D-xylose catabolism to D-xylulose 5-phosphate (oxidoreductase and isomerase routes)

D-xylose, the main pentose of plant hemicellulose, is channelled into the pentose phosphate pathway as D-xylulose 5-phosphate. The isomerisation of xylose to xylulose is achieved by one of two routes. In xylose-utilising yeasts and filamentous fungi, an aldo-keto reductase family xylose reductase reduces D-xylose to xylitol with NADPH (or NADH), and an NAD+-dependent medium-chain dehydrogenase, xylitol dehydrogenase (D-xylulose reductase), oxidises xylitol to D-xylulose. Because the two steps use different cofactors, the route can create a redox imbalance and xylitol accumulation under oxygen limitation. Bacteria, and some anaerobic fungi, instead use a single cofactor-free xylose isomerase. In both routes an FGGY-family xylulokinase then phosphorylates D-xylulose with ATP to D-xylulose 5-phosphate, which enters the non-oxidative pentose phosphate pathway. Xylose uptake and downstream pentose phosphate metabolism are outside this module.

MODULE:xylose_oxidoreductase_pathwayDRAFTCONCRETEMetabolic Pathwaymodules/xylose_oxidoreductase_pathway.yaml
D-xylose catabolic processGO:0042843
GO:0042843
D-xylose catabolic process
Module-level process concept.
GO:0005998
xylulose catabolic process
Process concept for the xylulokinase step, which also serves xylulose taken up directly.
YeastPathways:PWY3O-8
xylose metabolism (S. cerevisiae, SGD YeastPathways)
YeastCyc lists RXN-8773 (GRE3 aldose reductase, written in the xylitol oxidation direction) and D-XYLULOSE-REDUCTASE-RXN (XYL2). Both assignments were retained; the reductase reaction is modelled in its physiological direction (xylose to xylitol).
YeastPathways:PWY3O-5
xylulose degradation (S. cerevisiae, SGD YeastPathways)
YeastCyc lists XYLULOKIN-RXN with XKS1. Retained.
PMID:15184173
Endogenous xylose pathway in Saccharomyces cerevisiae.
Overexpression of the endogenous GRE3 aldose reductase and XYL2 xylitol dehydrogenase lets S. cerevisiae grow (slowly) on xylose; the S. stipitis XR and XDH work better.
We found that S. cerevisiae can grow on D-xylose when only the endogenous genes GRE3 (YHR104w), coding for a nonspecific aldose reductase, and XYL2 (YLR070c, ScXYL2), coding for a xylitol dehydrogenase (XDH), are overexpressed under endogenous promoters.
PMID:7747971
Purification and partial characterization of an aldo-keto reductase from Saccharomyces cerevisiae.
The cytosolic NADPH-specific aldo-keto reductase later identified as GRE3 resembles yeast and mammalian aldose reductases and has broad aldehyde specificity.
These data indicate that the S. cerevisiae aldo-keto reductase is a monomeric NADPH-specific reductase with strong similarities to the aldose reductases.
PMID:10486580
Evidence that the gene YLR070c of Saccharomyces cerevisiae encodes a xylitol dehydrogenase.
XYL2/YLR070C encodes an NADH-specific xylitol dehydrogenase.
Overexpression of this open reading frame in S. cerevisiae resulted in xylitol dehydrogenase activity. The enzyme is specific for NADH.
PMID:18555359
Direct evidence for a xylose metabolic pathway in Saccharomyces cerevisiae.
Wild-type S. cerevisiae has low xylose reductase and xylitol dehydrogenase activity and converts xylose mainly to xylitol.
In vivo conversion of (14)C-xylose in S. cerevisiae is demonstrated and xylitol is detected
PMID:9595677
The YGR194c (XKS1) gene encodes the xylulokinase from the budding yeast Saccharomyces cerevisiae.
XKS1 is required for growth on D-xylulose and encodes xylulokinase.
We report the finding of a Saccharomyces cerevisiae gene necessary for growth in culture media with D-xylulose as the sole carbon source.
PMID:10981687
The role of xylulokinase in Saccharomyces cerevisiae xylulose catabolism.
Xylulokinase is rate-limiting for xylulose use in S. cerevisiae; xks1 deletion prevents xylulose metabolism.
deletion of the gene prevented growth on D-xylulose and D-xylulose metabolism.
PMID:1756986
Cloning and expression in Saccharomyces cerevisiae of the NAD(P)H-dependent xylose reductase-encoding gene (XYL1) from the xylose-assimilating yeast Pichia stipitis.
Scheffersomyces (Pichia) stipitis XYL1 encodes the NADH/NADPH-dependent xylose reductase.
The XYL1 encodes a polypeptide of 35,927 Da that constitutes a NADH/NADPH-dependent xylose reductase (XR).
PMID:2127555
Isolation and characterization of the Pichia stipitis xylitol dehydrogenase gene, XYL2, and construction of a xylose-utilizing Saccharomyces cerevisiae transformant.
S. stipitis XYL2 encodes xylitol dehydrogenase; XYL1 plus XYL2 let S. cerevisiae grow on xylose.
S. cerevisiae cells transformed with a plasmid, pRD1, containing both the xylose reductase gene (XYL1) and the xylitol dehydrogenase gene (XYL2), were able to grow on xylose as a sole carbon source.
PMID:14554198
High-level functional expression of a fungal xylose isomerase: the key to efficient ethanolic fermentation of xylose by Saccharomyces cerevisiae?
The anaerobic fungus Piromyces sp. E2 uses xylose isomerase rather than the reductase/dehydrogenase route, so the isomerase variant is not restricted to bacteria.
Evidence is presented that xylose metabolism in the anaerobic cellulolytic fungus Piromyces sp. E2 proceeds via a xylose isomerase rather than via the xylose reductase/xylitol-dehydrogenase pathway found in xylose-metabolising yeasts.
PMID:20171088
Hemicelluloses for fuel ethanol: A review.
Background review on hemicellulose-derived xylose and its fermentation (cited by YeastPathways for both frames).
7Nodes
4Parts
1Variant Sets
2Variants
4Annotons
2Connections

Derived QC

Recommended-field compliance

100.0% recommended fields populated

All recommended fields populated.

Module deep research

✗ none found

No MODULE:xylose_oxidoreductase_pathway deep-research report alongside the module YAML.

Leaf nodes lacking representative members

✓ every leaf node grounds to a representative protein.

Template conformance

✓ every declared conforms_to bundle matches its template motif.

Reaction chaining (advisory)

✓ every PRECEDES step chains, or its break is acknowledged via chaining_status.

  • xylose_to_xylulose → xylulose_phosphorylation [NOT_CHECKED]
    D-xylulose from either route is the xylulokinase substrate; no CoA intermediate.

Function consistency and core coverage

4 gene-function check(s) reflected in core functions · 0 conflict(s) · 5 evidence or coverage gap(s) (12 comparison(s))

Retained noncore functions are reported separately; they do not require promotion to core functions. Support from representative genes does not establish conservation across the whole family. Missing reviews or identifiers leave a claim unverified.

AnnotonParticipantFunctionAssessment
xylose_reductase_activity GRE3 (Saccharomyces cerevisiae)
UniProtKB:P38715
Family Representative 		D-xylose reductase (NADPH) activity
GO:0032866 		Core Supported
The GO molecular function is reflected in core_functions (exact term or a reviewed subclass).
xylose_reductase_activity XYL1 (Scheffersomyces stipitis)
UniProtKB:P31867
Family Representative 		D-xylose reductase (NADPH) activity
GO:0032866 		Core Supported
The GO molecular function is reflected in core_functions (exact term or a reviewed subclass).
xylitol_dehydrogenase_activity XYL2 (Saccharomyces cerevisiae)
UniProtKB:Q07993
Family Representative 		D-xylulose reductase activity
GO:0046526 		Core Supported
The GO molecular function is reflected in core_functions (exact term or a reviewed subclass).
xylitol_dehydrogenase_activity XYL2 (Scheffersomyces stipitis)
UniProtKB:P22144
Family Representative 		D-xylulose reductase activity
GO:0046526 		Review Missing
No ai-review exists for this protein; its function and core-function coverage cannot yet be checked.
xylose_isomerase_activity xylA (Escherichia coli K-12)
UniProtKB:P00944
Family Representative 		xylose isomerase activity
GO:0009045 		Review Missing
No ai-review exists for this protein; its function and core-function coverage cannot yet be checked.
xylulokinase_activity XKS1 (Saccharomyces cerevisiae)
UniProtKB:P42826
Family Representative 		D-xylulokinase activity
GO:0004856 		Core Supported
The GO molecular function is reflected in core_functions (exact term or a reviewed subclass).
xylulokinase_activity xylB (Escherichia coli K-12)
UniProtKB:P09099
Family Representative 		D-xylulokinase activity
GO:0004856 		Review Missing
No ai-review exists for this protein; its function and core-function coverage cannot yet be checked.
xylose_reductase_activity AKR2-type aldose/xylose reductase
PANTHER:PTHR11732
Family 		D-xylose reductase (NADPH) activity
GO:0032866 		Family Scope Restricted
GO:0032866 is SUBFAMILY_ONLY; this family grounding does not establish the allowed subfamily scope. Aldose/xylose reductases such as GRE3 (SF218), as xylose_oxidoreductase_pathway asserts on the base family.
xylitol_dehydrogenase_activity medium-chain zinc dehydrogenase xylitol dehydrogenase subfamily
PANTHER:PTHR43161:SF9
Family 		D-xylulose reductase activity
GO:0046526 		Family Supported
Family review supports GO:0046526 for PANTHER:PTHR43161:SF9. The term the xylose oxidoreductase module requires of SF9 (XYL2). It is the primary activity of the dedicated xylitol dehydrogenases in SF9 (S. cerevisiae XYL2, S. stipitis XYL2, Aspergillus xdhA). The sorbitol dehydrogenases in SF9 also oxidise xylitol, but for them it is a secondary activity, and E. coli IdnD (SF9) is an L-idonate dehydrogenase. Plant SDH receives the term from its own node (PTN001707508).
xylose_isomerase_activity xylose isomerase (XylA)
Family 		xylose isomerase activity
GO:0009045 		Family Id Missing
The family has no identifier linking it to a curated family review.
xylulokinase_activity FGGY-family D-xylulokinase
PANTHER:PTHR10196:SF57
Family 		D-xylulokinase activity
GO:0004856 		Family Supported
Family review supports GO:0004856 for PANTHER:PTHR10196:SF57. Xylulokinases (SF57), as xylose_oxidoreductase_pathway selects for S. cerevisiae XKS1. The glycerol kinase and rhamnulokinase subfamilies do not phosphorylate xylulose.
xylulokinase_activity FGGY-family D-xylulokinase
PANTHER:PTHR43095:SF6
Family 		D-xylulokinase activity
GO:0004856 		Family Supported
Family review supports GO:0004856 for PANTHER:PTHR43095:SF6. D-xylulokinase for E. coli XylB (SF6), as xylose_oxidoreductase_pathway selects; SF5 contains XylB orthologs too, but mixed with other kinases.

Gene-review completeness (4/7 grounded genes reviewed)

4 complete review(s) · 4 with deep research · 3 missing review · 0 reviewed but lacking deep research

Gene Review Complete Deep research
GRE3 P38715 ✓ ✓ ✓
xylA (Escherichia coli K-12) P00944 ✗ — —
xylB (Escherichia coli K-12) P09099 ✗ — —
XYL2 (Scheffersomyces stipitis) P22144 ✗ — —
XKS1 P42826 ✓ ✓ ✓
XYL1 P31867 ✓ ✓ ✓
XYL2 Q07993 ✓ ✓ ✓

Details

D-xylose catabolism to D-xylulose 5-phosphateMetabolic Pathwayxylose_oxidoreductase_pathway
D-xylose catabolic processGO:0042843

NEW module (YeastPathways crosswalk, 2026-10) generalising PWY3O-8 (xylose metabolism) and PWY3O-5 (xylulose degradation). The crosswalk slug names the fungal oxidoreductase route, but the module also includes the bacterial and anaerobic-fungal xylose isomerase as an alternative entry variant, as the crosswalk scope asks; the xylulokinase step is common to both. Boundary: xylose transport (hexose transporters in S. cerevisiae) and the pentose phosphate pathway are outside. S. cerevisiae: all three enzymes are present, but wild-type cells grow on xylulose and not on xylose and convert xylose mainly to xylitol (PMID:18555359, PMID:15184173), because GRE3 and XYL2 activities are low, the NADPH-preferring reductase and NAD+-dependent dehydrogenase unbalance cofactors, and XKS1 expression is low (PMID:10981687). Other S. cerevisiae aldose reductases (for example YPR1) and sorbitol dehydrogenases SOR1/SOR2 can contribute to the two oxidoreductase steps (SOR1 is xylose-induced, PMID:15184173); they were not added as exemplars because their in vivo contribution is not established. YeastCyc corrections: the GRE3 reaction RXN-8773 is written in the xylitol-oxidation direction with EC 1.1.1.21; the module models the physiological reduction of D-xylose. GO-CAM disagreements: the YeastPathways PWY3O-8 model types GRE3 with the general aldose reductase (NADPH) term GO:0004032 and uses the broad process D-xylose metabolic process (GO:0042732); there is no GO-CAM for the XKS1 step. No PomBase GO-CAM is linked and S. pombe is not used as an exemplar.

Connections

Part 1: isomerisation of D-xylose to D-xylulose
D-xylose to D-xyluloseMetabolic Pathwayxylose_to_xylulose
Variant set: Two-step oxidoreductase route versus one-step isomerase route by route (One Or More)

Fungi typically use the oxidoreductase route and bacteria the isomerase route, but anaerobic fungi such as Piromyces use a xylose isomerase (PMID:14554198). Engineered S. cerevisiae strains use either.

Xylose reductase plus xylitol dehydrogenaseMetabolic Pathwayoxidoreductase_route

Connections

Part 1: xylose reduction
D-xylose + NAD(P)H to xylitol + NAD(P)+Reactionxylose_reduction

Annotons

Aldo-keto reductase family xylose reductase
xylose_reductase_activity
Participant: Family: AKR2-type aldose/xylose reductase
Family:
AKR2-type aldose/xylose reductasePANTHER:PTHR11732 Broad aldo-keto reductase family; xylose reductases are a subset and many members act on other aldehydes. Kept at family level: PANTHER 19.0 places GRE3 in SF218, but the dedicated S. stipitis xylose reductase XYL1 has no subfamily assignment, so no subfamily list can be shown to cover both exemplars.
Representative Members: GRE3 (Saccharomyces cerevisiae)UniProtKB:P38715 XYL1 (Scheffersomyces stipitis)UniProtKB:P31867
Required Function:
D-xylose reductase (NADPH) activityGO:0032866

Function

D-xylose reductase (NADPH) activityGO:0032866
Substrates: D-xylose NADPH
Products: xylitol NADP+

Locations

cytosolGO:0005829

First step of the fungal route. In S. cerevisiae GRE3 is a stress-induced broad-specificity aldose reductase rather than a dedicated xylose reductase.

Part 2: xylitol oxidation
xylitol + NAD+ to D-xylulose + NADHReactionxylitol_oxidation

Annotons

Xylitol dehydrogenase (D-xylulose reductase)
xylitol_dehydrogenase_activity
Participant: Family: medium-chain zinc dehydrogenase xylitol dehydrogenase subfamily
Family:
medium-chain zinc dehydrogenase xylitol dehydrogenase subfamilyPANTHER:PTHR43161:SF9 UniProt places S. cerevisiae XYL2 and S. stipitis XYL2 in this subfamily; it is named after the related sorbitol dehydrogenases.
Representative Members: XYL2 (Saccharomyces cerevisiae)UniProtKB:Q07993 XYL2 (Scheffersomyces stipitis)UniProtKB:P22144
Required Function:
D-xylulose reductase activityGO:0046526

Function

D-xylulose reductase activityGO:0046526
Substrates: xylitol NAD+
Products: D-xylulose NADH

Locations

cytosolGO:0005829

NAD+-dependent second step. Its cofactor preference differs from the NADPH-preferring reductase, which causes NADH accumulation and xylitol excretion under oxygen limitation.

Xylose isomeraseReactionisomerase_route

Annotons

Xylose isomerase
xylose_isomerase_activity
Participant: Family: xylose isomerase (XylA)
Family:
xylose isomerase (XylA) No PANTHER family is asserted: PANTHER's own classification places E. coli XylA in PTHR32176:SF92 (XYLOSE ISOMERASE) while UniProt's cross-reference gives PTHR48408, and the discrepancy was not resolved.
Representative Members: xylA (Escherichia coli K-12)UniProtKB:P00944
Required Function:
xylose isomerase activityGO:0009045

Function

xylose isomerase activityGO:0009045
Substrates: D-xylose
Products: D-xylulose
Cofactors: divalent metal cation (Mg2+ or Mn2+)

Locations

cytosolGO:0005829

Cofactor-free one-step alternative, typical of bacteria and also found in anaerobic fungi; S. cerevisiae lacks it.

Part 2: xylulose phosphorylation
D-xylulose + ATP to D-xylulose 5-phosphate + ADPReactionxylulose_phosphorylation
xylulose catabolic processGO:0005998

Annotons

FGGY-family D-xylulokinase
xylulokinase_activity
Participant: Family: FGGY-family D-xylulokinase
Family:
FGGY-family D-xylulokinase PANTHER 19.0 places S. cerevisiae XKS1 in PTHR10196:SF57 and E. coli XylB in PTHR43095:SF6; both are FGGY carbohydrate kinases.
Representative Members: XKS1 (Saccharomyces cerevisiae)UniProtKB:P42826 xylB (Escherichia coli K-12)UniProtKB:P09099
Required Function:
D-xylulokinase activityGO:0004856

Function

D-xylulokinase activityGO:0004856
Substrates: D-xylulose ATP
Products: D-xylulose 5-phosphate ADP

Locations

cytosolGO:0005829

Shared terminal step of both entry routes. Produces D-xylulose 5-phosphate for the non-oxidative pentose phosphate pathway (module pentose_phosphate_nonoxidative_branch).