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Citric acid cycle

Terms in this set (16)

- Pyruvate derived from glucose can be split into CO2 and a two carbon fragment that enters the cycle for oxidation as acetyl-CoA.

- Citric acid cycle is not merely a continuation of carbohydrate catabolism. The citric acid cycle is a central pathway for recovery energy from several metabolic fuels, including carbohydrates, fatty acids, and amino acids, that are broken down to acetyl-CoA for oxidation.

- Under some conditions, the principal function of the citric acid cycle is to recover energy from fatty acids. Citric acid cycle also supplies the reactants for a variety of biosynthesis pathways.

- The citric acid cylce is a multistep catalytic process that converts acetyl groups derived from carbohydrates, fatty acids, and amino acids to CO2 and produces NADH, FADH2 and GTP.

- series of 8 reactions that oxidizes the acetyl group of acetyl-CoA to 2 molecules of CP2 in a manner that conserves the liberated free energy in the reduced compounds NADH and FADH2.

- The cycle is named after the product of its first reaction, citrate.

-**one complete round of the cycle yields 2 molecules of CO2, 3 NADH, 1 FADH2 and 1 high energy compound (GTP and ATP).

- The circular pathway (aka Krebs cycle or TCA cycle), **oxidizes acetyl groups from many sources, not just pyruvate (as it also oxidizes carb, fatty acid and AA)

** Net reaction
3NAD+ + FAD + GDP + Pi + acetyl-CoA -> 3NADH + FADH2 + GTP + CoA + 2CO2.

- the oxaloacetate that is consumed in the first step of the citric acid cycle is regenerated in the last step of the cycle.

- ** in eukaryotes, all the enzymes of the citric acid cycles are located in the mitochondria, so all substrates, including NAD+ and GDP, must be generated in the mitochondria or be transported into mitochondria from the cytosol. Similarly, all the products of the citric acid cycle must be consumed in the mitochondria or transported into the cytosol.

- The carbon atoms of the two molecules of CO2 produced in one round of the cycle are not the 2 carbons of the acetyl group that began the rounds of the cycle. However, the net effect of each round of the cycle is the oxidation of one acetyl group to 2 CO2.

- Citric acid cycle intermediates are precursors for the biosynthesis of other compounds (eg oxaloacetate for gluconeogenesis).

- The oxidation of an acetyl group to 2 CO2 requires the transfer of four paris of electrons. The reduction of 2 NAD+ to 3 NADH accounts for three pairs of electrons; the reduction of FAD to FADH2 accounts for the fourth pair. Much of the free energy of oxidation of the acetyl group is conserved in these reduced coenzymes. Energy is also recovered as GTP (or ATP).
- given the large amount of ATP that can potentially be generated from carb catabolism via the citric acid cycle, the entry of acetyl units driven from carb sources is regulated. Since the decarboxylation of pyruvate by the pyruvate dehydrogenase complex is irreversible, and since there are no other pathways in mammals for the synthesis of acetyl-CoA from pyruvate, it is crucial that the rx be precisely controlled.

1. Product inhibition by NADH and acetyl-CoA - these compounds compete w/ NAD+ and CoA for binding sites on their respective enzymes. They also drive the reversible E2 and E3 rx backwards.
- high [NADH]/[NAD+] and [acetyl-CoA]/[CoA] ratios therefore maintain E2 in a acetylated form, incapable of accepting the hydroxyethyl group from the TPP on E1. This, in turn, ties up the TPP on the E1 subunit in its hydroxyethyl form, decreasing the rate of pyruvate decarboxylatin.

2. Covalent modification by phosphorylation/dephosphorylation on E1 - in eukaryotes, the products of the pyruvate dehydrogenase rx, NADH and acetyl-CoA, also activate the pyruvate dehydrogenase kinase associated with the enzyme complex.
- The resulting phosphorylation of a specific dehydrogenase Ser residue inactivates the pyruvate dehydrogenase complex.
- Insulin, the hormone that signals fuel abundance, reverses the inactivation by activating pyruvate dehydrogenase phosphatase, which removes the phosphate groups from pyruvate dehydrogenase. Recall hat insulin also activates glycogen synthesis by activating phosphoprotein phosphatase. Thus, in response to increases in blood glucose, insulin promotes the synthesis of acetyl-CoA as well as glycogen.
- Other regulators of the pyruvate dehydrogenase system include pyruvate and ADP, which inhibit pyurvate dehydrogenase kinase, and Ca2+, which inhibits pyruvate dehydrogenase kinase and activates pyruvate dehydrogenase phosphatase. **In contrast to the the glycogen metabolism control system, pyruvate dehydrogenase activity is unaffected by cAMP.
- to understand how a metabolic pathway is controlled, we must identify the enzymes that catalyze its rate determining steps.

- identifying the rate determining steps of the citric acid cycle is more difficult that it is for glycolysis bc most of the cylce's metabolites are present in both mitochondria and cytosol (hard to determine their distribution between compartments).

- if we assume that the compartments are in equilibrium and use the total cell concentration of these substances to estimate their mitochondrial concentrations. table 17.2

- From the table, three of the enzymes are likely to fx far from equilibrium under physiologcial conditions - citrate synthase, NAD+ - dependent isocitrate dehydrogenase, and a-ketogluterate dehydrogenase. **These are therefore rate determining steps.

- Unlike the rate-limiting enzymes of glycolysis and glycogen metabolism, which regulate flux by elaborate systems of allosteric control, substrate cycles, and covalent modification, the regulatory enzymes of the citric acid cycle seem to control flux primarily by 3 simple mechanisms: 1. substrate availability 2. product inhibition 3. competitive feedback inhibition by intermediates further along the cycle. ** There is no single flux-control point in the citric acid cycle: rather, flux control is distributed among several enzymes.

- Perhaps the most crucial regulator of the citric acid cycle are its substrates, acetyl-CoA and oxaloacetate, and its product, NADH. Both acetyl-CoA and oxaloacetate are present in mitochondria at concentrations that do not saturate citrate synthase. The metabolic flux through the enzyme therefore varies with substrate concentration and is controlled by substrate availability. **We have already seen that the production of acetyl-CoA from pyruvate is regulated by the activity of pyruvate dehydrogenase. The concentration of oxaloacetate, which is in equilibrium with malate, fluctuate with the [NADH]/[NAD+]. Equilibrium expression would be
k= [oxaloacetate][NADH]/[malate][NAD+].
- If, for example, the muscle workload and respiration rate increases, mitochondria [NADH] decreases. The consequent increase in [oxaloacetate] stimulates the citrate synthase rx, which controls the rate of citrate formation.
- Aconitase fx close to equilibrium, so the rate of citrate consumption depends on the activity of NAD+-dependent isocitrate dehydrogenase, which is strongly inhibited by NADH but is less sensitive than isocitrate dehydrogenase to changes in [NADH].
- Other instances of product inhibition in the citric acid cycle are the inhibition of citrate synthase by citrate (citrate competes with oxaloacetete) and the inhibition of a-ketoglutarate dehydrogenase by NADH and succinyl-CoA. Succinal-CoA also competes with acetyl-CoA in the citrate synthase rx (competitive feedback inhibition). This interlocking system helps keep the citric acid cycle coordinately regulated and the concentrations of its intermediates within reasonable bounds.