CPT2

CPT2 encodes the inner mitochondrial membrane carnitine palmitoyltransferase that reconverts fatty acyl-carnitines to fatty acyl-CoA, the committed step enabling mitochondrial long-chain fatty acid β-oxidation (PMID:2401367, PMID:38718533). Unlike the outer-membrane CPT1, CPT2 is intrinsically insensitive to malonyl-CoA and acquires this sensitivity only when reconstituted with an outer-membrane malonyl-CoA-binding component, establishing it as a topologically and functionally distinct enzyme (PMID:2401367, PMID:2809620). Catalytic activity depends on intact active-site architecture, as disease-associated missense substitutions (E174K, F383Y) sharply reduce activity and segregate with muscular versus hepatic clinical phenotypes, defining CPT2 as the gene underlying carnitine palmitoyltransferase II deficiency (PMID:9600456, PMID:30007356). CPT2 is a major node for post-translational control of fatty acid oxidation through reversible lysine acylation: SIRT5 de-succinylates K424 to relieve succinylation-mediated inactivation (PMID:38718533), SIRT3 deacetylates the enzyme to promote its catalytically active dimer (PMID:31866205), and acetylation at K79 conversely suppresses oxidation and drives long-chain acylcarnitine accumulation (PMID:35728063). Protein abundance is set by competing ubiquitin ligases—HRD1 stabilizes CPT2 via K48-linked ubiquitination to sustain oxidation, while MUL1, aided by SLC44A2, drives its degradation, and SIRT2-mediated K239 deacetylation promotes its ubiquitin-dependent turnover (PMID:33207079, PMID:40592838, PMID:39781464). At the transcriptional level CPT2 is repressed by E2F2 and induced through a FAM3A–calmodulin–FOXA2 axis (PMID:33771899, PMID:35995281). Functionally, loss of CPT2 activity causes accumulation of long-chain acylcarnitines and mitochondrial dysfunction across heart, liver, muscle, and platelets, whereas its activity is dispensable for B-cell development and humoral immunity (PMID:32896106, PMID:35728063, PMID:39258879).

1. 1989 Medium

   Established that CPT2 is a protein biochemically and genetically separable from CPT1, resolving whether muscle and liver carnitine palmitoyltransferase deficiencies arise from distinct enzymes.

   Evidence Isotope exchange enzymatic assays distinguishing CPT1 and CPT2 activities in patient muscle biopsies and fibroblasts

   PMID:2809620

   Open questions at the time

   • Did not define the molecular basis of the residual activity or the gene sequence
   • Tissue-specific CPT1 isoform hypothesis not resolved at the protein level
2. 1990 High

   Defined the topological logic of malonyl-CoA regulation by showing CPT2 is intrinsically malonyl-CoA-insensitive and only acquires sensitivity from an outer-membrane binding partner.

   Evidence In vitro reconstitution of cholate-extracted inner and outer mitochondrial membrane fractions with malonyl-CoA binding and CPT activity assays

   PMID:2401367

   Open questions at the time

   • Molecular identity of the malonyl-CoA-binding outer-membrane component not defined here
   • Stoichiometry and physical interaction with CPT2 not mapped
3. 1998 Medium

   Connected specific missense mutations to loss of catalytic activity and to genotype-phenotype correlation, establishing CPT2 as the causal gene for carnitine palmitoyltransferase II deficiency.

   Evidence Site-directed mutagenesis of CPT2 with expression and enzymatic activity assays in COS-1 cells

   PMID:9600456

   Open questions at the time

   • No structural explanation of how E174K/F383Y impair catalysis
   • Mechanism linking specific mutations to hepatic vs muscular tissue phenotype unresolved
4. 2018 Medium

   Showed that CPT2-deficient FAO can be pharmacologically corrected by raising CPT2 protein, opening a therapeutic route in patient cells.

   Evidence AMPK activator (GSK773) treatment of CPT2-mutant patient-derived myotubes with FAO flux, acylcarnitine profiling, and pathway perturbation

   PMID:30007356

   Open questions at the time

   • Whether correction depends on mutant residual activity vs increased abundance not separated
   • PGC-1α/ROS/p38 contributions defined only pharmacologically
5. 2019 Medium

   Identified deacetylation as an activating switch by showing a palmitate-CDK1-SIRT3 cascade deacetylates CPT2 and drives its active dimer.

   Evidence Kinase and deacetylase assays, co-IP, acetylation and dimerization analysis in liver cells plus a CCl4 mouse model

   PMID:31866205

   Open questions at the time

   • Specific deacetylated lysines not mapped in this study
   • Quantitative link between dimerization and catalytic rate not established
6. 2020 Medium

   Demonstrated that ubiquitination can stabilize rather than degrade CPT2, linking metabolic stress to CPT2 abundance via HRD1.

   Evidence Reciprocal co-IP, K48-linkage-specific ubiquitination and stability assays in TNBC cells in vitro and in vivo

   PMID:33207079

   Open questions at the time

   • Ubiquitinated lysine residues on CPT2 not mapped
   • How K48 chains stabilize rather than target CPT2 mechanistically unexplained
7. 2020 Medium

   Established physiologically that CPT2 flux is required to clear long-chain acylcarnitines and protect mitochondrial and cardiac function during inflammation.

   Evidence Pharmacological CPT2 inhibition (aminocarnitine) in LPS endotoxaemia with cardiac respiration, acylcarnitine, and function readouts

   PMID:32896106

   Open questions at the time

   • Inhibitor specificity for CPT2 not orthogonally confirmed genetically
   • Direct toxicity mechanism of accumulated acylcarnitines not dissected
8. 2021 Medium

   Placed CPT2 under direct transcriptional repression by E2F2 in the fatty-liver-to-cancer transition.

   Evidence ChIP showing E2F2 binding to the Cpt2 promoter with E2f2 knockout, knockdown, and overexpression mouse models and FAO flux

   PMID:33771899

   Open questions at the time

   • E2F1 contribution relative to E2F2 not fully separated
   • Coregulators at the Cpt2 promoter not identified
9. 2022 Medium

   Defined an inhibitory acetylation site (K79) whose accumulation under NAD+/SIRT3 decline causes acylcarnitine buildup and organelle damage.

   Evidence Acetylation site identification, SIRT3 activity and FAO flux assays in platelet storage and transfusion models

   PMID:35728063

   Open questions at the time

   • Whether K79 acetylation alters catalysis directly vs via stability not resolved
   • Acetyltransferase responsible for K79 not identified
10. 2022 Medium

    Mapped a transcriptional activation route to CPT2 through a FAM3A-ATP-P2 receptor-calmodulin-FOXA2 signaling axis.

    Evidence RNA-seq, FOXA2 ChIP/promoter assay, CaM nuclear translocation imaging in FAM3A-deficient hepatocytes and mice

    PMID:35995281

    Open questions at the time

    • Direct FOXA2 binding to the CPT2 promoter vs indirect effect not fully distinguished
    • Which P2 receptor subtype mediates the signal not pinned down
11. 2022 Low

    Linked a CPT2 variant (R631C) to reduced protein abundance and loss of a UCP2 interaction without altered localization.

    Evidence Fluorescence intensity comparison of WT vs mutant CPT2, co-IP with UCP2, and localization imaging in transfected cells

    PMID:35372350

    Open questions at the time

    • UCP2 interaction shown by a single method without reciprocal or endogenous validation
    • Functional consequence of lost UCP2 binding for FAO not measured
12. 2024 High

    Identified succinylation at K424 as an inactivating modification reversed by SIRT5, establishing a desuccinylation-dependent activity switch validated by mutational rescue.

    Evidence Succinylomics, K424R mutagenesis, enzymatic activity and acylcarnitine metabolomics with Sirt5 KO/overexpression diabetic mouse models

    PMID:38718533

    Open questions at the time

    • Succinyltransferase or non-enzymatic source of K424 succinylation not defined
    • Structural basis for K424 succinylation-induced inactivation not solved
13. 2024 High

    Showed CPT2-dependent long-chain FAO is dispensable for humoral immunity, delimiting where CPT2 function is metabolically required.

    Evidence Lymphocyte-specific CPT2 knockout mice with 13C isotope tracing and comprehensive B-cell developmental and functional assays

    PMID:39258879

    Open questions at the time

    • Whether other immune lineages depend on CPT2 not addressed
    • Compensatory metabolic pathways in B cells not characterized
14. 2025 Medium

    Expanded the SIRT3-CPT2 relationship to a de-lactylation mechanism that stabilizes CPT2 and links FAO to granulosa cell proliferation.

    Evidence Co-IP, half-life measurement, lactylation/de-lactylation and FAO assays with CPT2 overexpression in goat granulosa cells

    PMID:40671144

    Open questions at the time

    • Lactylated residues on CPT2 not mapped
    • Causal link between CPT2 stabilization and β-catenin/cyclin D1 induction not mechanistically traced
15. 2025 Medium

    Defined SIRT2 as a deacetylase that destabilizes CPT2 by promoting K239-dependent ubiquitination, showing deacetylation can reduce rather than increase CPT2 function.

    Evidence Co-IP, K239 acetylation site identification, deacetylation and ubiquitination assays with cardiac-specific SIRT2 manipulation in diabetic mice

    PMID:39781464

    Open questions at the time

    • Ubiquitin ligase coupling K239 deacetylation to degradation not identified
    • Reconciliation with SIRT3 activating deacetylation at other residues not addressed
16. 2025 Medium

    Identified MUL1 as a degradative E3 ligase for CPT2 whose action is potentiated by SLC44A2, providing a mitochondrial route to suppress FAO.

    Evidence Co-IP, ubiquitination and stability assays with SLC44A2 manipulation and FAO flux in colorectal cancer cells in vitro and in vivo

    PMID:40592838

    Open questions at the time

    • MUL1-ubiquitinated lysines on CPT2 not mapped
    • How SLC44A2 enhances the MUL1-CPT2 interaction structurally unknown
17. 2025 Low

    Linked the mitochondrial CoA transporter SLC25A42 to increased CPT2 acetylation, expression and FAO with reduced ferroptosis in gastric cancer.

    Evidence CPT2 acetylation analysis, co-IP, Seahorse FAO flux, and ROS/free fatty acid measurements with SLC25A42 KD/OE

    PMID:40246810

    Open questions at the time

    • Mechanistic detail of how acetylation increases CPT2 activity here is thin and not site-resolved
    • Apparent activation by acetylation conflicts with inactivating acetylation reports and is unreconciled

• It remains unresolved how the competing acylation marks (succinylation, acetylation at distinct lysines, lactylation) and opposing ubiquitin ligases are integrated to set net CPT2 activity and abundance in a given tissue.
• No unified structural model of how lysine modifications at K79, K239, K424 jointly control catalysis and turnover
• Cross-talk between HRD1 stabilization and MUL1/SIRT2 degradation not mapped
• Identity of the malonyl-CoA-binding outer-membrane partner from the original reconstitution remains undefined in the corpus