Krebs Cycle BSc Nursing 1st Year, Steps, Products & Functions

The Krebs Cycle, also called the Citric Acid Cycle or TCA Cycle, is a core stage of cellular respiration responsible for releasing energy from nutrients. Inside the mitochondrial matrix, it breaks down Acetyl-CoA derived from carbohydrates, fats, and proteins to generate high-energy electron carriers (NADH and FADH₂) and GTP, which ultimately help produce ATP. Because it connects multiple metabolic pathways and continuously regenerates its starting molecule, the Krebs Cycle acts as the central hub of aerobic metabolism.

Importance of the Krebs Cycle

The Krebs Cycle is a very important topic in university exams, frequently appearing as a direct question (10-15 marks) asking for a detailed explanation. It is considered a high-yield, important, or hot topic that students should never skip as it would result in significant loss of marks.

Alternative Names of the Krebs Cycle

The Krebs Cycle is also known by other names:

• TCA Cycle (Tricarboxylic Acid Cycle)

• Citric Acid Cycle

Discovery and Naming of the Krebs Cycle

This cycle was discovered by Hans Adolf Krebs in 1937 and is named the Krebs Cycle after its discoverer.

Connection to Glycolysis

Following glycolysis, where one molecule of glucose is converted into two molecules of pyruvate, pyruvate is further converted into Acetyl Coenzyme A (Acetyl CoA) under aerobic conditions. The Krebs Cycle then begins with Acetyl CoA entering the cycle.

Definition and Overview of the Krebs Cycle

The Krebs Cycle is a common oxidative pathway. In this cycle, carbohydrates, fats, and proteins converge after being broken down into Acetyl CoA. This Acetyl CoA combines with Oxaloacetate to form Citric Acid (or Citrate), initiating a series of steps involving the conversion of various intermediates.

Also Check: BSc Nursing Syllabus 2026

Location of the Krebs Cycle

Unlike glycolysis, which occurs in the cytoplasm of all cells, the Krebs Cycle takes place in the mitochondrial matrix.

Nature of the Krebs Cycle

The Krebs Cycle is an aerobic pathway, meaning oxygen is indirectly required for its progression.

Starting Point of the Krebs Cycle

The starting point for the Krebs Cycle is Acetyl CoA, which is derived from pyruvate under aerobic conditions.

Energy Carriers in the Krebs Cycle (NAD+ and FAD)

During the Krebs Cycle, reducing equivalents are repeatedly regenerated through the involvement of key energy carriers:

• NAD+ (Nicotinamide Adenine Dinucleotide): This is the oxidized form. It accepts two electrons and one proton, reducing to form NADH. 1 NADH yields 3 ATP.

• FAD (Flavin Adenine Dinucleotide): This is the oxidized form. It accepts two electrons and two protons, reducing to form FADH2. 1 FADH2 yields 2 ATP.

(Memory Tip: Both NAD+ and FAD are high-power electron carriers that provide energy. Ultimately, both contribute to ATP production. Think of them as different ways of getting to the same energy currency, ATP.)

NAD+ vs. FAD

Steps of the Krebs Cycle

The Krebs Cycle involves a series of reactions that continuously regenerate oxaloacetate.

Step 1: Formation of Citrate

• Reactants: Acetyl CoA (2-carbon molecule) + Oxaloacetate (4-carbon molecule).

• Enzyme: Citrate Synthase.

• Product: Citrate (also called Citric Acid) (6-carbon molecule).

• Reaction Type: Condensation reaction.

• Coenzyme A is released temporarily from the cycle. (Memory Tip: Think of Coenzyme A as going "to rest" after initiating the cycle, returning later when needed.)

• Significance: This step introduces Acetyl CoA into the cycle.

Step 2: Isomerization to Isocitrate

• Reactant: Citrate (6-carbon molecule).

• Enzyme: Aconitase.

• Product: Isocitrate (6-carbon molecule).

• Reaction Type: Isomerization (rearrangement of the -OH group). The carbon count remains the same.

• Significance: Isocitrate is more easily oxidized in the subsequent step.

Step 3: Oxidative Decarboxylation to α-Ketoglutarate

• Reactant: Isocitrate (6-carbon molecule).

• Enzyme: Isocitrate Dehydrogenase.

• Products: α-Ketoglutarate (5-carbon molecule), NADH, and CO2.

• Reaction Type: Oxidative Decarboxylation.

• Energy Release: NAD+ is reduced to NADH.

• Carbon Change: One molecule of CO2 is released, reducing the carbon count from 6 to 5.
  (Memory Tip: When you see "Dehydrogenase" in an enzyme name, it indicates the involvement of NAD+ (which gets reduced to NADH) due to the release of hydrogen atoms.)

Step 4: Oxidative Decarboxylation to Succinyl CoA

• Reactant: α-Ketoglutarate (5-carbon molecule).

• Enzyme: α-Ketoglutarate Dehydrogenase.

• Products: Succinyl CoA (4-carbon molecule), NADH, and CO2.

• Coenzyme A: The Coenzyme A that was released in Step 1 re-enters here.

• Reaction Type: Oxidative Decarboxylation.

• Energy Release: NAD+ is reduced to NADH.

• Carbon Change: Another molecule of CO2 is released, reducing the carbon count from 5 to 4.

Step 5: Formation of Succinate

• Reactant: Succinyl CoA (4-carbon molecule).

• Enzyme: Succinyl CoA Synthetase.

• Products: Succinate (4-carbon molecule) + Coenzyme A.

• Energy Release: GTP (Guanosine Triphosphate) is produced. (Memory Tip: GTP is equivalent to ATP in energy terms. Think of it as ATP having "different names" depending on the context, like how you might have different nicknames with different family members.)

• Reaction Type: Substrate-level phosphorylation.

Step 6: Formation of Fumarate

• Reactant: Succinate (4-carbon molecule).

• Enzyme: Succinate Dehydrogenase.

• Products: Fumarate (4-carbon molecule) + FADH2.

• Energy Release: FAD is reduced to FADH2.

• Note: This is the only step in the Krebs Cycle where FAD is the electron acceptor, not NAD+.

Step 7: Formation of Malate

• Reactant: Fumarate (4-carbon molecule).

• Enzyme: Fumarase.

• Product: Malate (4-carbon molecule).

• Reaction Type: Hydration (addition of water). (Memory Tip: "Hydration" means water is added, just like you get hydrated when you drink water.)

Step 8: Formation of Oxaloacetate

• Reactant: Malate (4-carbon molecule).

• Enzyme: Malate Dehydrogenase.

• Products: Oxaloacetate (4-carbon molecule) + NADH.

• Energy Release: NAD+ is reduced to NADH.

• Significance: Oxaloacetate is regenerated, allowing the cycle to continue.

Overall Products of Krebs Cycle (per Glucose Molecule)

For one glucose molecule, which yields two Acetyl CoA molecules, the Krebs Cycle produces:

• NADH: 6 molecules (3 per Acetyl CoA x 2)

• FADH2: 2 molecules (1 per Acetyl CoA x 2)

• GTP: 2 molecules (1 per Acetyl CoA x 2)

• CO2: 4 molecules (2 per Acetyl CoA x 2)

Significance and Roles of the Krebs Cycle

The Krebs Cycle plays multiple vital roles in cellular metabolism:

1. Energy Production: Its primary role is energy production through the generation of NADH and FADH2. These reducing equivalents subsequently feed into the electron transport chain to produce a large amount of ATP.

2. Biosynthetic Role:

• α-Ketoglutarate is a precursor for the synthesis of certain amino acids.

• Succinyl CoA is involved in heme synthesis.

• Oxaloacetate is crucial for gluconeogenesis (synthesis of new glucose molecules) and provides precursors for aspartate family amino acids.

• Citrate is a precursor for fatty acid synthesis.

3. Metabolic Hub: The Krebs Cycle serves as a central metabolic hub where catabolic (breakdown) pathways for carbohydrates, fats, and proteins converge, and anabolic (synthesis) pathways diverge.

4. Replenishment of Intermediates: The cycle constantly regenerates Oxaloacetate, ensuring its continuous operation. Intermediates can also be replenished from pyruvate, making the cycle highly dynamic.

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