Biochemistry—Metabolic Pathway Logic In the enzyme-catalyzed pathway below, predict the effect of inactivating enzyme E2 on the appearance of intermediates and final product. Assume a unidirectional metabolic sequence under steady supply of A. A ---(E1)---> B ---(E2)---> C ---(E3)---> D

Introduction / Context:
Enzyme-catalyzed pathways convert substrates into products through a series of discrete, ordered steps. Each step is typically catalyzed by a specific enzyme. This question examines pathway logic: what happens upstream and downstream when one enzyme (E2) is specifically inactivated, while the rest remain functional and substrate A is continuously available.

Given Data / Assumptions:

• Linear pathway: A --(E1)--> B --(E2)--> C --(E3)--> D.
• Only E2 is inactivated; E1 and E3 retain full activity.
• Substrate A input continues; no alternative bypass routes exist.
• Products accumulate according to flux permitted by available enzymes.

Concept / Approach:
In a linear pathway, blocking a step prevents formation of all downstream intermediates and products beyond that step, but upstream reactions may continue and cause accumulation of intermediates just before the block. Therefore, activity prior to E2 (E1) proceeds, while any reaction requiring C cannot occur. E3 remains idle due to lack of its substrate C.

Step-by-Step Solution:

Verification / Alternative check:
Consider steady-state flux J. If a step is completely blocked, J downstream becomes zero. Upstream steps may still run, leading to B accumulation until regulatory feedback or substrate limitation reduces E1 flux.

Why Other Options Are Wrong:

• A, B, C, and D all present: impossible without E2 to make C.
• A, B, C present but not D: C cannot form without E2.
• A only: E1 is still active, so B forms.
• None present: feed A is still present and E1 still forms B.

Common Pitfalls:
Assuming E3 can act on B directly; assuming spontaneous conversion of B to C without catalysis; overlooking that upstream reactions can continue despite a downstream block.

Final Answer:
A and B will still be produced, but not C or D