In the non-oxidative phase of the pentose phosphate pathway, transketolase catalyzes the transfer of a 2-carbon unit from which sugar to another?

Explanation

## Non-Oxidative Phase of the Pentose Phosphate Pathway **Key Point:** Transketolase catalyzes the transfer of a 2-carbon ketol unit (from a ketose) to an aldose sugar. The primary reaction involves transfer from xylulose-5-phosphate (a ketose) to ribose-5-phosphate (an aldose). ### Transketolase Mechanism ```mermaid flowchart LR A["Xylulose-5-phosphate<br/>(ketose)"]:::outcome --> B["Transketolase<br/>+ TPP cofactor"]:::action B --> C["2-carbon unit<br/>transfer"]:::action C --> D["Ribose-5-phosphate<br/>(aldose)"]:::outcome E["Glyceraldehyde-3-phosphate<br/>product"]:::outcome -.-> B ``` **High-Yield:** Transketolase requires **thiamine pyrophosphate (TPP)** as a cofactor. Thiamine deficiency (Wernicke-Korsakoff syndrome) impairs transketolase activity. ### Transketolase Reactions in PPP | Reaction | Donor (Ketose) | Acceptor (Aldose) | Products | | --- | --- | --- | --- | | 1st | Xylulose-5-P | Ribose-5-P | Glyceraldehyde-3-P + Sedoheptulose-7-P | | 2nd | Xylulose-5-P | Erythrose-4-P | Glyceraldehyde-3-P + Fructose-6-P | **Mnemonic:** **Transketolase = Transfer of 2-carbon Ketol** — it moves a 2-carbon ketol unit from a ketose to an aldose. **Clinical Pearl:** Transketolase deficiency or thiamine deficiency impairs the non-oxidative phase, reducing the recycling of pentose sugars back to glucose-6-phosphate and reducing the net yield of NADPH.