Question

In germinating oilseeds, which of the following metabolic end product of the glyoxylate pathway is transported out of glyoxysomes for further utilization in the TCA cycle?

• A. Malate
• B. Acetyl CoA
• C. Succinate
• D. $\alpha$-ketoglutarate

Hint:

Sequence: Fat $\rightarrow$ Acetyl CoA $\rightarrow$ Succinate (Leaves Glyoxysome) $\rightarrow$ Oxaloacetate (Mitochondria) $\rightarrow$ Sugar (Cytosol).
Remember: Animals cannot do this because they lack the enzyme Isocitrate Lyase. This is why we can't turn fat into sugar!

Answer 1

The Correct Option is C

Step 1: Understanding the Question:
Germinating oilseeds convert stored lipids (fats) into sugars (carbohydrates) to fuel growth. This process uses the glyoxylate cycle. The question asks for the specific intermediate that leaves the glyoxysome to enter the mitochondria.

Step 2: Detailed Explanation:

• The Glyoxylate Cycle: This is a modified version of the TCA cycle that bypasses the two decarboxylation steps, allowing the net synthesis of 4-carbon units from 2-carbon Acetyl-CoA units.

• Enzymatic Steps in Glyoxysomes: Inside the glyoxysome, fatty acids are broken down into Acetyl-CoA via $\beta$-oxidation. Acetyl-CoA enters the glyoxylate cycle. Isocitrate is cleaved by isocitrate lyase into Succinate and Glyoxylate.

• Product Fate: Glyoxylate remains in the glyoxysome to be recycled into malate. However, the Succinate produced is the key metabolic "output" of the organelle.

• Transport and TCA Integration: Succinate is transported across the glyoxysomal membrane and enters the Mitochondria. Inside the mitochondria, it enters the regular TCA cycle.

• Gluconeogenesis: Succinate is converted to Fumarate $\rightarrow$ Malate $\rightarrow$ Oxaloacetate. This Oxaloacetate then leaves the mitochondria and enters the cytosol to be converted into glucose through gluconeogenesis.

• Biological Importance: Without this "succinate export," plants could not turn fat into the sugar needed for the seedling to grow before it can perform photosynthesis.

Step 3: Final Answer:
Succinate is the 4-carbon intermediate produced in the glyoxysome that is exported to the mitochondria for further metabolism.