Eukaryotic lipid metabolism — In which cellular compartment of eukaryotes does the bulk of fatty acid β-oxidation (fatty acid breakdown) occur before acetyl-CoA enters the TCA cycle?
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Amitochondrial matrix
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Bcytosol
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Ccell membrane (plasma membrane)
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Dendoplasmic reticulum (ER) lumen
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Eperoxisomal matrix only, with no mitochondrial involvement
Answer
Correct Answer: mitochondrial matrix
Explanation
Introduction:Understanding where fatty acids are catabolized inside eukaryotic cells is fundamental to biochemistry and physiology. This question targets the location of β-oxidation, the core pathway that converts long-chain fatty acids into acetyl-CoA for the citric acid (TCA) cycle and oxidative phosphorylation.
Given Data / Assumptions:
- Eukaryotic cells possess mitochondria and, in many tissues, peroxisomes.
- β-oxidation sequentially removes two-carbon units as acetyl-CoA.
- Carnitine shuttles long-chain fatty acyl groups into mitochondria.
Concept / Approach:The dominant site of fatty acid β-oxidation in eukaryotes is the mitochondrial matrix. Very-long-chain fatty acids (especially > C20) undergo initial chain-shortening in peroxisomes, but full energy extraction (NADH/FADH2 feeding the electron transport chain, and acetyl-CoA oxidation in the TCA cycle) relies on mitochondria. Thus, the most accurate general answer is the mitochondrial matrix.
Step-by-Step Solution:
1) Activation: fatty acids → fatty acyl-CoA (cytosolic/outer mitochondrial membrane enzymes).2) Transport: carnitine shuttle transfers fatty acyl groups across the inner mitochondrial membrane.3) β-Oxidation proper: acyl-CoA dehydrogenase etc. operate in the mitochondrial matrix, producing acetyl-CoA, NADH, and FADH2.4) Energy harvest: acetyl-CoA enters the TCA cycle; reduced cofactors drive ATP synthesis in oxidative phosphorylation.Verification / Alternative check:Disorders of the carnitine shuttle or mitochondrial acyl-CoA dehydrogenases (e.g., MCAD deficiency) cause characteristic hypoketotic hypoglycemia, confirming the mitochondrial localization and physiological importance of matrix β-oxidation.
Why Other Options Are Wrong:
- Cytosol: fatty acids are activated here, but β-oxidation occurs in mitochondria, not freely in cytosol.
- Plasma membrane: not a site of oxidative catabolism.
- ER lumen: involved in lipid synthesis and desaturation, not β-oxidation.
- Peroxisome only: peroxisomes shorten very-long chains and generate H2O2; complete energy extraction requires mitochondrial β-oxidation.
Common Pitfalls:Overgeneralizing peroxisomal involvement. Remember: peroxisomes start the job for very-long chains; mitochondria finish it and make ATP.
Final Answer:mitochondrial matrix.