id: A4F7P3
gene_symbol: eryCIII
product_type: PROTEIN
status: COMPLETE
taxon:
  id: NCBITaxon:405948
  label: Saccharopolyspora erythraea (strain ATCC 11635 / DSM 40517 / JCM 4748 / NBRC
    13426 / NCIMB 8594 / NRRL 2338)
description: >-
  EryCIII (SACE_0726) is the desosaminyl glycosyltransferase (EC 2.4.1.278) of
  erythromycin biosynthesis in Saccharopolyspora erythraea. It transfers D-desosamine
  from the nucleotide sugar donor TDP-D-desosamine onto 3-alpha-mycarosylerythronolide
  B, converting it to erythromycin D, and is highly active and selective for its
  natural donor and macrolide acceptor (PMID:15303858). EryCIII works in concert with
  its activating partner EryCII, with which it forms an elongated heterotetramer;
  EryCII stabilizes and allosterically activates the glycosyltransferase
  (PMID:22056329; PDB 2YJN). EryCIII is a member of the GT-B superfamily desosaminyl
  transferase family.
references:
- id: PMID:15303858
  title: 'Reconstitution and characterization of a new desosaminyl transferase, EryCIII,
    from the erythromycin biosynthetic pathway.'
  findings:
  - statement: >-
      EryCIII transfers TDP-D-desosamine onto alpha-mycarosyl erythronolide B to form
      erythromycin D, with high activity and donor/acceptor selectivity.
    supporting_text: >-
      EryCIII converts alpha-mycarosyl erythronolide B into erythromycin D using
      TDP-d-desosamine as the glycosyl donor.
  - statement: >-
      EryCIII is highly active on its own (kcat > 100 min-1), implying it does not
      require an auxiliary partner for catalysis.
    supporting_text: >-
      The enzyme was found to be highly active with a kcat greater than 100 min-1
    finding_review:
      finding_status: DISPUTED
      superseded_by:
      - PMID:22056329
      review_notes: >-
        The 2012 structural study (PMID:22056329) found purified EryCIII inactive without
        EryCII and proposed that earlier apparent standalone activity arose from residual
        contaminating activator. The desosaminyltransferase activity itself is not in
        question - only the claim that it proceeds efficiently without the EryCII partner.
        Disputed (credible primary papers disagree; may be preparation-dependent) rather
        than fully overturned.
  reference_review:
    relevance: HIGH
    correctness: VERIFIED
    review_notes: >-
      PubMed-verified in vitro reconstitution; the IDA source for the hexosyltransferase
      and antibiotic-biosynthesis annotations. Establishes the TDP (not UDP) sugar donor.
      One finding (standalone high activity) is marked DISPUTED at the finding level (see
      finding_review) while the catalytic activity itself remains VERIFIED.
- id: PMID:22056329
  title: Structure of the glycosyltransferase EryCIII in complex with its activating
    P450 homologue EryCII.
  findings:
  - statement: >-
      EryCIII, with its partner EryCII, attaches a nucleotide-activated sugar to the
      macrolide scaffold; the two form a heterotetramer.
    supporting_text: >-
      the glycosyltransferase (GT) EryCIII, in concert with its partner EryCII, attaches
      a nucleotide-activated sugar to the macrolide scaffold with high specificity
  - statement: >-
      In this study purified EryCIII was inactive on its own and its GT activity was
      restored by EryCII, indicating the catalytic activity is realized in the
      EryCIII-EryCII complex.
    supporting_text: >-
      Purified EryCIII was used for in vitro glycosylation assays as described
      previously, but the enzyme was inactive under all conditions tried.
  reference_review:
    relevance: HIGH
    correctness: VERIFIED
    review_notes: >-
      PDB 2YJN; defines the alpha2-beta2 EryCIII-EryCII heterotetramer and EryCII's
      activating role. Note a literature nuance: this study found EryCIII inactive
      without EryCII, whereas the earlier IDA paper (PMID:15303858) reported high
      standalone activity - so EryCIII is the catalytic subunit, but reliable activity
      requires the EryCII activator.
- id: file:SACEN/eryCIII/eryCIII-notes.md
  title: eryCIII review notes (BGC project) with predicted-complex evidence
- id: GO_REF:0000002
  title: Gene Ontology annotation through association of InterPro records with GO terms
- id: GO_REF:0000117
  title: Gene Ontology annotation based on Ensembl/EnsemblGenomes orthology
existing_annotations:
- term:
    id: GO:0008194
    label: UDP-glycosyltransferase activity
  evidence_type: IEA
  original_reference_id: GO_REF:0000002
  qualifier: enables
  review:
    summary: >-
      InterPro-based (IEA) UDP-glycosyltransferase activity. EryCIII uses
      TDP-D-desosamine, a dTDP-sugar, as its donor (PMID:15303858), not a UDP-sugar, so
      the UDP-specific term is incorrect. The accurate activity is hexosyltransferase
      (already IDA-supported below).
    action: REMOVE
    reason: >-
      Wrong donor specificity (TDP-D-desosamine, a dTDP-sugar, not a UDP-sugar). The accurate term
      GO:0016758 (hexosyltransferase activity) is already present (IEA + IDA), so this incorrect
      UDP-specific term should be removed rather than modified into a duplicate.
    supported_by:
    - reference_id: PMID:15303858
      supporting_text: >-
        EryCIII converts alpha-mycarosyl erythronolide B into erythromycin D using
        TDP-d-desosamine as the glycosyl donor.
- term:
    id: GO:0016757
    label: glycosyltransferase activity
  evidence_type: IEA
  original_reference_id: GO_REF:0000002
  qualifier: enables
  review:
    summary: >-
      IEA general glycosyltransferase activity. Correct broad parent of the desosaminyl
      transferase reaction.
    action: ACCEPT
    reason: Accurate general molecular function (true parent of the specific activity).
    supported_by:
    - reference_id: PMID:15303858
      supporting_text: >-
        a new desosaminyl transferase, EryCIII
- term:
    id: GO:0016758
    label: hexosyltransferase activity
  evidence_type: IEA
  original_reference_id: GO_REF:0000117
  qualifier: enables
  review:
    summary: >-
      IEA hexosyltransferase activity (orthology). Accurate for the desosaminyl
      (deoxyhexose) transfer; also experimentally supported (IDA) below.
    action: ACCEPT
    reason: Accurate molecular function, corroborated by direct experimental evidence.
    supported_by:
    - reference_id: PMID:15303858
      supporting_text: >-
        EryCIII converts alpha-mycarosyl erythronolide B into erythromycin D using
        TDP-d-desosamine as the glycosyl donor.
- term:
    id: GO:0017000
    label: antibiotic biosynthetic process
  evidence_type: IEA
  original_reference_id: GO_REF:0000117
  qualifier: involved_in
  review:
    summary: >-
      IEA antibiotic biosynthetic process. Correct (erythromycin is an antibiotic) but
      general; the more specific GO:1901115 (erythromycin biosynthetic process) is
      available and is captured in core_functions.
    action: ACCEPT
    reason: >-
      Accurate biological process; retained. A more specific child term (GO:1901115
      erythromycin biosynthetic process) is proposed as the core annotation rather than
      replacing this correct general term.
    supported_by:
    - reference_id: PMID:15303858
      supporting_text: >-
        EryCIII converts alpha-mycarosyl erythronolide B into erythromycin D
- term:
    id: GO:0016758
    label: hexosyltransferase activity
  evidence_type: IDA
  original_reference_id: PMID:15303858
  qualifier: enables
  review:
    summary: >-
      Direct experimental (IDA) hexosyltransferase activity from in vitro reconstitution
      of EryCIII (kcat > 100 min-1, selective for TDP-D-desosamine).
    action: ACCEPT
    reason: >-
      Core molecular function with direct experimental support; the defining catalytic
      activity of EryCIII (EC 2.4.1.278).
    supported_by:
    - reference_id: PMID:15303858
      supporting_text: >-
        The enzyme was found to be highly active with a kcat greater than 100 min-1
- term:
    id: GO:0017000
    label: antibiotic biosynthetic process
  evidence_type: IDA
  original_reference_id: PMID:15303858
  qualifier: involved_in
  review:
    summary: >-
      Direct experimental (IDA) involvement in antibiotic (erythromycin) biosynthesis:
      EryCIII produces erythromycin D from alpha-mycarosyl erythronolide B.
    action: ACCEPT
    reason: >-
      Core biological process with experimental support. (The more specific GO:1901115
      erythromycin biosynthetic process is proposed as the replacement for the IEA
      duplicate above and is captured in core_functions.)
    supported_by:
    - reference_id: PMID:15303858
      supporting_text: >-
        EryCIII converts alpha-mycarosyl erythronolide B into erythromycin D
core_functions:
- description: >-
    Desosaminyl transferase (EC 2.4.1.278): transfers D-desosamine from TDP-D-desosamine
    to 3-alpha-mycarosylerythronolide B, forming erythromycin D - the desosaminylation
    step of erythromycin biosynthesis. Acts within the alpha2-beta2 EryCIII-EryCII
    heterotetramer, on which full activity depends (PMID:15303858, PMID:22056329; PDB 2YJN).
  molecular_function:
    id: GO:0016758
    label: hexosyltransferase activity
  directly_involved_in:
    - id: GO:1901115
      label: erythromycin biosynthetic process
  in_complex:
    id: GO:0032991
    label: protein-containing complex
  supported_by:
  - reference_id: PMID:15303858
    supporting_text: >-
      EryCIII converts alpha-mycarosyl erythronolide B into erythromycin D using
      TDP-d-desosamine as the glycosyl donor.
  - reference_id: PMID:22056329
    supporting_text: >-
      The structure reveals a heterotetramer with a distinctive, elongated quaternary
      organization.
proposed_new_terms:
- proposed_name: 3-alpha-mycarosylerythronolide B desosaminyltransferase activity
  proposed_definition: >-
    Catalysis of the transfer of D-desosamine from TDP-D-desosamine to
    3-alpha-mycarosylerythronolide B, producing erythromycin D and TDP. Corresponds to
    EC 2.4.1.278.
  justification: >-
    EC 2.4.1.278 has no specific GO molecular function term; EryCIII is currently
    annotated only with the general parents (glycosyltransferase / hexosyltransferase
    activity). A specific term would precisely capture this characterized activity.
suggested_questions:
- question: >-
    Per GO guidelines the desosaminyltransferase activity (EC 2.4.1.278) is annotated
    to the catalytic member EryCIII (enables), with EryCII captured separately as
    enzyme activator activity and both as subunits of the protein-containing complex.
    Given that EryCIII is inactive without EryCII, is an additional contributes_to from
    EryCII to the GT activity warranted?