161. The reduction phase of Calvin-Benson cycle in Arabidopsis is inhibited. This can be attributed to the
inactivation of:
1. Aldolase
2. Triose phosphate isomerase
3. Fructose-1,6-bisphosphatase
4. 3-Phosphoglycerate kinase
Understanding the Calvin-Benson Cycle and Its Inhibition in Arabidopsis
The Calvin-Benson cycle is the fundamental carbon fixation pathway in plants, converting atmospheric CO₂ into sugars using ATP and NADPH generated by the light reactions. This cycle operates in three major phases:
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Carboxylation
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Reduction
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Regeneration of RuBP
The reduction phase is particularly important for generating glyceraldehyde-3-phosphate (G3P), the precursor to sugars.
Reduction Phase: A Closer Look
In the reduction phase, the steps are:
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3-Phosphoglycerate (3-PGA) is converted to 1,3-bisphosphoglycerate by 3-phosphoglycerate kinase (PGK) using ATP.
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Then, 1,3-bisphosphoglycerate is reduced to glyceraldehyde-3-phosphate (G3P) by G3P dehydrogenase, consuming NADPH.
Which Enzyme’s Inactivation Inhibits the Reduction Phase?
Let’s evaluate the given options:
1. Aldolase
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Involved in the regeneration phase, not the reduction phase.
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Not responsible for halting the reduction.
2. Triose phosphate isomerase
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Converts G3P ↔ DHAP (interconversion of triose phosphates).
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Not the main step in reduction, but after it.
3. Fructose-1,6-bisphosphatase
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Functions in regeneration, especially in the conversion of fructose-1,6-bisphosphate to fructose-6-phosphate.
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Not part of reduction.
4. 3-Phosphoglycerate kinase
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Catalyzes the first step of the reduction phase, converting 3-PGA into 1,3-bisphosphoglycerate.
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If this enzyme is inhibited, the entire reduction phase halts, as downstream processes cannot occur.
Conclusion
In Arabidopsis, if the reduction phase of the Calvin-Benson cycle is inhibited, it is primarily due to the inactivation of 3-phosphoglycerate kinase (PGK). This enzyme catalyzes the ATP-dependent phosphorylation of 3-phosphoglycerate, a critical step in the conversion of fixed carbon into sugar precursors.
