- Engineering of metabolic pathways in plants can be achieved by introduction and overexpression of appropriate candidate gene(s) using transgenic technology. The figure given below represents a biochemical pathway in plants where a precursor molecule ‘A’ is converted in to products ‘T’ and ‘X’ through a series of enzymatic reactions. Enzymes 1-5 are involved in this pathway. Scientists attempted to increase the levels of ‘X’ by introducing additional copy
of the gene for enzyme ‘5’ under transcriptional control of a strong constitutive promoter. However, the developed transgenic plants did not display a proportionate increase in the level of ‘X’.The following statements were proposed for explaining the above results:
(A) Enzyme ‘4’ has greater affinity for D than enzyme ‘3’
(B) Feedback inhibition of enzyme ‘5’ by compound X
(C) Substrate limitation for enzyme ‘5’
Which of the above statement could represent probable reasons for NOT obtaining proportionate increase in the amount of ‘X’ in the transgenic plants?
(1) Only C (2) Only A and B
(3) Only A (4) A, B and CThe correct answer is (4) A, B and C – all three statements can explain why overexpressing enzyme 5 did not give a proportionate increase in product X.
Pathway context from the figure
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A → C → D → E → X via enzymes 1, 2, 3, 5.
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At D, there is a branch: D → T via enzyme 4.
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Scientists overexpress enzyme 5 (E → X step) but X does not increase proportionally.
Explanation of each proposed reason
(A) Enzyme 4 has greater affinity for D than enzyme 3
If enzyme 4 has a lower Km for D than enzyme 3, it will capture much of intermediate D, channeling it toward T rather than toward E and then X.
Even if enzyme 5 is overexpressed, the flux into E (and thus X) is limited because substrate D is preferentially consumed by enzyme 4.
So A is a valid reason for not seeing a proportional increase in X.(B) Feedback inhibition of enzyme 5 by compound X
If product X inhibits enzyme 5 (feedback inhibition), then as X starts to accumulate in the transgenic plants, it will slow the activity of enzyme 5.
This self‑limiting control prevents further increase of flux to X despite higher amounts of enzyme 5, so B is also a plausible explanation.(C) Substrate limitation for enzyme 5
Overexpressing enzyme 5 increases catalytic capacity, but if its substrate E is limiting (due to finite upstream flux through enzymes 1–3 or siphoning at D via enzyme 4), then reaction rate cannot rise proportionally.
In other words, enzyme 5 becomes in excess relative to its substrate, making substrate supply the new bottleneck.
Thus C is a valid explanation too.
Why option (4) is correct
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All three mechanisms—strong competition at a branch point (A), feedback inhibition by product (B), and limited substrate supply to the overexpressed step (C)—are classic reasons in metabolic engineering for why increasing one enzyme’s expression does not give a proportional increase in end product.
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Therefore the correct choice is (4) A, B and C.
SEO‑oriented introduction (for article use)
In metabolic engineering of plants, simply overexpressing a single downstream enzyme, such as enzyme 5 in a branched pathway from precursor A to product X, often fails to yield a proportional increase in the target metabolite. Competitive branch reactions (enzyme 4 using intermediate D), feedback inhibition of the engineered enzyme by product X, and substrate limitation upstream can all restrict flux, explaining why transgenic plants overexpressing enzyme 5 do not accumulate X as much as expected.
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