4.2: Tricarboxylic acid cycle (TCA)

The TCA cycle is responsible for generating over half of the ATP from the oxidation of fuels. This is primarily because the substrate for the TCA cycle, acetyl-CoA, is generated by the oxidation of fatty acids, glucose, amino acids, and ketone bodies. With each turn of the TCA cycle, there is a net production of three NADH, FADH₂, two \(\ce{CO2}\), and one GTP for every acetyl-CoA that enters (figure 4.10).

Figure 4.10: Overview of the TCA cycle.

The other major role of the TCA cycle is to provide substrates for other synthetic pathways. Malate is shuttled out of the TCA cycle and used as a substrate for gluconeogenesis and oxaloacetate (OAA) and \(\alpha\)-ketoglutarate are used as substrates for amino acid synthesis. Through transamination reactions, these two keto-acids can be converted into aspartate and glutamate, respectively. \(\alpha\)-ketoglutarate is also a key substrate for the synthesis of neurotransmitters, and succinyl-CoA is the substrate for heme synthesis. Citrate is also a key compound as it is both an intermediate of the TCA cycle and can be shuttled into the cytosol to provide acetyl-CoA for both cholesterol and fatty acid synthesis (figure 4.11).

Figure 4.11: Substrates produced by the TCA cycle.

In order to maintain a pool of TCA cycle intermediates, as substrates are removed from the cycle, there are several key reactions (anaplerotic reactions) that are responsible for the addition of intermediates. These reactions are illustrated in figure 4.12. Notice all of these reactions add carbon back to the cycle from amino acids (reactions 1, 2, 3, 4, 5). These will become very important in the discussion of gluconeogenesis where these substrates will provide the majority of carbon for glucose production. Odd chain fatty acid oxidation can also provide carbon in the from of propionyl-CoA (3) although this is not a primary source of TCA cycle intermediates.

Figure 4.12: Anaplerotic reactions of the TCA cycle.

Regulation of the TCA cycle

Throughout the cycle, there are two key regulatory and irreversible steps to be aware of. The first is the conversion of isocitrate to \(\alpha\)-ketoglutarate by isocitrate dehydrogenase, and the second is the conversion of \(\alpha\)-ketoglutarate to succinyl-CoA by \(\alpha\)-ketoglutarate dehydrogenase. The two key regulatory points are:

Isocitrate dehydrogenase, which can be activated by Ca²⁺ and ADP to increase flux through the cycle, and inhibited by NADH, which would suggest adequate energy in the cell.

Likewise, \(\alpha\)-ketoglutarate dehydrogenase can be activated by Ca²⁺ and inhibited by NADH (and succinyl-CoA) (figure 4.13).

Figure 4.13: Regulation of the TCA cycle.

Malate dehydrogenase can also be inhibited by NADH, however, the reaction is reversible depending on levels of NADH. The oxidation of malate to OAA requires NAD⁺, and under certain pathological situations the lack of free NAD⁺ within the mitochondria will reduce the rate of this reaction (this is common in the case of alcohol metabolism).

Keep in mind that with the addition of each acetyl-CoA (comprised of 2 carbons) to the TCA cycle, two molecules of \(\ce{CO2}\) are released, thus there is no net gain or loss of carbons in the cycle. The process moves forward driven by energetics and substrate availability. The pathway can be active in both the fed and fasted states. In the fed state, acetyl-CoA is generated primarily through glucose oxidation. In contrast, in the fasted state acetyl-CoA is generated primarily from \(\beta\)-oxidation, and the majority of acetyl-CoA is used to synthesize ketones.

Summary of pathway regulation

Metabolic pathway	Major regulatory enzyme(s)	Allosteric effectors	Hormonal effects
TCA cycle	Isocitrate dehydrogenase	ADP, Ca+ (+) NADH (-)
	Alpha-ketoglutarate dehydrogenase	Ca+ (+) NADH (-)

Table 4.2: Summary of pathway regulation.

References and resources

Text

Ferrier, D. R., ed. Lippincott Illustrated Reviews: Biochemistry, 7th ed. Philadelphia: Wolters Kluwer Health/Lippincott Williams & Wilkins, 2017, Chapter 6: Bioenergetics and Oxidative Phosphorylation: Section V, VI, Chapter 8: Introduction to Metabolism and Glycolysis, Chapter 9: TCA Cycle and Pyruvate Dehydrogenase Complex: Section IIA, IIB, Chapter 11: Glycogen Metabolism: Section V, VI, Chapter 16: Fatty Acid Ketone Body and TAG Metabolism: Section II, IV, V, Chapter 23: Metabolic Effect of Insulin and Glucagon, Chapter 25: Diabetes Mellitus.

Le, T., and V. Bhushan. First Aid for the USMLE Step 1, 29th ed. New York: McGraw Hill Education, 2018, 72–78, 85–89.

Lieberman, M., and A. Peet, eds. Marks' Basic Medical Biochemistry: A Clinical Approach, 5th ed. Philadelphia: Wolters Kluwer Health/Lippincott Williams & Wilkins, 2018, Chapter 2: The Fed or Absorptive State, Chapter 19: Basic Concepts of Regulation: Section IV.A.1.2, Chapter 20: Cellular Bioenergetics, Chapter 22: Generation of ATP from Glucose: Section I.A.B.C, III, Chapter 24: Oxidative Phosphorylation and the ETC: Section I.E, II, III, Chapter 31: Synthesis of Fatty Acids: Section I.A.B, IV, V.

Figures

Grey, Kindred, Figure 4.10 Overview of the TCA cycle. 2021. https://archive.org/details/4.10-new. CC BY 4.0.

Grey, Kindred, Figure 4.11 Substrates produced by the TCA cycle. 2021. https://archive.org/details/4.11_20210924. CC BY 4.0.

Lieberman M, Peet A. Figure 4.12 Anaplerotic reactions of the TCA cycle. Adapted under Fair Use from Marks' Basic Medical Biochemistry. 5th Ed. pp 473. Figure 23.18 Major anaplerotic pathways of the tricarboxylic acid (TCA) cycle. 2017.

Lieberman M, Peet A. Figure 4.13 Regulation of the TCA cycle. Adapted under Fair Use from Marks' Basic Medical Biochemistry. 5th Ed. pp 468. Figure 23.11 Major regulatory interaction in the tricarboxylic acid (TCA) cycle. 2017. Added ion channel by Léa Lortal from the Noun Project.