Question

Arrange the stages of fatty acid oxidation in sequence.
1. $\beta$-Oxidation proper in the mitochondrial matrix.
2. Activation of fatty acids occurring in the cytosol.
3. Transport of fatty acids into mitochondria.
Choose the correct answer from the options given below:

• A. 1,2,3
• B. 2,3,1
• C. 3,2,1
• D. 1,3,2

Hint:

Remember that fatty acids cannot enter the powerhouses of the cell (mitochondria) in their raw form.
They must first be activated in the cytosol (2), escorted across the membrane by carnitine (3), and then broken down inside the matrix (1).

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
$\beta$-Oxidation is the primary metabolic pathway by which fatty acids are broken down to generate acetyl-CoA, which then enters the citric acid cycle to produce ATP.
Key Principle:
This biochemical pathway is highly compartmentalized, requiring sequential activation, transport, and enzymatic degradation steps.

Step 2: Detailed Explation:
The breakdown of fatty acids occurs in three sequential stages:
1. Activation (Stage 2): Free fatty acids in the cytosol are biochemically ictive.
Before they can be metabolized, they must be activated by the enzyme acyl-CoA synthetase on the outer mitochondrial membrane.
This reaction couples the fatty acid to coenzyme A, consuming ATP to form fatty acyl-CoA.
2. Transport (Stage 3): The inner mitochondrial membrane is impermeable to long-chain fatty acyl-CoA molecules.
They must be transported into the mitochondrial matrix via the carnitine shuttle system.
This shuttle utilizes carnitine palmitoyltransferase-I (CPT-I) to replace CoA with carnitine, translocates the acylcarnitine across the membrane, and uses CPT-II to reconstitute fatty acyl-CoA inside the matrix.
3. Beta-Oxidation proper (Stage 1): Once inside the mitochondrial matrix, the fatty acyl-CoA undergoes $\beta$-oxidation proper.
This cycle consists of four recurring enzymatic steps: oxidation, hydration, oxidation, and thiolytic cleavage, releasing acetyl-CoA, DH, and $\text{FADH}_2$.
Therefore, the correct chronological sequence is 2 $\rightarrow$ 3 $\rightarrow$ 1, which corresponds to Option (B).

Step 3: Fil Answer
The correct order of stages for fatty acid oxidation is 2, 3, 1.