The citric acid cycle (a.k.a. tricarboxylic acid [TCA] cycle or Krebs cycle) is arguably one of the most difficult topics to try and master in medical school or any medical profession. It's about time for a better way to study it! Follow our Krebs cycle study guide below using the MedStudy Method to remember it for the long term.Preview these questions on the Krebs cycle
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CITRIC ACID CYCLE — OVERVIEW
The citric acid cycle (a.k.a. tricarboxylic acid [TCA] cycle or Krebs cycle, for Hans Adolph Krebs who identified it in 1937) is a series of biochemical reactions used by all aerobic organisms to produce energy from carbohydrates, protein, and fat. This occurs through the oxidation of acetyl-CoA, leading to ATP production. In contrast to glycolysis, the citric acid cycle occurs in mitochondria.
This process was named the citric acid cycle because it begins with conversion of acetyl-CoA to citric acid (a.k.a. citrate). The citrate is consumed and eventually regenerated to continue the cycle. Citrate is a type of TCA.
Think of the citric acid cycle as the biochemical crossroads of the cell. First, it is the final common pathway for the oxidation of fuel, meaning carbohydrates, amino acids, and fatty acids. Any molecule that can be broken down into an acetyl group or dicarboxylic acid can enter this pathway to produce ATP. Recall that glycolysis produces only a small amount of ATP from glucose. Further processing in the citric acid cycle (in conjunction with oxidative phosphorylation) is the main source of ATP generated in metabolism, accounting for about 95% of energy in aerobic cells. Second, there are many important byproducts of the reactions in the citric acid cycle. These include precursors for amino acids, nucleotide bases, cholesterol, and porphyrin, as well as NADH and other forms of energy.
The cycle is a series of 8 enzymatic oxidation-reduction, or redox, reactions. Oxidation is a chemical reaction in which a molecule loses electrons, whereas reduction is a chemical reaction in which a molecule gains electrons. Redox reactions involve the transfer of electrons between molecules during the same reaction. Chemically, the citric acid cycle produces high-energy electrons from carbon fuel (see figure). It does this by removing electrons from acetyl-CoA and using them to generate NADH and the reduced form of flavin adenine dinucleotide (FADH2) (see figure). NADH and FADH2 transfer their electrons in the electron transport chain, and through oxidative phosphorylation, generate most of the ATP in aerobic cells. Oxygen is not used directly in the citric acid cycle. It is the final electron acceptor at the end of the electron transport chain, regenerating NAD+ and FAD.
NADH and FADH2 production in the citric acid cycle
Electron production in the citric acid cycle and use in oxidative phosphorylation
SUBSTRATE FOR THE CITRIC ACID CYCLE
Once acetyl-CoA is in a mitochondrion, it cannot be transported out. There are 2 main sources of acetyl-CoA:
1) Glucose: At the end of glycolysis, the PDC converts pyruvate to acetyl-CoA.
2) Fatty acids: β-oxidation of fatty acids yields acetyl-CoA. Note that β-oxidation of some fatty acids produces propionyl-CoA, which is converted to succinyl- CoA. Succinyl-CoA can enter the citric acid cycle as an intermediate.
Did you notice that we did not mention proteins? So, how do proteins enter the citric acid cycle? Proteases break proteins down into their constituent amino acids. Their carbon skeletons can enter the citric acid cycle in a few different ways:
• By conversion into intermediates; for example, glutamine is processed to form α-ketoglutarate.
• By conversion into acetyl-CoA; this is possible for several amino acids, including leucine, isoleucine, lysine, phenylalanine, tryptophan, and tyrosine.
• By conversion into pyruvate; this is possible for alanine, cysteine, glycine, serine, and threonine.
STEPS OF THE CITRIC ACID CYCLE
Review the 8 reactions of the citric acid cycle:
The citric acid cycle
• Step 1: A 2-carbon unit of acetyl-CoA condenses with a 4-carbon unit of oxaloacetate to form a 6-carbon unit of citrate. ◦ Citrate synthase catalyzes this reaction.
• Step 2: Aconitase catalyzes the isomerization of citrate to isocitrate. ◦ Isocitrate is much more readily oxidized than citrate.
• Step 3: Isocitrate dehydrogenase catalyzes the oxidative decarboxylation of isocitrate. This removes a unit of CO2, resulting in a 5-carbon unit of α-ketoglutarate. ◦ This reaction requires the input of NAD+ and produces NADH.
• Step 4: The α-ketoglutarate dehydrogenase complex catalyzes the oxidative decarboxylation of α-ketoglutarate. This removes another unit of CO2, resulting in a 4-carbon unit of succinyl-CoA, which is an energy-rich compound. ◦ This reaction also requires the input of NAD+ and produces NADH.
• Step 5: Succinyl-CoA synthetase cleaves succinyl-CoA to produce succinate. ◦ This reaction is coupled with phosphorylation of guanosine diphosphate (GDP) to produce guanosine triphosphate (GTP). (Note that it is possible to convert GTP to ATP. The enzyme nucleoside diphosphokinase catalyzes the reaction of GTP + ADP → GDP + ATP.)
• Step 6: Succinate dehydrogenase oxidizes succinate to fumarate. ◦ This is a difficult oxidation reaction, so it utilizes FAD, which is a more powerful oxidant than NAD+. The FAD is reduced to FADH2.
• Step 7: Fumarase hydrates (adds H2O) fumarate to produce malate.
• Step 8: Malate dehydrogenase oxidizes malate to produce oxaloacetate. ◦ This reaction requires the input of NAD+ and produces NADH.
CITRIC ACID CYCLE INTERMEDIATES
Some of the intermediates formed in the citric acid cycle leave the cycle to enter other biosynthetic pathways. Although acetyl-CoA cannot leave the mitochondrion, citrate can. Once it is in the cytoplasm, the citrate can be converted back into acetyl-CoA for use in fatty acid and cholesterol synthesis. Likewise, malate can leave the mitochondrion. In the cytosol, malate is oxidized back to oxaloacetate, which enters the gluconeogenesis pathway.
Some intermediates provide the carbon skeletons for nonessential amino acids (i.e., amino acids the body can make, which means they do not have to be consumed in the diet). α-ketoglutarate is processed to form glutamine, proline, and arginine. Oxaloacetate is processed to form aspartate and asparagine.
Succinyl-CoA provides the carbon needed for synthesis of porphyrins, which are heterocyclic organic compounds. Porphyrins are important components of hemoglobin, myoglobin, and cytochromes.
Citric acid cycle intermediates are depleted in the reactions involving synthesis of fatty acids, cholesterol, glucose, nonessential amino acids, and porphyrins. Other processes that replenish the supply of intermediates of metabolic pathways are referred to as anaplerotic pathways. The flow of intermediates into and out of the citric acid cycle is balanced, so that concentrations of these molecules within the mitochondria remain constant over time.
SUMMARY OF ENERGY GENERATION
In a single turn of the citric acid cycle:
• 2 carbon atoms enter the cycle in the form of acetyl-CoA, and 2 molecules of CO2 are released.
• 3 NADH and 1 FADH2 are generated.
• 1 GTP is produced, which ultimately is converted to ATP.
The chemical equation representing the sum of the 8 reactions in a single turn of the citric acid cycle is:
Acetyl-CoA + 2 H2O + 3 NAD+ + FAD + GDP + Pi →
2 CO2 + 3 NADH + 3H+ + FADH2 + uncombined coenzyme A (CoASH) + GTP
Note that 1 molecule of glucose yields 2 molecules of acetyl-CoA, so you will actually want to double the above numbers to show the yield for each molecule of glucose. So, for 1 glucose molecule, the energy output for the citric acid cycle is 2 ATP, 6 NADH, and 2 FADH2. You can see that at this point we have not generated that much more ATP than we did in glycolysis. But look at all the energy-rich NADH and FADH2 we generated! The big payoff arrives in the electron transport chain, which generates about 3 ATP per NADH oxidized and 2 ATP per FADH2 oxidized. In total, aerobic cellular respiration yields approximately 38 ATP from the oxidation of 1 molecule of glucose!
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