metabolism v It are Controlled17.3. The Citric acid Cycle Is a resource of Biosynthetic Precursors17.4. The Glyoxylate Cycle allows Plants and Bacteria to prosper on AcetateSummaryProblemsSelected Readings

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