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Regulation of TCA cycle- Lecture-3
A) Entry of substrates into the cycle – Fuel enters the TCA cycle primarily as acetyl-CoA. The generation of acetyl-CoA from carbohydrates is, therefore, a major control point of the cycle. This is the reaction catalyzed by the PDH complex.
Regulation of PDH Complex– PDH complex is inhibited by acetyl-CoA and NADH and activated by non-acetylated CoA (CoASH) and NAD+. The pyruvate dehydrogenase activities of the PDH complex are regulated by their state of phosphorylation (Figure-1) . This modification is carried out by a specific kinase (PDH kinase) and the phosphates are removed by a specific phosphatase (PDH phosphatase). The phosphorylation of PDH inhibits its activity and, therefore, leads to decreased oxidation of pyruvate. PDH kinase is activated by NADH and acetyl-CoA and inhibited by pyruvate, ADP, CoASH, Ca2+ and Mg2+. The PDH phosphatase, in contrast, is activated by Mg2+ and Ca2+. In a tissue such as brain, which is largely dependent on carbohydrate to supply acetyl-CoA, control of the citric acid cycle may occur at pyruvate dehydrogenase.
Figure -1-Regulation of PDH complex by covalent modification. Activation of PDH kinase causes inactivation of PDH, whereas activation of PDH phosphatase bring about activation of enzyme
B) Key reactions of the cycle– Key regulation of the cycle occurs by regulation of the individual enzymes of TCA cycle and by respiratory control-
i) Regulation of TCA cycle enzymes-The most likely sites for regulations are the nonequilibrium reactions catalyzed citrate synthase, isocitrate dehydrogenase, and α-ketoglutarate dehydrogenase. The dehydrogenases are activated by Ca2+, which increases in concentration during muscular contraction and secretion, when there is increased energy demand.
a) Citrate synthase– There is allosteric inhibition of citrate synthase by ATP and long-chain fatty acyl-CoA .
b) Isocitrate dehydrogenase- is allosterically stimulated by ADP, which enhances the enzyme’s affinity for substrates. In contrast, NADH inhibits iso-citrate dehydrogenase by directly displacing NAD+. ATP, too, is inhibitory (Figure-2) .
c) α-ketoglutarate dehydrogenase -α- Ketoglutarate dehydrogenase is inhibited by succinyl CoA and NADH, the products of the reaction that it catalyzes. In addition, α-ketoglutarate dehydrogenase is inhibited by a high energy charge. Thus, the rate of the cycle is reduced when the cell has a high level of ATP (Figure-2).
d) Succinate dehydrogenase is inhibited by oxaloacetate, and the availability of oxaloacetate, as controlled by malate dehydrogenase, depends on the [NADH]/[NAD+] ratio.
Figure-2- Regulation of TCA cycle. Excess of ATP depicts energy rich state of the cell, hence TCA cycle is inhibited while reverse occurs when the cell is in a low energy state with excess of ADP.
ii) Respiratory control of TCA cycle- Since three reactions of the TCA cycle as well as PDH utilize NAD+ as co-factor it is not difficult to understand why the cellular ratio of NAD+/NADH has a major impact on the flux of carbon through the TCA cycle. Thus, activity of TCA cycle is immediately dependent on the supply of NAD+, which in turn, because of the tight coupling between oxidation and phosphorylation, is dependent on the availability of ADP and hence, ultimately on the rate of utilization of ATP in chemical and physical work. Thus, respiratory control via the respiratory chain and oxidative phosphorylation primarily regulates citric acid cycle activity.
a) Low energy state- Under conditions of low energy state, TCA cycle activity is stimulated to restore the energy balance. This mechanism of regulation can be explained as follows-
- Low ATP/High ADP concentration
- More ADP available for phosphorylation
- Higher rate of oxidative phosphorylation
- Higher rate of electron transport
- More availability of oxidized coenzymes
- More active TCA cycle enzymes
b) High energy state- Under conditions of high energy state, TCA cycle activity is inhibited . This mechanism of regulation can be explained as follows-
- High ATP/Low ADP concentration
- Less ADP available for phosphorylation
- Lower rate of oxidative phosphorylation
- Decreased rate of electron transport
- Less availability of oxidized coenzymes
- TCA cycle enzymes are inhibited.