L-cysteine biosynthesis I

MetaCyc:CYSTSYN-PWY

L-cysteine biosynthesis I converts L-serine to L-cysteine through O-acetyl-L-serine.

Pathway type

amino acid biosynthesis

Organism scope

Escherichia coli NCBITaxon:562

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Pathway network

16 nodes · 18 asserted directed edges

Arrows show recorded subject → object relationships, not necessarily metabolic flow. Select a node for details or an arrow for evidence. Scroll, zoom or find a node to explore.

Reaction / activity
Chemical / cofactor
Protein / complex
Other participant
Cell / process context
Solid arrow: mechanistic relation
Dashed arrow: membership, location or cofactor relation

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Components and activities

Declared biological components and source reaction directions
Node	Kind or role	Source direction
Escherichia coli `NCBITaxon:562`	taxon
serine O-acetyltransferase `EC:2.3.1.30`	molecular activity
cysteine synthase `EC:2.5.1.47`	molecular activity
L-serine `CHEBI:33384`	small molecule
acetyl-CoA `CHEBI:57288`	small molecule
O-acetyl-L-serine `CHEBI:58340`	small molecule
CoA `CHEBI:57287`	small molecule
hydrosulfide `CHEBI:29919`	small molecule
L-cysteine `CHEBI:35235`	small molecule
acetate `CHEBI:30089`	small molecule
Serine acetyltransferase `UniProtKB:P0A9D4`	protein
cytoplasm `GO:0005737`	cellular component
Cysteine synthase A `UniProtKB:P0ABK5`	protein
pyridoxal 5'-phosphate(2-) `CHEBI:597326`	cofactor
Cysteine synthase B `UniProtKB:P16703`	protein
serine O-acetyltransferase reaction `RHEA:24561`	reaction	left to right
O-acetyl-L-serine sulfhydrylase reaction `RHEA:14830`	reaction	left to right

Mechanistic edges

Mechanistic edges and their cited evidence
Subject	Predicate and description	Object	Evidence
L-serine `CHEBI:33384`	consumes	serine O-acetyltransferase reaction `RHEA:24561`	RHEA:24561 Source assertion: Rhea lists CHEBI:33384 on the substrates side of RHEA:24561 in this directed reaction. Source location `rdf:Description[@rdf:about='http://rdf.rhea-db.org/24561']/rh:substrates; rh:contains/rh:compound/rh:accession`
acetyl-CoA `CHEBI:57288`	consumes	serine O-acetyltransferase reaction `RHEA:24561`	RHEA:24561 Source assertion: Rhea lists CHEBI:57288 on the substrates side of RHEA:24561 in this directed reaction. Source location `rdf:Description[@rdf:about='http://rdf.rhea-db.org/24561']/rh:substrates; rh:contains/rh:compound/rh:accession`
serine O-acetyltransferase reaction `RHEA:24561`	produces	CoA `CHEBI:57287`	RHEA:24561 Source assertion: Rhea lists CHEBI:57287 on the products side of RHEA:24561 in this directed reaction. Source location `rdf:Description[@rdf:about='http://rdf.rhea-db.org/24561']/rh:products; rh:contains/rh:compound/rh:accession`
serine O-acetyltransferase reaction `RHEA:24561`	produces	O-acetyl-L-serine `CHEBI:58340`	RHEA:24561 Source assertion: Rhea lists CHEBI:58340 on the products side of RHEA:24561 in this directed reaction. Source location `rdf:Description[@rdf:about='http://rdf.rhea-db.org/24561']/rh:products; rh:contains/rh:compound/rh:accession`
hydrosulfide `CHEBI:29919`	consumes	O-acetyl-L-serine sulfhydrylase reaction `RHEA:14830`	RHEA:14830 Source assertion: Rhea lists CHEBI:29919 on the substrates side of RHEA:14830 in this directed reaction. Source location `rdf:Description[@rdf:about='http://rdf.rhea-db.org/14830']/rh:substrates; rh:contains/rh:compound/rh:accession`
O-acetyl-L-serine `CHEBI:58340`	consumes	O-acetyl-L-serine sulfhydrylase reaction `RHEA:14830`	RHEA:14830 Source assertion: Rhea lists CHEBI:58340 on the substrates side of RHEA:14830 in this directed reaction. Source location `rdf:Description[@rdf:about='http://rdf.rhea-db.org/14830']/rh:substrates; rh:contains/rh:compound/rh:accession`
O-acetyl-L-serine sulfhydrylase reaction `RHEA:14830`	produces	acetate `CHEBI:30089`	RHEA:14830 Source assertion: Rhea lists CHEBI:30089 on the products side of RHEA:14830 in this directed reaction. Source location `rdf:Description[@rdf:about='http://rdf.rhea-db.org/14830']/rh:products; rh:contains/rh:compound/rh:accession`
O-acetyl-L-serine sulfhydrylase reaction `RHEA:14830`	produces	L-cysteine `CHEBI:35235`	RHEA:14830 Source assertion: Rhea lists CHEBI:35235 on the products side of RHEA:14830 in this directed reaction. Source location `rdf:Description[@rdf:about='http://rdf.rhea-db.org/14830']/rh:products; rh:contains/rh:compound/rh:accession`
serine O-acetyltransferase `EC:2.3.1.30`	catalyzes	serine O-acetyltransferase reaction `RHEA:24561`	RHEA:24560 Source assertion: Rhea associates EC:2.3.1.30 with the reaction family containing RHEA:24561. Source location `rh:ec` PMID:12627979 Random-order ternary complex reaction mechanism of serine acetyltransferase from Escherichia coli. Source location `PubMed ArticleTitle`
cysteine synthase `EC:2.5.1.47`	catalyzes	O-acetyl-L-serine sulfhydrylase reaction `RHEA:14830`	RHEA:14829 Source assertion: Rhea associates EC:2.5.1.47 with the reaction family containing RHEA:14830. Source location `rh:ec` PMID:16546401 O-Acetylserine sulfhydrylase-B (OASS-B, EC 2.5.1.47) is one of the two isozymes produced by Escherichia coli that catalyze the synthesis of L-cysteine from O-acetyl-L-serine and sulfide. Source location `PubMed Abstract`
serine O-acetyltransferase reaction `RHEA:24561`	precedes	O-acetyl-L-serine sulfhydrylase reaction `RHEA:14830`	MetaCyc:CYSTSYN-PWY Source assertion: MetaCyc pathway ordering places RHEA:24561 before RHEA:14830 in this pathway. Shared intermediate: CHEBI:58340. Source location `Pathway/reaction-ordering and reaction-layout; Rhea-to-MetaCyc cross-references`
Serine acetyltransferase `UniProtKB:P0A9D4`	enables	serine O-acetyltransferase reaction `RHEA:24561`	UniProtKB:P0A9D4 Source assertion: UniProt annotates UniProtKB:P0A9D4 with catalytic activity for the reaction family of RHEA:24561. Annotation evidence: curated source annotation; organism taxon: NCBITaxon:83333. Source location `comments[commentType='CATALYTIC ACTIVITY'].reaction.reactionCrossReferences`
L-cysteine `CHEBI:35235`	inhibits	Serine acetyltransferase `UniProtKB:P0A9D4`	UniProtKB:P0A9D4 Source assertion: UniProt activity-regulation annotation: Sensitive to feedback inhibition by L-cysteine Source location `comments[commentType='ACTIVITY REGULATION'].texts`
Serine acetyltransferase `UniProtKB:P0A9D4`	located_in	cytoplasm `GO:0005737`	UniProtKB:P0A9D4 Source assertion: UniProt locates UniProtKB:P0A9D4 in Cytoplasm (SL-0086, mapped by UniProt to GO:0005737). Annotation evidence: curated source annotation. Source location `comments[commentType='SUBCELLULAR LOCATION'].subcellularLocations; UniProt location vocabulary GO mapping`
Cysteine synthase A `UniProtKB:P0ABK5`	enables	O-acetyl-L-serine sulfhydrylase reaction `RHEA:14830`	UniProtKB:P0ABK5 Source assertion: UniProt annotates UniProtKB:P0ABK5 with catalytic activity for the reaction family of RHEA:14830. Annotation evidence: ECO:0000269; organism taxon: NCBITaxon:83333. Source location `comments[commentType='CATALYTIC ACTIVITY'].reaction.reactionCrossReferences`
Cysteine synthase A `UniProtKB:P0ABK5`	has_cofactor	pyridoxal 5'-phosphate(2-) `CHEBI:597326`	UniProtKB:P0ABK5 Source assertion: UniProt lists CHEBI:597326 as a cofactor of UniProtKB:P0ABK5 (evidence: ECO:0000269). Source location `comments[commentType='COFACTOR'].cofactors and note`
Cysteine synthase B `UniProtKB:P16703`	enables	O-acetyl-L-serine sulfhydrylase reaction `RHEA:14830`	UniProtKB:P16703 Source assertion: UniProt annotates UniProtKB:P16703 with catalytic activity for the reaction family of RHEA:14830. Annotation evidence: curated source annotation; organism taxon: NCBITaxon:83333. Source location `comments[commentType='CATALYTIC ACTIVITY'].reaction.reactionCrossReferences`
Cysteine synthase B `UniProtKB:P16703`	has_cofactor	pyridoxal 5'-phosphate(2-) `CHEBI:597326`	UniProtKB:P16703 Source assertion: UniProt lists CHEBI:597326 as a cofactor of UniProtKB:P16703 (evidence: curated annotation). Source location `comments[commentType='COFACTOR'].cofactors and note`

References

MetaCyc:CYSTSYN-PWY MetaCyc L-cysteine biosynthesis I pathway (source version: 22.5)
RHEA:24561 Rhea biosynthetic serine O-acetyltransferase direction (source version: 139)
RHEA:14829 Rhea cysteine synthase reaction (source version: 139)
RHEA:14830 Rhea left-to-right cysteine synthase direction (source version: 139)
PMID:12627979 Hindson and Shaw, Biochemistry, 2003
PMID:16546401 Zhao et al., Protein Expression and Purification, 2006
RHEA:24560 RHEA:24560 authority record (source version: 139)
UniProtKB:P0A9D4 UniProtKB:P0A9D4 authority record (source version: 2026_03)
UniProtKB:P0ABK5 UniProtKB:P0ABK5 authority record (source version: 2026_03)
UniProtKB:P16703 UniProtKB:P16703 authority record (source version: 2026_03)

Curation history

2026-10-05T06:25:52Z codex
- EDIT — Review causal chemistry and add sourced molecular context
  Reviewed the entire record against MetaCyc 22.5 native pathway XML (including nested pathways), Rhea 139 explicit sides and directions, ENZYME 02-Sep-2026, PubMed records, and reviewed UniProt 2026_03 annotations within the record's taxon scope. Replaced generated structured-source quotations with source_assertion and source_locator. Typed molecular activities and participants. Reaction count 2 to 2; 0 directed-ID corrections; 3 protein annotations, 2 cofactor annotations and 1 sourced locations. All Rhea reaction substrate/product sets checked against independent Rhea authority; native pathway leaf coverage and source exceptions recorded in reports/causal_graph_review/metacyc-review.json. Full source URLs, versions and byte hashes are in metacyc-sources.json. All 50 MetaCyc records passed native schema validation; six targeted chemistry/evidence regression tests passed. Missing protein or cofactor annotations are not treated as biological absence; qualifications and exclusions are preserved in the ledger.
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