Q.51 Write the correct sequence of intermediates formed during Pentose Phosphate pathway.
A. 6-phosphogluconate
B. Ribose-5-phosphate
C. Sedoheptulose-7-phosphate
D. Glucose-6-phosphate
E. Ribulose-5-phosphate
Choose the correct answer from the options given below:
1. B, C, D, E, A
2. A, D, B, E, C
3. A, B, C, D, E
4. D, A, B, E, C
Correct Answer: 4. D, A, B, E, C
The Pentose Phosphate Pathway (PPP), also known as the hexose monophosphate shunt, begins with glucose-6-phosphate and generates NADPH and ribose-5-phosphate through oxidative and non-oxidative phases.
Pathway Sequence
Oxidative Phase Intermediates
Glucose-6-phosphate (D) is oxidized by glucose-6-phosphate dehydrogenase to 6-phosphogluconolactone, then hydrolyzed to 6-phosphogluconate (A), and finally decarboxylated to ribulose-5-phosphate (E), producing NADPH.
Ribulose-5-phosphate (E) is then isomerized to ribose-5-phosphate (B), a key product for nucleotide synthesis.
Non-Oxidative Phase Intermediates
Ribose-5-phosphate (B) and xylulose-5-phosphate (from epimerization of E) rearrange via transketolase and transaldolase to form sedoheptulose-7-phosphate (C) and other glycolytic intermediates like glyceraldehyde-3-phosphate.
Option Analysis
| Option | Sequence (D=Glucose-6-P, A=6-PG, B=Rib5P, E=RibU5P, C=Sed7P) | Correct? | Reason |
|---|---|---|---|
| 1 | B, C, D, E, A | No | Starts with B (mid-pathway product), skips initial steps from D. |
| 2 | A, D, B, E, C | No | Begins with A before substrate D; illogical order. |
| 3 | A, B, C, D, E | No | Places D after early intermediates; pathway starts with D. |
| 4 | D, A, B, E, C | Yes | Matches oxidative flow: D → A → E → B; C follows in non-oxidative rearrangements. |
This sequence reflects the core progression in most tissues, prioritizing NADPH production before pentose interconversions.
