During the pentose phosphate pathway reaction, the following – Free A7
During the pentose phosphate pathway reaction, the following reactions involving various sugar rearrangements are possible EXCEPT
Answer
Pentose Phosphate Pathway: Sugar Rearrangement Analysis
Reaction ①: Transketolase transfers a 2-carbon unit between two 5-carbon sugars.
Example: Xylulose-5P (C₅) + Ribose-5P (C₅) → Sedoheptulose-7P (C₇) + Glyceraldehyde-3P (C₃)
Carbon Balance: 5 + 5 = 10 → 7 + 3 = 10 ✅
Status: Valid Reaction
Reaction ②: Another transketolase reaction between two C₅ sugars (same as Reaction ①).
Status: Valid Reaction
Reaction ③: Transketolase transfers a 2-carbon unit from a C₅ to a C₄ sugar.
Example: Xylulose-5P (C₅) + Erythrose-4P (C₄) → Fructose-6P (C₆) + Glyceraldehyde-3P (C₃)
Carbon Balance: 5 + 4 = 9 → 6 + 3 = 9 ✅
Status: Valid Reaction
Reaction ④: Transaldolase transfers a 3-carbon unit from a C₇ sugar to a C₃ sugar.
Example: Sedoheptulose-7P (C₇) + Glyceraldehyde-3P (C₃) → Fructose-6P (C₆) + Erythrose-4P (C₄)
Carbon Balance: 7 + 3 = 10 → 6 + 4 = 10 ✅
Status: Valid Reaction
Invalid Reaction Identified:
While the carbon numbers are balanced, one rearrangement does not occur in the established pathway. It is not biochemically supported despite numerical validity.
Status: ❌ Not a Valid Reaction
Summary:
- ✅ Transketolase → transfers 2-carbon units
- ✅ Transaldolase → transfers 3-carbon units
- ✅ All valid reactions maintain carbon balance and follow known enzyme mechanisms
- ❌ One reaction is theoretically balanced but not part of the real biochemical pathway