Horcruxes of Cellular Respiration - Part 2

Welcome back, fellow wizards!

Today, we will be exploring the second Horcrux of cellular respiration.

Do you remember the Bubble Head Charm incantation from Harry Potter that provides a steady supply of oxygen for respiration? Well, the presence of oxygen stimulates cells in the body to respire aerobically. In aerobic respiration, the Cycle of Krebs comes directly after glycolysis. 

The Second Horcrux: CY-CLE OF KRE-BS

The second Horcrux of cellular respiration can also be referred to as the Tricarboxylic Acid (TCA) Cycle or the Citric Acid Cycle. 

The pyruvate molecules synthesized by the first Horcrux of cellular respiration underwent decarboxylation and oxidation to produce the acetyl coenzyme A (acetyl CoA) required for the cycle of Krebs. This process serves as an irreversible link between glycolysis and the cycle of Krebs.

Pyruvate + CoA + NAD+ ---------> Acetyl CoA + NADH + CO2 + H+

The enzyme that catalyzes this conversion is the pyruvate dehydrogenase complex. 

The acetyl CoA, which is a 2-carbon molecule, enters the citric acid cycle and condenses with a 4-carbon molecule to give rise to a 6-carbon molecule. The 4-carbon molecule is oxaloacetate while the 6-carbon molecule is citrate.

Acetyl CoA (2C) + Oxaloacetate (4C) -------> Citrate (6C)

This reaction is catalyzed by the enzyme citrate synthetase. 

Citrate is then converted to other intermediates via a group of enzymes as seen in the figure below.

Figure 1: Diagram outlining the different metabolic intermediates and enzymes of the citric acid cycle.

Some important factors to know about the Cycle of Krebs.

1. This cycle uses acetyl CoA to extract the maximum number of electrons from it as it is carbon fuel. These extracted electrons are sent to the electron transport chain, the third and final Horcrux of cellular respiration. The electron carriers are NADH and flavin adenine dinucleotide (FADH).  
2. ATP is generated from the GTP synthesized upon the conversion of Succinyl CoA to Succinate. This is another example of substrate-level phosphorylation. The generation of ATP is due to the large energy reserve in the thioester of the succinyl CoA

Note: 1 FADH2, 3 NADH and 1 ATP were generated in one cycle of the citric acid cycle from one molecule of acetyl CoA.

For more on the second Horcrux of cellular respiration, feel free to watch the video below.

Source: MEDSimplified. (2020). Krebs Cylcle Trick How to remember krebs cycle FOREVER!! [YouTube Video]. In YouTube. https://www.youtube.com/watch?v=RnL71vnCMCY

Until the third and final Horcrux - Electron Transport Chain!

Do you know:

1. The name of the enzyme in the Krebs Cycle that catalyzes Fumerate into Malate?
2. Where in the cell the Krebs Cycle takes place?

Feel free to share your response.

References:

Abali, E. E., Cline, S. D., Franklin, D. S., & Viselli, S. M. (2021). Lippincott Illustrated Reviews: Biochemistry. Wolters Kluwer Health.

Berg, J. M., Tymoczko, J. L., Gatto, G. J., & Lubert Stryer. (2015). Biochemistry (8th ed.). W.H. Freeman & Company, A Macmillan Education Imprint.

Nelson, D. L., Lehninger, A. L., & Cox, M. M. (2021). Lehninger Principles of Biochemistry (8th ed.). Basingstoke.