Chemical Reactions in the Krebs Cycle
The Krebs cycle, also known as the citric acid cycle or the tricarboxylic acid cycle, is a vital component of cellular respiration. This biochemical pathway occurs inside the mitochondria of cells and helps generate energy for various cellular processes. Through a series of chemical reactions, the Krebs cycle converts the energy stored in the bonds of carbohydrates, fats, and proteins into usable energy in the form of adenosine triphosphate (ATP). Let’s take a closer look at the chemical reactions that occur in the Krebs cycle.
1. Acetyl-CoA Formation: The cycle begins when a two-carbon molecule called acetyl-CoA combines with a four-carbon molecule known as oxaloacetate. This reaction is catalyzed by the enzyme citrate synthase, resulting in the formation of a six-carbon molecule called citrate.
2. Citrate Isomerization: The enzyme aconitase converts citrate into its isomer, isocitrate.
3. Isocitrate Oxidation: Isocitrate is oxidized by the enzyme isocitrate dehydrogenase, leading to the release of a carbon dioxide molecule and the formation of a five-carbon molecule called alpha-ketoglutarate. At the same time, NAD+ (nicotinamide adenine dinucleotide) is reduced to NADH.
4. Alpha-Ketoglutarate Decarboxylation: Alpha-ketoglutarate is decarboxylated by the enzyme alpha-ketoglutarate dehydrogenase. This results in the release of another carbon dioxide molecule, as well as the conversion of NAD+ to NADH. The remaining four-carbon molecule is called succinyl-CoA.
5. Succinyl-CoA Reaction: The enzyme succinyl-CoA synthetase facilitates the conversion of succinyl-CoA into succinate. This process generates one molecule of ATP and involves the release of an enzyme-bound CoA molecule.
6. Succinate Dehydrogenation: The enzyme succinate dehydrogenase oxidizes succinate to form fumarate. In this process, the molecule FAD (flavin adenine dinucleotide) is reduced to FADH2.
7. Fumarate Hydration: The enzyme fumarase adds a water molecule to fumarate, creating malate.
8. Malate Dehydrogenation: Malate is oxidized by malate dehydrogenase to form oxaloacetate. At the same time, NAD+ is reduced to NADH.
The oxaloacetate produced in the last step is then used to start the cycle again by reacting with acetyl-CoA, completing the Krebs cycle.
Now, let’s move on to some questions and answers related to the chemical reactions in the Krebs cycle:
1. What is the Krebs cycle?
The Krebs cycle is a series of chemical reactions that occur in the mitochondria, responsible for the conversion of carbohydrates, fats, and proteins into energy-rich molecules.
2. Which molecule combines with acetyl-CoA to initiate the Krebs cycle?
Oxaloacetate combines with acetyl-CoA to start the Krebs cycle.
3. What are the products of the conversion of citrate to isocitrate?
The isomerization of citrate to isocitrate does not produce any new molecules but changes the molecular structure.
4. What are the products of isocitrate oxidation?
Isocitrate oxidation results in the release of a carbon dioxide molecule and the formation of alpha-ketoglutarate.
5. Which coenzyme gets reduced during the conversion of isocitrate to alpha-ketoglutarate?
NAD+ (nicotinamide adenine dinucleotide) is reduced to NADH.
6. Which enzyme decarboxylates alpha-ketoglutarate?
The enzyme alpha-ketoglutarate dehydrogenase decarboxylates alpha-ketoglutarate.
7. What are the products of alpha-ketoglutarate decarboxylation?
Alpha-ketoglutarate decarboxylation releases a carbon dioxide molecule and generates succinyl-CoA.
8. Which enzyme converts succinyl-CoA into succinate?
Succinyl-CoA synthetase converts succinyl-CoA into succinate.
9. What energy molecule is produced during the conversion of succinyl-CoA to succinate?
One molecule of ATP is generated during this reaction.
10. What coenzyme is reduced during the conversion of succinate to fumarate?
FAD (flavin adenine dinucleotide) is reduced to FADH2.
11. Which enzyme hydrates fumarate?
Fumarase hydrates fumarate to form malate.
12. What coenzyme gets reduced during malate dehydrogenation?
NAD+ is reduced to NADH.
13. What is the final product of the Krebs cycle?
The final product of the Krebs cycle is oxaloacetate, which is then used to initiate a new cycle.
14. How many NADH molecules are produced during one turn of the Krebs cycle?
Three molecules of NADH are produced.
15. How many FADH2 molecules are produced during one turn of the Krebs cycle?
One molecule of FADH2 is produced.
16. What is the net ATP production from one turn of the Krebs cycle?
One turn of the Krebs cycle results in one ATP molecule.
17. How many carbon dioxide molecules are released during one turn of the Krebs cycle?
Four carbon dioxide molecules are released.
18. How many water molecules are involved in the Krebs cycle?
Two water molecules are utilized during the hydration reactions.
19. What is the role of the Krebs cycle in cellular respiration?
The Krebs cycle is an essential part of cellular respiration as it generates high-energy molecules (NADH and FADH2) that are used in the electron transport chain to produce ATP.
20. What happens to the oxaloacetate produced at the end of the Krebs cycle?
The oxaloacetate is used to start the next round of the Krebs cycle by reacting with acetyl-CoA.
