Pentose phosphate pathway, non-oxidative branch

Route: Ribulose 5-phosphate → Ribose 5-phosphate + Xylulose 5-phosphate → Sedoheptulose 7-phosphate + Glyceraldehyde 3-phosphate → Erythrose 4-phosphate + Fructose 6-phosphate

The branch rearranges sugar phosphates and makes no net NADPH. An epimerase flips one hydroxyl group of ribulose 5-phosphate to give xylulose 5-phosphate. Transketolase, which needs thiamine pyrophosphate, moves a two-carbon unit from xylulose 5-phosphate to ribose 5-phosphate. This gives the seven-carbon sedoheptulose 7-phosphate and glyceraldehyde 3-phosphate. Transaldolase moves a three-carbon unit from sedoheptulose 7-phosphate to glyceraldehyde 3-phosphate, which gives erythrose 4-phosphate and fructose 6-phosphate. Transketolase acts a second time: it moves a two-carbon unit from xylulose 5-phosphate to erythrose 4-phosphate and gives fructose 6-phosphate and glyceraldehyde 3-phosphate. Fructose 6-phosphate and glyceraldehyde 3-phosphate are intermediates of glycolysis, so a cell can burn the sugars it does not need for nucleotides. The reactions run in both directions, so the branch can also make ribose 5-phosphate from glycolysis intermediates without the oxidative branch. Erythrose 4-phosphate is the start of the aromatic amino acid pathways. All three enzymes shown are human.

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Steps

Enzyme names and EC numbers are those used on the interactive board. Checked against sources: see the claim table in the repository (tools/pathway_data/checks).