19.4
The Citric Acid Cycle: Overview
In aerobic organisms, the citric acid cycle is the second stage of cellular respiration wherein molecules derived from the breakdown of carbohydrates,…
The citric acid cycle, also known as the tricarboxylic acid cycle or the Krebs cycle, is the second phase of cellular respiration that oxidizes biomolecules to produce energy.
It occurs in the mitochondrial matrix in eukaryotes and within the cytosol in prokaryotes.
Pyruvate, the end product of glycolysis, combines with coenzyme A, generating acetyl coenzyme A or acetyl-CoA.
Then, the enzyme citrate synthase initiates the cycle by condensing acetyl-CoA and oxaloacetate to form the first product, citric acid.
In the second step, aconitase rearranges citric acid to its easily oxidizable isomer, isocitrate.
The third step involves oxidation of isocitrate to α-ketoglutarate by isocitrate dehydrogenase.
Next, α-ketoglutarate dehydrogenase decarboxylates and oxidizes α-ketoglutarate in the presence of coenzyme A to form succinyl-CoA.
In the fifth step, succinyl-CoA synthetase converts succinyl-CoA to succinate, releasing coenzyme A.
Further, succinate is oxidized to fumarate by succinate dehydrogenase.
Fumarase hydrates the double bond of fumarate in the seventh step to yield malate.
Finally, the malate dehydrogenase enzyme oxidizes malate, regenerating oxaloacetate for the next round of the cycle.
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Q1: What is the citric acid cycle and where does it occur?
The citric acid cycle, also called the tricarboxylic acid (TCA) or Krebs cycle, is the second phase of cellular respiration that oxidizes biomolecules to produce energy. In eukaryotes, it occurs in the mitochondrial matrix, while in prokaryotes it takes place within the cytosol. This cycle is central to aerobic energy production.
Q2: How does acetyl-CoA enter the citric acid cycle?
Pyruvate, the end product of glycolysis, combines with coenzyme A to generate acetyl-CoA. The enzyme citrate synthase then initiates the cycle by condensing acetyl-CoA with oxaloacetate, a four-carbon molecule, to form citric acid, the first six-carbon product of the cycle.
Q3: What are the key enzymatic steps in the citric acid cycle?
The cycle involves eight major enzymatic steps: citrate synthase forms citric acid, aconitase rearranges it to isocitrate, isocitrate dehydrogenase oxidizes isocitrate to α-ketoglutarate, α-ketoglutarate dehydrogenase forms succinyl-CoA, succinyl-CoA synthetase produces succinate, succinate dehydrogenase creates fumarate, fumarase yields malate, and malate dehydrogenase regenerates oxaloacetate.
Q4: What happens to carbon atoms during the citric acid cycle?
During each cycle, two carbon atoms from citric acid are removed and released as two molecules of carbon dioxide. This oxidation process also releases four electrons that are captured by coenzyme carriers. At the cycle's end, oxaloacetate is regenerated to accept another acetyl group.
Q5: Why are intermediates in the citric acid cycle important for other pathways?
The majority of citric acid cycle intermediates are components of other biochemical pathways that produce metabolites such as porphyrins, fatty acids, and amino acids. If these intermediates are diverted away from the cycle, the cycle's integrity is compromised and the cycle halts, disrupting energy production.
Q6: How does the citric acid cycle connect to energy production?
The citric acid cycle oxidizes molecules derived from carbohydrates, proteins, and fats into carbon dioxide while releasing electrons. These electrons are captured and transferred to the electron transport chain, where they drive the production of ATP through oxidative phosphorylation and energy capture.
Q7: What is the relationship between oxaloacetate and cycle continuity?
Oxaloacetate is a four-carbon molecule that combines with acetyl groups to initiate each cycle turn. At the cycle's end, malate dehydrogenase regenerates oxaloacetate, enabling the cycle to continue. This regeneration is essential for sustained energy production and metabolic function in aerobic organisms.