Krebs Cycle Or Citric Acid Cycle: Biochemistry, Metabolism, Enzymes

What Is The Krebs Cycle?

The Krebs cycle, also identified with the terms citric acid cycle and tricarboxylic acid (TCA) cycle, is this important fundamental metabolic pathway that carries much significance in cellular respiration. After glycolysis, the second step regarding aerobic respiration is the Krebs cycle. The cycle takes place in the mitochondrial matrix of all eukaryotic cells. So this is the cycle that converts carbohydrates, fats, and proteins into carbon dioxide, water, and energy.

Location of Krebs Cycle

In eukaryotic cells, it occurs in the mitochondrial matrix. However, in the prokaryotic cells, the cycle takes place in cytosol and membrane bound enzyme complexes.

Steps of the Krebs Cycle (Enzyme-Wise)

The Krebs cycle includes the following 8 steps:

Step 1 — Citrate Formation

• Acetyl-CoA + Oxaloacetate → Citrate

• Enzyme: Citrate synthase

Step 2 — Isomerization

• Citrate → Isocitrate

• Enzyme: Aconitase

Step 3 — First Oxidative Decarboxylation

• Isocitrate → α-ketoglutarate + CO₂ + NADH

• Enzyme: Isocitrate dehydrogenase

Step 4 — Second Oxidative Decarboxylation

• α-ketoglutarate → Succinyl-CoA + CO₂ + NADH

• Enzyme: α-ketoglutarate dehydrogenase

Step 5 — Substrate-Level Phosphorylation

• Succinyl-CoA → Succinate + GTP/ATP

• Enzyme: Succinyl-CoA synthetase

Step 6 — Oxidation

• Succinate → Fumarate + FADH₂

• Enzyme: Succinate dehydrogenase

Step 7 — Hydration

• Fumarate → Malate

• Enzyme: Fumarase

Step 8 — Final Oxidation

• Malate → Oxaloacetate + NADH

• Enzyme: Malate dehydrogenase

• Oxaloacetate regenerated → cycle repeats

Products of the Krebs Cycle

The products of Krebs Cycle are:

Per turn (1 Acetyl-CoA):

• 3 NADH

• 1 FADH₂

• 1 GTP/ATP

• 2 CO₂

Per glucose molecule (2 Acetyl-CoA):

• 6 NADH

• 2 FADH₂

• 2 GTP/ATP

• 4 CO₂

Krebs Cycle Equation

The overall Krebs cycle can be appropriately represented in the following equation:

Acetyl-CoA + 3 NAD⁺ + FAD + GDP + Pᵢ + 2 H₂O → 2 CO₂ + 3 NAD

Importance Of Krebs Cycle

The importance of Krebs cycle is:

Energy Production

The most important point regarding the significance of the Krebs cycle, is the line of production for the high-energy electron carriers NADH and FADH2, which, in turn, produce ATP through the electron transport chain.

Carbon Dioxide Production

The cycle is a producer of carbon dioxide, forming a waste product that can be excreted from the body after respiration.

Biosynthesis

Various intermediates of the Krebs cycle serve as a source or precursor of the synthesis of important biomolecules like amino acids and nucleotides.

Regulation Of Krebs Cycle

Tightly regulated, the Krebs cycle ensures that the production of energy is accessible in amounts required for metabolic needs. Easily identified control enzymes of the cycle include:

1. Citrate Synthase: Controls entry of acetyl-CoA into the cycle.

2. Isocitrate Dehydrogenase: The activity of this enzyme, for the conversion of isocitrate to alpha-ketoglutarate, is controlled by levels of NADH and ATP.

3. Alpha-Ketoglutarate Dehydrogenase: Logic identical to that of isocitrate dehydrogenase, for a proper balance of energy production about the availability of the substrate.

Relationship Between Glycolysis And Krebs Cycle

The Krebs cycle is directly connected to glycolysis through pyruvate transformation into acetyl-CoA. After the reception of the end product of glycolysis within the cytoplasm, pyruvate diffused in the mitochondria undergoes oxidative decarboxylation, forming acetyl-CoA and NADH. The acetyl-CoA formed is assimilated into the Krebs cycle and is completely oxidized with the release of energy.

Krebs Cycle NEET MCQs (With Answers & Explanations)

Important topics for NEET are:

• Steps of Krebs Cycle

• Regulation of Krebs Cycle

Practice Questions for NEET

Q1. The only step in the Krebs cycle, where FAD is involved is

1. Conversion of succinate into fumarate

2. Conversion of citrate into isocitrate

3. Conversion of fumarate into malate

4. Conversion of alpha-ketoglutarate into succinyl C0-A

Correct answer: 1) Conversion of succinate into fumarate

Explanation:

If aconitase becomes nonfunctional due to the deletion of its gene, then the Krebs cycle will be disrupted at that step where citrate gets converted into isocitrate. Thus, citrate will accumulate because it will not be able to further process it.
Aconitase catalyzes the reversible isomerization of citrate to isocitrate by an intermediate, cis-aconitate.
Without a functional aconitase enzyme, this conversion cannot take place, and citrate accumulates while subsequent steps of the Krebs cycle are stopped.
This disruption reduces the efficiency of the cycle in producing NADH, FADH₂, and ATP.

Q2. Which of the following sentences is incorrect about Krebs Cycle.

1. The Krebs Cycle is the major metabolic mechanism by which aerobic energy is produced in a usable form from carbs, proteins, and lipids.

2. The Krebs Cycle is the process by which anaerobic cellular metabolism takes place

3. The Krebs Cycle is often referred to as the Citric Acid Cycle or the Tricarboxylic Acid (TCA) Cycle.

4. The Krebs Cycle converts acetyl-CoA into energy.

Correct answer: 2) The Krebs Cycle is the process by which anaerobic cellular metabolism takes place.

Explanation:

The Krebs Cycle is the principal metabolic route by which aerobic energy is released in a usable form from carbs, proteins, and lipids. The energy produced by the Krebs cycle is measured in molecules of ATP (Adenosine triphosphate) per molecule of glucose.

The Krebs Cycle which is additionally referred to as the Citric Acid or Tricarboxylic Acid (TCA) cycle is the metabolic process that happens in aerobic cells.

The Krebs Cycle generates energy from acetyl-CoA, but the biological problem is to do it gradually and in usable forms.

Hence, the correct answer is option 2) The Krebs Cycle is the process by which anaerobic cellular metabolism takes place.

Q3. Which of the following is the correct sequence of the stages of cellular respiration?

1. Glycolysis, Krebs cycle, electron transport chain

2. Krebs cycle, glycolysis, electron transport chain

3. Electron transport chain, glycolysis, Krebs cycle

4. Krebs cycle, electron transport chain, glycolysis

Correct answer: 1) Glycolysis, Krebs cycle, electron transport chain

Explanation:

In order to convert the biochemical energy from nutrients into adenosine triphosphate (ATP) and later release waste products, cellular respiration is the collection of metabolic activities and processes that occur in the cells of organisms.

The final product of glycolysis (pyruvate) from the TCA cycle is used in aerobic respiration, also known as cell respiration in the presence of oxygen, to produce significantly more energy in the form of ATP. The electron transport chain, which is followed by the TCA cycle, creates the proton gradient required for ATP production.

Hence, the correct answer is option 1) Glycolysis, Krebs cycle, electron transport chain.

Also Read:

• MCQs on Krebs Cycle

• Link Reaction

• Alcoholic Fermentation

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