Thiamine (B1) deficiency causes Wernicke-Korsakoff syndrome in alcoholics. Beyond its known role in pyruvate dehydrogenase and alpha-ketoglutarate dehydrogenase, which other thiamine-dependent enzyme, when deficient, explains the elevated pentose phosphates and ribose-5-phosphate found in Wernicke encephalopathy?
- A Transketolase — catalyzes two reversible steps in the non-oxidative HMP shunt, transferring 2-carbon units between sugar phosphates ✓
- B Pyruvate kinase — converts PEP to pyruvate, which requires TPP
- C Alpha-ketoacid dehydrogenase — catabolizes branched-chain keto acids
- D 2-hydroxyacyl-CoA lyase — an elongation enzyme in fatty acid synthesis
Explanation
Thiamine pyrophosphate (TPP) is required by transketolase in the non-oxidative phase of the HMP shunt. Transketolase transfers 2-carbon (glycolaldehyde) units in two steps: sedoheptulose-7-P + glyceraldehyde-3-P → xylulose-5-P + ribose-5-P, and fructose-6-P + glyceraldehyde-3-P → xylulose-5-P + erythrose-4-P. Thiamine deficiency reduces transketolase activity, disrupting pentose interconversion. Erythrocyte transketolase activity stimulation with TPP (>20% = thiamine deficiency) is a functional test for thiamine status. The elevated pentose phosphates reflect non-oxidative phase backup.
Reference: Harper's Illustrated Biochemistry, 32nd ed.
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