23. TCA Cycle II: How Does the TCA Cycle Oxidize Acetyl-CoA?

TL;DR

The TCA cycle oxidizes two-carbon units by combining acetyl-CoA with four-carbon oxaloacetate to make six-carbon citrate, then regenerating oxaloacetate while releasing two CO2 molecules. Occurring in the mitochondrial matrix, the cycle processes carbon derived from glucose, ethanol, acetate, and fat while releasing energy and producing metabolic intermediates. Read on to understand pyruvate conversion, required cofactors, and why the pathway forms a cycle.

Transcript

[SQUEAKING] [RUSTLING] [CLICKING] MATTHEW VANDER HEIDEN: Hello, everybody. Last time, I introduced the idea of the TCA cycle, tricarboxylic acid cycle. Also known as the citric acid cycle, because citric acid is a tricarboxylic acid, as you'll see later today. Also known as the Krebs cycle, named after Hans Krebs, who discovered it in the early par... Read More

Key Insights

• 🏍️ The TCA cycle is a central metabolic pathway that enables the complete oxidation of carbon and provides energy for the cell.
• 🏍️ The cycle is regulated based on the energy needs of the cell, with high levels of ATP and NADH inhibiting the cycle while low levels of ADP activating it.
• 🤩 The TCA cycle not only releases energy but also generates key intermediates for other metabolic processes.
• 🖤 Humans lack the ability to convert two-carbon units into four-carbon oxaloacetate, limiting the ability to convert acetate or acetyl-CoA into glucose.
• 😒 The glyoxylate cycle is an alternative pathway that certain microbes use to convert two-carbon units into four-carbon intermediates for anabolic processes.

Questions & Answers

Q: What is the TCA cycle?

The TCA cycle is a series of reactions in the mitochondrial matrix that enables the complete oxidation of two-carbon units to CO2. It releases energy and generates intermediates that cells can use to make other molecules.

Q: Why is the TCA cycle also called the citric acid cycle or Krebs cycle?

It is called the citric acid cycle because citrate, or citric acid, is a tricarboxylic acid formed in the pathway. It is also called the Krebs cycle after Hans Krebs, who discovered it in the early part of the last century.

Q: How does acetyl-CoA enter the TCA cycle?

The two-carbon group from acetyl-CoA combines with four-carbon oxaloacetate. The reaction releases the CoA group and produces six-carbon citrate.

Q: Why is the TCA pathway a cycle?

Six-carbon citrate is oxidized while two CO2 molecules are released, ultimately reforming four-carbon oxaloacetate. The regenerated oxaloacetate can accept another two-carbon acetyl-CoA group, allowing the reactions to repeat.

Q: Where do the two-carbon units processed by the TCA cycle come from?

They can come from pyruvate produced through glucose metabolism and glycolysis. The transcript also identifies acetate, vinegar, alcohol, and fat breakdown as sources that can ultimately provide two-carbon units.

Q: How is pyruvate converted into acetyl-CoA?

Three-carbon pyruvate is decarboxylated, releasing one carbon as CO2. Its remaining carbon group is oxidized as NAD is reduced to NADH, and a CoA group is added to form acetyl-CoA.

Q: Which cofactors are involved in converting pyruvate to acetyl-CoA?

NAD accepts electrons and is reduced to NADH during oxidation. Thiamine pyrophosphate is required for the alpha decarboxylation of pyruvate, and the lecturer notes that additional cofactors are also needed.

Q: What metabolic products and intermediates does the TCA cycle provide?

The cycle enables carbon from several molecules to be completely oxidized to CO2 while releasing energy. It also produces intermediates for other cellular processes, including citrate that can be used to make fatty acids.

Summary & Key Takeaways

• Definition: The TCA cycle is a mitochondrial-matrix reaction series that completely oxidizes two-carbon units to CO2.

• Who: Hans Krebs discovered the pathway in the early part of the last century, giving rise to the name Krebs cycle.

• Step 1: Three-carbon pyruvate is decarboxylated, releasing one carbon as CO2 during its conversion toward acetyl-CoA.

• Step 2: The remaining pyruvate-derived group is oxidized as NAD is reduced to NADH, and CoA is added to form acetyl-CoA.

• Step 3: Two-carbon acetyl-CoA combines with four-carbon oxaloacetate and releases its CoA group.

• Step 4: This condensation produces six-carbon citrate, a molecule containing three carboxylic acids.

• Step 5: Citrate is oxidized, two CO2 molecules are released, and four-carbon oxaloacetate is regenerated.

• Number: The cycle has a net entry of two carbons from acetate and a release of two carbons as CO2.

• Tool: Thiamine pyrophosphate supports the alpha decarboxylation involved in converting pyruvate toward acetyl-CoA.

• Compare: Glucose-derived pyruvate, acetate, vinegar, alcohol, and fat breakdown can provide carbon that enters this oxidative pathway.

• Definition: TCA intermediates support other metabolism, including the use of citrate to make fatty acids.