The correct answer is (1) K inhibits F→G and L inhibits F→H; D→E is inhibited at equal amounts of K and L.

Introduction

Metabolic pathways often branch to produce multiple end products, each with distinct cellular roles. To maintain cellular balance, these pathways are tightly regulated—one crucial mechanism is concerted feedback inhibition, where end products collectively modulate key steps, ensuring stoichiometric output. This article explores the logic and biochemical basis for achieving balanced end product formation using feedback inhibition, applying these concepts to a branched pathway schematic for advanced life science learners.

Pathway Layout and Branch Points

Given pathway:

• D → E → F

• F → G → I → K

• F → H → J → L

End products: K and L, each formed via separate branches from the common intermediate F.

Principle of Concerted Feedback Inhibition

• Concerted inhibition involves regulation by both end products.

• The aim: prevent over-production of one branch’s product, secure equal (stoichiometric) formation.

• Each product inhibits its branch and together they control the common upstream entry.

Correct Regulatory Logic

• K inhibits F→G

  • This blocks excess K branch flux when K accumulates.

• L inhibits F→H

  • This blocks excess L branch flux when L accumulates.

• D→E inhibited at equal amounts of K and L

  • This ensures entry of substrate (D) is downregulated only when both products accumulate, enforcing stoichiometric output.

Why This Achieves Stoichiometric Output

• If only K or L could inhibit the main entry, the pathway could produce an excess of one product if the other’s demand is low.

• By inhibiting at F→G or F→H individually, each branch is self-regulated.

• Only when both K and L build up is the initial entry (D→E) reduced—this ensures neither product is far in excess of the other.

Why Other Choices Are Incorrect

• Option 2/3/4:

  • Suggest additional upstream branch inhibition or misplacement of regulatory points—not as efficient for enforcing strict stoichiometric (equal) end product formation.

Summary Table

Biological Significance

• Maintains critical cellular ratios of products.

• Protects cell against metabolic imbalance or wasteful excess of specific molecules.

• Exemplifies elegant regulatory design in biochemistry.

Conclusion

Concerted feedback inhibition at key nodes ensures stoichiometric production of two pathway end products. In this schematic, K and L inhibit their respective branches, and together their combined build-up impedes pathway entry, maintaining equal product formation. Hence, option (1) correctly describes this balanced biochemical logic.