Question

For transport of acyl CoA into mitochondria, select the correct order of steps involved:
A. Acyl- carnitine to acyl CoA
B. Acyl- carnitine transported to mitochondrial matrix
C. Acyl group of acyl CoA transferred to carnitine
D. Carnitine released and return to cytosol

• A. C, B, A, D
• B. B, C, D, A
• C. C, A, D, B
• D. D, C, A, B

Hint:

Carnitine Shuttle Order:
1. CPT-I attaches carnitine to acyl group (C).
2. Translocase shuttles acyl-carnitine into matrix (B).
3. CPT-II converts acyl-carnitine back to acyl-CoA (A).
4. Free carnitine shuttles back to cytosol (D).

Answer 1

The Correct Option is A

Step 1: Understanding the Question:
The question asks for the correct temporal sequence of biochemical steps in the carnitine shuttle system, which transports long-chain fatty acyl-CoA molecules across the impermeable inner mitochondrial membrane into the matrix for $\beta$-oxidation.

Step 2: Key Formula or Approach:
The carnitine shuttle operates via a four-step enzymatic sequence across the outer membrane, intermembrane space, and inner mitochondrial membrane: Transesterification to Carnitine (C) $\rightarrow{}$ Translocation into Matrix (B) $\rightarrow{}$ Re-esterification to CoA (A) $\rightarrow{}$ Carnitine Recycling (D)

Step 3: Detailed Explation:

• 1. Transfer of Acyl group to Carnitine (C): Fatty acyl-CoA generated in the cytosol cannot directly cross the inner mitochondrial membrane. The outer membrane enzyme Carnitine Palmitoyltransferase-I (CPT-I) transfers the fatty acyl group from acyl-CoA to carnitine, forming acyl-carnitine and releasing free CoA.

• 2. Transport across Inner Membrane (B): Acyl-carnitine is translocated across the inner mitochondrial membrane into the mitochondrial matrix by Carnitine-Acylcarnitine Translocase (CACT) via antiport exchange.

• 3. Conversion back to Acyl-CoA (A): Inside the matrix, Carnitine Palmitoyltransferase-II (CPT-II) transfers the acyl group from acyl-carnitine back onto matrix Coenzyme A, reforming acyl-CoA for $\beta$-oxidation.

• 4. Release and Return of Carnitine (D): The freed carnitine molecule is transported back across the inner membrane into the intermembrane space/cytosol by the translocase to participate in subsequent transport cycles.

Step 4: Fil Answer:
The correct step sequence for fatty acyl transport into mitochondria is C, B, A, D.