id: P12277
gene_symbol: CKB
product_type: PROTEIN
status: COMPLETE
taxon:
  id: NCBITaxon:9606
  label: Homo sapiens
description: >-
  CKB encodes the B-type (brain-type) cytosolic creatine kinase, a 381-residue
  enzyme (EC 2.7.3.2) of the ATP:guanido phosphotransferase (phosphagen kinase)
  family. It reversibly catalyzes transfer of the high-energy phosphoryl group
  between ATP and creatine (creatine + ATP = N-phosphocreatine + ADP + H+),
  interconverting ATP and the phosphocreatine energy reservoir. The active enzyme
  is a dimer: B-CK assembles into CK-BB homodimers and, with the muscle-type
  subunit CKM, into CK-MB heterodimers. As the cytosolic limb of the
  phosphocreatine/creatine-kinase shuttle, B-CK is functionally coupled to
  ATP-consuming processes (e.g. membrane ion transporters and other ATPases),
  locally regenerating ATP from phosphocreatine and providing temporal and spatial
  energy buffering in cells with high, fluctuating energy demands. B-CK is the
  predominant cytosolic creatine kinase of brain and is broadly expressed in smooth
  muscle and most non-muscle tissues. Beyond catalysis it acts as an
  ATP-regenerating partner in membrane microcompartments — for example binding the
  C-terminal domain of the K-Cl cotransporter KCC3 (SLC12A6) and supporting its
  transport activity — and is a substrate of the ECS(ASB9) Cullin-RING E3 ubiquitin
  ligase, which targets it for proteasomal degradation.
references:
- id: GO_REF:0000002
  title: Gene Ontology annotation through association of InterPro records with GO
    terms
  findings: []
- id: GO_REF:0000024
  title: Manual transfer of experimentally-verified manual GO annotation data to orthologs
    by curator judgment of sequence similarity
  findings: []
- id: GO_REF:0000033
  title: Annotation inferences using phylogenetic trees
  findings: []
- id: GO_REF:0000044
  title: Gene Ontology annotation based on UniProtKB/Swiss-Prot Subcellular Location
    vocabulary mapping, accompanied by conservative changes to GO terms applied by
    UniProt
  findings: []
- id: GO_REF:0000052
  title: Gene Ontology annotation based on curation of immunofluorescence data
  findings: []
- id: GO_REF:0000107
  title: Automatic transfer of experimentally verified manual GO annotation data to
    orthologs using Ensembl Compara
  findings: []
- id: GO_REF:0000120
  title: Combined Automated Annotation using Multiple IEA Methods
  findings: []
- id: PMID:11487543
  title: Intestinal epithelial cells secrete exosome-like vesicles.
  findings: []
  reference_review:
    relevance: LOW
    correctness: VERIFIED
    review_notes: Proteomic exosome study; supports detection of CKB in secreted
      vesicles but not a functional extracellular role.
- id: PMID:16189514
  title: Towards a proteome-scale map of the human protein-protein interaction network.
  findings: []
  reference_review:
    relevance: LOW
    correctness: VERIFIED
    review_notes: High-throughput Y2H interactome map; generic 'protein binding'
      support, low functional informativeness for CKB.
- id: PMID:18566107
  title: HMSN/ACC truncation mutations disrupt brain-type creatine kinase-dependant
    activation of K+/Cl- co-transporter 3.
  findings: []
  reference_review:
    relevance: HIGH
    correctness: VERIFIED
    review_notes: Establishes CK-B (CKB) binding to the KCC3/SLC12A6 C-terminal
      domain, co-localization at the membrane, and a kinase-dependent functional
      role in cotransport.
- id: PMID:19056867
  title: Large-scale proteomics and phosphoproteomics of urinary exosomes.
  findings: []
  reference_review:
    relevance: LOW
    correctness: VERIFIED
    review_notes: Exosome proteomic detection of CKB; supports exosome localization
      annotation, not a functional role.
- id: PMID:19060904
  title: An empirical framework for binary interactome mapping.
  findings: []
  reference_review:
    relevance: LOW
    correctness: VERIFIED
    review_notes: High-throughput binary interactome; generic protein-binding hit.
- id: PMID:19264780
  title: Involvement of creatine kinase B in hepatitis C virus genome replication
    through interaction with the viral NS4A protein.
  findings: []
  reference_review:
    relevance: MEDIUM
    correctness: VERIFIED
    review_notes: Shows CKB interacts with HCV NS4A and is recruited to the viral
      replication complex; a moonlighting host-factor role, not a core function.
- id: PMID:19725078
  title: Proteomic analysis of increased Parkin expression and its interactants provides
    evidence for a role in modulation of mitochondrial function.
  findings: []
  reference_review:
    relevance: LOW
    correctness: VERIFIED
    review_notes: Parkin (E3 ligase) interactant proteomics; supports CKB as an E3
      ligase-binding substrate, not an independent CKB function.
- id: PMID:21145461
  title: Dynamics of cullin-RING ubiquitin ligase network revealed by systematic quantitative
    proteomics.
  findings: []
  reference_review:
    relevance: LOW
    correctness: VERIFIED
    review_notes: CRL network proteomics; CKB recorded as a CUL3-associated protein
      (consistent with ECS/Cullin-RING substrate role).
- id: PMID:22926577
  title: Quantitative proteomic analysis of human substantia nigra in Alzheimer's
    disease, Huntington's disease and Multiple sclerosis.
  findings: []
  reference_review:
    relevance: LOW
    correctness: MISCITED
    review_notes: A disease-state differential-abundance proteomic survey of substantia
      nigra; it does not demonstrate a role for CKB in substantia nigra development.
- id: PMID:23533145
  title: In-depth proteomic analyses of exosomes isolated from expressed prostatic
    secretions in urine.
  findings: []
  reference_review:
    relevance: LOW
    correctness: VERIFIED
    review_notes: Exosome proteomic detection; supports exosome localization, not a
      functional extracellular role.
- id: PMID:25416956
  title: A proteome-scale map of the human interactome network.
  findings: []
  reference_review:
    relevance: LOW
    correctness: VERIFIED
    review_notes: High-throughput interactome map; generic protein-binding support.
- id: PMID:28514442
  title: Architecture of the human interactome defines protein communities and disease
    networks.
  findings: []
  reference_review:
    relevance: LOW
    correctness: VERIFIED
    review_notes: High-throughput interactome (BioPlex); generic protein-binding hits
      including CKM and ASB9.
- id: PMID:3034271
  title: 'Human creatine kinase: isolation and sequence analysis of cDNA clones for
    the B subunit, development of subunit specific probes and determination of gene
    copy number.'
  findings: []
  reference_review:
    relevance: HIGH
    correctness: VERIFIED
    review_notes: Original cloning/sequence of the human CKB (B-subunit) cDNA;
      cited for creatine kinase activity (TAS).
- id: PMID:32296183
  title: A reference map of the human binary protein interactome.
  findings: []
  reference_review:
    relevance: LOW
    correctness: VERIFIED
    review_notes: High-throughput binary interactome (HuRI); generic protein-binding
      hit (ASB9).
- id: PMID:33961781
  title: Dual proteome-scale networks reveal cell-specific remodeling of the human
    interactome.
  findings: []
  reference_review:
    relevance: LOW
    correctness: VERIFIED
    review_notes: High-throughput interactome; generic protein-binding hits (CKM,
      ASB9).
- id: PMID:35271311
  title: 'OpenCell: Endogenous tagging for the cartography of human cellular organization.'
  findings: []
  reference_review:
    relevance: LOW
    correctness: VERIFIED
    review_notes: Endogenous-tagging interactome/localization map; generic protein-binding
      hit.
- id: PMID:37207277
  title: Using brain cell-type-specific protein interactomes to interpret neurodevelopmental
    genetic signals in schizophrenia.
  findings: []
  reference_review:
    relevance: LOW
    correctness: VERIFIED
    review_notes: Brain cell-type interactome study; generic protein-binding hits
      (TCF4, CUL3).
- id: PMID:40205054
  title: Multimodal cell maps as a foundation for structural and functional genomics.
  findings: []
  reference_review:
    relevance: LOW
    correctness: VERIFIED
    review_notes: Multimodal cell-map interactome; generic protein-binding hit (ASB9).
- id: PMID:8186255
  title: Determination of the catalytic site of creatine kinase by site-directed mutagenesis.
  findings:
  - statement: Site-directed mutagenesis of human B-CK shows Cys-283 and Arg-292
      are essential for catalysis; mutants abolish homodimer enzymatic activity
      without preventing dimerization.
    supporting_text: The results showed residues Cys-283 and Arg-292 are essential
      for enzyme catalysis.
    reference_section_type: ABSTRACT
  reference_review:
    relevance: HIGH
    correctness: VERIFIED
    review_notes: Direct functional evidence for human CKB creatine kinase activity
      and its catalytic residues; basis for the GO:0004111 IDA and GO:0046314 IDA.
- id: PMID:18977227
  title: Structural studies of human brain-type creatine kinase complexed with the
    ADP-Mg2+-NO3--creatine transition-state analogue complex.
  findings: []
  reference_review:
    relevance: HIGH
    correctness: VERIFIED
    review_notes: Crystal structure of human B-CK with transition-state analogue;
      confirms active site, substrate binding, and the dimer.
- id: PMID:21448658
  title: The creatine kinase system and pleiotropic effects of creatine.
  findings: []
  reference_review:
    relevance: HIGH
    correctness: VERIFIED
    review_notes: Authoritative review of the PCr/CK shuttle and energy-buffering
      roles of cytosolic creatine kinases.
- id: Reactome:R-HSA-200318
  title: creatine + ATP => phosphocreatine + ADP [CKB,CKM]
  findings: []
  reference_review:
    relevance: HIGH
    correctness: VERIFIED
    review_notes: Reactome reaction for the creatine kinase activity of CKB/CKM.
existing_annotations:
- term:
    id: GO:0005576
    label: extracellular region
  evidence_type: IBA
  original_reference_id: GO_REF:0000033
  qualifier: is_active_in
  review:
    summary: Phylogenetic (IBA) inference that B-CK is active in the extracellular
      region. B-CK is a cytosolic enzyme; it is detected in exosomes/secretome but
      is not active extracellularly. The "is_active_in extracellular region" call
      is an over-propagation and is biologically incorrect as a site of activity.
    action: REMOVE
    reason: Creatine kinase catalysis requires cytosolic ATP/ADP and creatine; the
      enzyme is not known to function in the extracellular region. Detection in
      secreted vesicles does not make the extracellular region a site of activity,
      so an "is_active_in extracellular region" assertion is incorrect. Aligned with
      the parallel CKM review, which REMOVEs the identical IBA annotation; genuine
      extracellular/exosome detection is retained separately under the
      extracellular-exosome annotations.
- term:
    id: GO:0004111
    label: creatine kinase activity
  evidence_type: IBA
  original_reference_id: GO_REF:0000033
  qualifier: enables
  review:
    summary: Creatine kinase activity is the core molecular function of CKB,
      supported phylogenetically (IBA) and experimentally. Accept.
    action: ACCEPT
    reason: This is the defining, experimentally established activity of CKB
      (EC 2.7.3.2); the IBA inference is consistent with direct assays.
    supported_by:
    - reference_id: PMID:8186255
      supporting_text: The results showed residues Cys-283 and Arg-292 are essential
        for enzyme catalysis.
- term:
    id: GO:0046314
    label: phosphocreatine biosynthetic process
  evidence_type: IBA
  original_reference_id: GO_REF:0000033
  qualifier: involved_in
  review:
    summary: The biological process corresponding to the forward (ATP -> phosphocreatine)
      direction of the creatine kinase reaction. Core process for CKB. Accept.
    action: ACCEPT
    reason: Directly reflects the physiological left-to-right reaction direction
      catalyzed by CKB; consistent with the experimentally demonstrated activity.
- term:
    id: GO:0003824
    label: catalytic activity
  evidence_type: IEA
  original_reference_id: GO_REF:0000002
  qualifier: enables
  review:
    summary: Generic root catalytic-activity term assigned by InterPro2GO. Correct
      but uninformative; subsumed by the specific creatine kinase activity annotation.
    action: MARK_AS_OVER_ANNOTATED
    reason: A high-level ancestor of GO:0004111; adds no information beyond the
      specific molecular function and should not be retained as a meaningful
      annotation.
- term:
    id: GO:0004111
    label: creatine kinase activity
  evidence_type: IEA
  original_reference_id: GO_REF:0000120
  qualifier: enables
  review:
    summary: Electronic (IEA) assignment of the core creatine kinase activity,
      consistent with experimental evidence. Accept (redundant with the experimental
      annotation but correct).
    action: ACCEPT
    reason: Same correct core molecular function; electronic support via RHEA/EC
      mapping aligns with direct evidence.
- term:
    id: GO:0005739
    label: mitochondrion
  evidence_type: IEA
  original_reference_id: GO_REF:0000120
  qualifier: located_in
  review:
    summary: Electronic mitochondrion localization transferred by similarity from the
      mouse ortholog (Q04447) in the context of thermogenic-fat / futile-creatine-cycle
      biology. Human CKB is predominantly cytosolic; a mitochondrial pool is not
      directly demonstrated for human B-CK.
    action: MARK_AS_OVER_ANNOTATED
    reason: B-CK is the cytosolic creatine kinase isoform; mitochondrial creatine
      kinase is a distinct gene product (uMtCK/CKMT1). The mitochondrial call is a
      by-similarity transfer specific to adipocyte thermogenesis and is not the core
      localization of human CKB.
- term:
    id: GO:0005829
    label: cytosol
  evidence_type: IEA
  original_reference_id: GO_REF:0000044
  qualifier: located_in
  review:
    summary: Electronic cytosol localization from the UniProt subcellular-location
      mapping. Correct core localization. Accept.
    action: ACCEPT
    reason: B-CK is a cytosolic enzyme; consistent with IDA (HPA) and Reactome TAS
      annotations to cytosol.
- term:
    id: GO:0005886
    label: plasma membrane
  evidence_type: IEA
  original_reference_id: GO_REF:0000044
  qualifier: located_in
  review:
    summary: Electronic plasma-membrane call from UniProt subcellular-location
      mapping, reflecting peripheral recruitment of B-CK to membrane ATPase
      microcompartments (e.g. with KCC3). Real but peripheral, not the core compartment.
    action: KEEP_AS_NON_CORE
    reason: CKB is not an integral membrane protein; it is recruited to the membrane
      to supply ATP to transporters such as KCC3 (PMID:18566107). Retain as a
      non-core peripheral localization.
- term:
    id: GO:0016301
    label: kinase activity
  evidence_type: IEA
  original_reference_id: GO_REF:0000002
  qualifier: enables
  review:
    summary: Generic kinase-activity parent from InterPro2GO; subsumed by the
      specific creatine kinase activity. Uninformative.
    action: MARK_AS_OVER_ANNOTATED
    reason: Ancestor of GO:0004111; carries no information beyond the specific
      molecular function.
- term:
    id: GO:0016772
    label: transferase activity, transferring phosphorus-containing groups
  evidence_type: IEA
  original_reference_id: GO_REF:0000002
  qualifier: enables
  review:
    summary: Generic phosphotransferase parent from InterPro2GO; subsumed by the
      specific creatine kinase activity. Uninformative.
    action: MARK_AS_OVER_ANNOTATED
    reason: High-level ancestor of GO:0004111; redundant generic term.
- term:
    id: GO:0016775
    label: phosphotransferase activity, nitrogenous group as acceptor
  evidence_type: IEA
  original_reference_id: GO_REF:0000002
  qualifier: enables
  review:
    summary: The immediate parent class of creatine kinase activity (phosphotransfer
      to a nitrogenous/guanidino acceptor). Correct but redundant with the specific
      term.
    action: MARK_AS_OVER_ANNOTATED
    reason: Direct ancestor of GO:0004111; the specific creatine kinase activity term
      is preferred and makes this generic term redundant.
- term:
    id: GO:0046314
    label: phosphocreatine biosynthetic process
  evidence_type: IEA
  original_reference_id: GO_REF:0000002
  qualifier: involved_in
  review:
    summary: Electronic assignment of the core phosphocreatine biosynthetic process,
      consistent with experimental evidence. Accept.
    action: ACCEPT
    reason: Correct core process for CKB; redundant with the IDA/IBA annotations but
      not erroneous.
- term:
    id: GO:0005515
    label: protein binding
  evidence_type: IPI
  original_reference_id: PMID:16189514
  qualifier: enables
  review:
    summary: Generic 'protein binding' from a high-throughput interactome screen.
      Uninformative as a molecular function; the biologically meaningful interactions
      are captured by more specific annotations.
    action: KEEP_AS_NON_CORE
    reason: Bare protein binding does not describe a function; retained as non-core
      evidence of interaction only.
- term:
    id: GO:0005515
    label: protein binding
  evidence_type: IPI
  original_reference_id: PMID:19060904
  qualifier: enables
  review:
    summary: Generic 'protein binding' from a high-throughput binary interactome.
      Uninformative molecular function.
    action: KEEP_AS_NON_CORE
    reason: Bare protein binding is not a core function; retained as non-core.
- term:
    id: GO:0005515
    label: protein binding
  evidence_type: IPI
  original_reference_id: PMID:19264780
  qualifier: enables
  review:
    summary: Interaction with hepatitis C virus NS4A, recruiting CKB to the viral
      replication complex. A genuine moonlighting interaction but represented only as
      generic 'protein binding'.
    action: KEEP_AS_NON_CORE
    reason: Biologically real host-factor interaction (PMID:19264780) but a
      non-core, condition-specific role; the bare term is uninformative as a core
      function.
    supported_by:
    - reference_id: PMID:19264780
      supporting_text: CKB interacts with HCV NS4A protein and forms a complex with
        NS3-4A, which possesses multiple enzyme activities.
- term:
    id: GO:0005515
    label: protein binding
  evidence_type: IPI
  original_reference_id: PMID:21145461
  qualifier: enables
  review:
    summary: Generic 'protein binding' from a cullin-RING ligase network proteomic
      study (CUL3 association). Uninformative as a function.
    action: KEEP_AS_NON_CORE
    reason: Bare protein binding; relates to CKB's role as a CRL substrate, retained
      as non-core.
- term:
    id: GO:0005515
    label: protein binding
  evidence_type: IPI
  original_reference_id: PMID:25416956
  qualifier: enables
  review:
    summary: Generic 'protein binding' from a high-throughput interactome map.
      Uninformative molecular function.
    action: KEEP_AS_NON_CORE
    reason: Bare protein binding is not a core function; retained as non-core.
- term:
    id: GO:0005515
    label: protein binding
  evidence_type: IPI
  original_reference_id: PMID:28514442
  qualifier: enables
  review:
    summary: Generic 'protein binding' from a BioPlex interactome (includes CKM and
      ASB9 partners). Uninformative as a standalone function.
    action: KEEP_AS_NON_CORE
    reason: Bare protein binding; the meaningful partners (CKM dimerization, ASB9 E3
      ligase) are better captured specifically. Retained as non-core.
- term:
    id: GO:0005515
    label: protein binding
  evidence_type: IPI
  original_reference_id: PMID:32296183
  qualifier: enables
  review:
    summary: Generic 'protein binding' from the HuRI binary interactome (ASB9).
      Uninformative molecular function.
    action: KEEP_AS_NON_CORE
    reason: Bare protein binding is not a core function; retained as non-core.
- term:
    id: GO:0005515
    label: protein binding
  evidence_type: IPI
  original_reference_id: PMID:33961781
  qualifier: enables
  review:
    summary: Generic 'protein binding' from a dual proteome-scale interactome (CKM,
      ASB9). Uninformative molecular function.
    action: KEEP_AS_NON_CORE
    reason: Bare protein binding is not a core function; retained as non-core.
- term:
    id: GO:0005515
    label: protein binding
  evidence_type: IPI
  original_reference_id: PMID:35271311
  qualifier: enables
  review:
    summary: Generic 'protein binding' from the OpenCell endogenous-tagging map.
      Uninformative molecular function.
    action: KEEP_AS_NON_CORE
    reason: Bare protein binding is not a core function; retained as non-core.
- term:
    id: GO:0005515
    label: protein binding
  evidence_type: IPI
  original_reference_id: PMID:37207277
  qualifier: enables
  review:
    summary: Generic 'protein binding' from a brain cell-type interactome (TCF4,
      CUL3). Uninformative molecular function.
    action: KEEP_AS_NON_CORE
    reason: Bare protein binding is not a core function; retained as non-core.
- term:
    id: GO:0005515
    label: protein binding
  evidence_type: IPI
  original_reference_id: PMID:40205054
  qualifier: enables
  review:
    summary: Generic 'protein binding' from a multimodal cell-map interactome (ASB9).
      Uninformative molecular function.
    action: KEEP_AS_NON_CORE
    reason: Bare protein binding is not a core function; retained as non-core.
- term:
    id: GO:0140651
    label: futile creatine cycle
  evidence_type: IEA
  original_reference_id: GO_REF:0000107
  qualifier: involved_in
  review:
    summary: Electronic transfer (from mouse Q04447) of involvement in the futile
      creatine cycle (adipocyte thermogenesis). This role is characterized for the
      thermogenic-fat mitochondrial creatine cycle and has not been demonstrated for
      human B-CK; retain as a non-core, by-similarity possibility.
    action: KEEP_AS_NON_CORE
    reason: The futile creatine cycle is a specialized thermogenic-fat process
      inferred by orthology; it is plausible but not the core energy-buffering
      function of cytosolic B-CK in humans.
- term:
    id: GO:0005829
    label: cytosol
  evidence_type: IDA
  original_reference_id: GO_REF:0000052
  qualifier: located_in
  review:
    summary: Direct immunofluorescence (HPA) localization to the cytosol. This is the
      core, experimentally supported localization of B-CK. Accept.
    action: ACCEPT
    reason: B-CK is a cytosolic enzyme; IDA from HPA directly supports the core
      localization.
- term:
    id: GO:0005739
    label: mitochondrion
  evidence_type: ISS
  original_reference_id: GO_REF:0000024
  qualifier: located_in
  review:
    summary: ISS mitochondrion localization transferred from mouse Q04447
      (thermogenic-fat creatine cycle). Not the core compartment of human cytosolic
      B-CK.
    action: MARK_AS_OVER_ANNOTATED
    reason: By-similarity mitochondrial localization tied to adipocyte thermogenesis;
      human B-CK is the cytosolic isoform and a mitochondrial pool is not directly
      established.
- term:
    id: GO:0005886
    label: plasma membrane
  evidence_type: EXP
  original_reference_id: PMID:18566107
  qualifier: located_in
  review:
    summary: Experimental co-localization of CK-B with KCC3 at the plasma membrane,
      consistent with recruitment of B-CK to membrane ATPase microcompartments.
      Real but peripheral, not the core cytosolic compartment.
    action: KEEP_AS_NON_CORE
    reason: CKB associates with the membrane via interaction with the KCC3
      transporter to supply ATP locally; it is not an integral membrane protein.
      Retain as a non-core peripheral localization.
    supported_by:
    - reference_id: PMID:18566107
      supporting_text: immunofluorescence labeling showed that CK-B co-localizes with
        wild-type KCC3
- term:
    id: GO:0004111
    label: creatine kinase activity
  evidence_type: IDA
  original_reference_id: PMID:8186255
  qualifier: enables
  review:
    summary: Direct experimental evidence (site-directed mutagenesis of human B-CK)
      for creatine kinase activity, including identification of essential catalytic
      residues. The strongest support for the core molecular function. Accept.
    action: ACCEPT
    reason: Direct assay of human CKB enzymatic activity; defines the core molecular
      function (EC 2.7.3.2).
    supported_by:
    - reference_id: PMID:8186255
      supporting_text: The results showed residues Cys-283 and Arg-292 are essential
        for enzyme catalysis.
- term:
    id: GO:0046314
    label: phosphocreatine biosynthetic process
  evidence_type: IDA
  original_reference_id: PMID:8186255
  qualifier: involved_in
  review:
    summary: Direct evidence for the phosphocreatine-forming (forward) direction of
      the creatine kinase reaction. Core process for CKB. Accept.
    action: ACCEPT
    reason: Reflects the physiological left-to-right reaction catalyzed by CKB,
      directly demonstrated for the human enzyme.
- term:
    id: GO:0004111
    label: creatine kinase activity
  evidence_type: ISS
  original_reference_id: GO_REF:0000024
  qualifier: enables
  review:
    summary: ISS support (from ortholog) for the core creatine kinase activity.
      Consistent with direct evidence. Accept.
    action: ACCEPT
    reason: Correct core molecular function; redundant with the experimental
      annotation but not erroneous.
- term:
    id: GO:0005739
    label: mitochondrion
  evidence_type: ISS
  original_reference_id: GO_REF:0000024
  qualifier: is_active_in
  review:
    summary: ISS 'is_active_in mitochondrion' transferred from mouse Q04447. Human
      B-CK is the cytosolic isoform; an active mitochondrial pool is not directly
      established.
    action: MARK_AS_OVER_ANNOTATED
    reason: By-similarity mitochondrial activity tied to thermogenic-fat biology;
      not the core localization or site of activity of human cytosolic B-CK.
- term:
    id: GO:0070062
    label: extracellular exosome
  evidence_type: HDA
  original_reference_id: PMID:11487543
  qualifier: located_in
  review:
    summary: High-throughput proteomic detection of CKB in exosomes. A real
      observation but reflects passive secretion/vesicle loading rather than a
      functional compartment.
    action: KEEP_AS_NON_CORE
    reason: Exosome detection is common for abundant cytosolic enzymes; retain as a
      non-core localization, not a site of creatine kinase function.
- term:
    id: GO:0005576
    label: extracellular region
  evidence_type: ISS
  original_reference_id: GO_REF:0000024
  qualifier: located_in
  review:
    summary: ISS extracellular-region localization transferred from an ortholog.
      B-CK is a cytosolic enzyme; extracellular detection (exosomes/secretome) does
      not establish a functional extracellular localization.
    action: MARK_AS_OVER_ANNOTATED
    reason: Over-propagated localization for a cytosolic enzyme; not a site of
      activity. Exosomal detection is better captured by the exosome annotation.
- term:
    id: GO:0031625
    label: ubiquitin protein ligase binding
  evidence_type: IPI
  original_reference_id: PMID:19725078
  qualifier: enables
  review:
    summary: CKB binds an E3 ubiquitin ligase component (Parkin/E3 interactant
      proteomics), consistent with CKB being a substrate of the ECS(ASB9)
      Cullin-RING E3 ligase that targets it for degradation. A real binding activity
      but reflecting CKB's role as a degradation substrate rather than a core enzyme
      function.
    action: KEEP_AS_NON_CORE
    reason: Informative interaction (E3 ligase/degradation axis) but not part of
      CKB's core energy-metabolism function; retain as non-core.
- term:
    id: GO:0070062
    label: extracellular exosome
  evidence_type: HDA
  original_reference_id: PMID:23533145
  qualifier: located_in
  review:
    summary: High-throughput proteomic detection of CKB in prostatic-secretion
      exosomes. Same interpretation as other exosome detections.
    action: KEEP_AS_NON_CORE
    reason: Proteomic exosome detection; non-core localization, not a functional
      compartment for catalysis.
- term:
    id: GO:0021762
    label: substantia nigra development
  evidence_type: HEP
  original_reference_id: PMID:22926577
  qualifier: involved_in
  review:
    summary: HEP annotation derived from a disease-state differential-abundance
      proteomic survey of human substantia nigra. Differential expression in
      neurodegenerative disease does not establish a role in substantia nigra
      development.
    action: MARK_AS_OVER_ANNOTATED
    reason: The supporting study compares protein abundance in disease vs control
      substantia nigra; it does not provide evidence that CKB is involved in the
      developmental process. Over-interpretation of an expression observation.
- term:
    id: GO:0070062
    label: extracellular exosome
  evidence_type: HDA
  original_reference_id: PMID:19056867
  qualifier: located_in
  review:
    summary: High-throughput proteomic detection of CKB in urinary exosomes. Same
      interpretation as other exosome detections.
    action: KEEP_AS_NON_CORE
    reason: Proteomic exosome detection; non-core localization.
- term:
    id: GO:0005829
    label: cytosol
  evidence_type: TAS
  original_reference_id: Reactome:R-HSA-200318
  qualifier: located_in
  review:
    summary: Traceable author statement (Reactome) placing the creatine kinase
      reaction in the cytosol. Consistent with the core cytosolic localization.
      Accept.
    action: ACCEPT
    reason: Correct core localization; the creatine kinase reaction catalyzed by CKB
      occurs in the cytosol.
- term:
    id: GO:0004111
    label: creatine kinase activity
  evidence_type: TAS
  original_reference_id: PMID:3034271
  qualifier: enables
  review:
    summary: Traceable author statement (original CKB cDNA cloning paper) for
      creatine kinase activity. Core molecular function. Accept.
    action: ACCEPT
    reason: Correct core molecular function, supported by the foundational
      cloning/characterization reference.
core_functions:
- description: >-
    B-type cytosolic creatine kinase reversibly transfers the high-energy phosphoryl
    group between ATP and creatine (creatine + ATP = N-phosphocreatine + ADP + H+),
    acting in the cytosol to interconvert ATP and the phosphocreatine energy
    reservoir. The forward direction generates phosphocreatine (phosphocreatine
    biosynthetic process), buffering cellular ATP in brain and non-muscle tissues.
  molecular_function:
    id: GO:0004111
    label: creatine kinase activity
  directly_involved_in:
  - id: GO:0046314
    label: phosphocreatine biosynthetic process
  locations:
  - id: GO:0005829
    label: cytosol
  supported_by:
  - reference_id: PMID:8186255
    supporting_text: The results showed residues Cys-283 and Arg-292 are essential
      for enzyme catalysis.
suggested_questions:
- question: Does human cytosolic B-CK have any direct, functionally relevant
    mitochondrial or futile-creatine-cycle role, or are the mitochondrion/futile-cycle
    annotations entirely by-similarity from the mouse thermogenic-fat ortholog?
- question: How general is the role of B-CK as an ATP-supplying partner physically
    coupled to membrane ATPases/transporters (e.g. KCC2/KCC3) across tissues, and is
    its kinase activity strictly required for transporter function?
suggested_experiments:
- description: Determine the subcellular distribution of endogenous human B-CK
    (cytosol vs mitochondrion vs membrane microcompartments) across brain, smooth
    muscle, and adipose tissue using fractionation and quantitative imaging, to test
    whether any mitochondrial/futile-cycle role exists for the human protein.
  hypothesis: Human B-CK is overwhelmingly cytosolic, with membrane and mitochondrial
    pools being minor and context-specific.
- description: Reconstitute B-CK with KCC2/KCC3 in a controlled system and measure
    cotransport activity with catalytically active versus catalytically dead B-CK to
    test whether local ATP regeneration, rather than mere binding, drives transporter
    activation.
  hypothesis: B-CK kinase activity is required to sustain KCC transporter activity by
    locally regenerating ATP at the membrane microcompartment.