In the HMP shunt, transketolase requires thiamine pyrophosphate (TPP) as cofactor and transfers 2-carbon units. In thiamine deficiency, which specific reaction step in the non-oxidative phase of the pentose phosphate pathway is PRIMARILY impaired?
- A Conversion of ribulose-5-phosphate to ribose-5-phosphate by phosphopentose isomerase
- B Transfer of a 3-carbon unit from sedoheptulose-7-phosphate to glyceraldehyde-3-phosphate by transaldolase
- C Oxidative decarboxylation of 6-phosphogluconate by 6-phosphogluconate dehydrogenase
- D Transfer of a 2-carbon unit from xylulose-5-phosphate to ribose-5-phosphate by transketolase ✓
Explanation
Transketolase is the TPP-dependent enzyme in the non-oxidative phase of the pentose phosphate pathway. In thiamine deficiency, transketolase activity is specifically reduced, impairing the transfer of 2-carbon units (glycolaldehyde units) from ketoses (xylulose-5-phosphate) to aldoses (ribose-5-phosphate or erythrose-4-phosphate). Transaldolase transfers 3-carbon units and does not require TPP. Phosphopentose isomerase requires no cofactor. The oxidative phase (6-phosphogluconate dehydrogenase) uses NADP+ not TPP. RBC transketolase activity measurement, with and without added TPP (the TPP effect), is a classic biochemical test for thiamine deficiency.
Reference: Harper's Illustrated Biochemistry, 32nd ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.