Mcad (CG12262) is the Drosophila melanogaster ortholog of human ACADM (MCAD), a mitochondrial medium-chain specific acyl-CoA dehydrogenase (EC 1.3.8.7). It is synthesized with an N-terminal mitochondrial transit peptide and, after import, catalyzes the first, committed step of each cycle of mitochondrial fatty acid beta-oxidation for medium-chain (C6-C12) acyl-CoA esters: the FAD-dependent alpha,beta-dehydrogenation of a saturated medium-chain acyl-CoA to the corresponding trans-2-enoyl-CoA, with electrons passed to the electron-transfer flavoprotein (ETF) and thence to the respiratory chain. Each subunit carries one non-covalently bound FAD, and the mammalian ortholog assembles as a soluble homotetramer. The enzyme resides principally in the mitochondrial matrix, with a secondary cytosolic pool detected biochemically in flies. Loss of Mcad function in flies (CRISPR deletion, deficiency transheterozygotes, or RNAi knockdown) elevates medium-chain acylcarnitines (C6, C8, C10:1), the same biochemical signature seen in human MCAD deficiency, establishing Mcad as the medium-chain beta-oxidation enzyme in the fly. Mcad is also phosphorylated at Ser347 in a PINK1-dependent manner; this modification contributes to Pink1-related metabolic and neuromuscular physiology through a mechanism that is independent of its acyl-CoA dehydrogenase activity.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0005737 cytoplasm | IBA GO_REF:0000033 | MARK AS OVER ANNOTATED | Summary: Phylogenetically inferred cytoplasmic localization. Mcad is in fact a mitochondrially targeted protein acting principally in the mitochondrial matrix; "cytoplasm" is an over-general parent (the mitochondrion is within the cytoplasm) that does not capture the precise compartment. Reason: The precise, experimentally supported location is the mitochondrial matrix (GO:0005759). The IBA "cytoplasm" call is an uninformative over-general placeholder. |
| GO:0005739 mitochondrion | IBA GO_REF:0000033 | KEEP AS NON CORE | Summary: Phylogenetically inferred mitochondrial localization, fully consistent with the fly experimental evidence. Correct but less precise than the mitochondrial matrix annotation. Reason: Mitochondrial localization is well established; the more specific mitochondrial matrix term is the core location annotation. Supporting Evidence: PMID:29563254 both endogenous and transgenic MCAD protein localized to the mitochondria as well as the cytosol, consistent with a previous report in mammals |
| GO:0051793 medium-chain fatty acid catabolic process | IBA GO_REF:0000033 | ACCEPT | Summary: Phylogenetically inferred role in medium-chain fatty acid catabolism, directly matching Mcad's biochemical function and its canonical role in breaking down C6-C12 fatty acids. This is a core process for Mcad, corroborated in fly by the deficiency-model acylcarnitine phenotype. Reason: Mcad catabolizes medium-chain fatty acids via beta-oxidation; this term accurately captures its core biological process. Supporting Evidence: PMID:29563254 MCAD's canonical function is to break down medium-chain fatty acids (C6-12) |
| GO:0033539 fatty acid beta-oxidation using acyl-CoA dehydrogenase | IBA GO_REF:0000033 | ACCEPT | Summary: Phylogenetically inferred participation in the acyl-CoA-dehydrogenase-dependent step of beta-oxidation. This is the defining process for ACAD-family enzymes and the most precise process term for Mcad's role. Independently supported by the fly IMP annotation below. Reason: Mcad performs the acyl-CoA dehydrogenase step of beta-oxidation; this is a core process annotation. Supporting Evidence: PMID:29563254 an enzyme critical to fatty acid Ξ²-oxidation, medium-chain acyl-coenzyme A dehydrogenase (MCAD) |
| GO:0070991 medium-chain fatty acyl-CoA dehydrogenase activity | IBA GO_REF:0000033 | ACCEPT | Summary: Phylogenetically inferred medium-chain acyl-CoA dehydrogenase activity. This is Mcad's defining molecular function, at the correct level of specificity, and is independently supported in fly by the experimental EC 1.3.8.7 assignment and the deficiency-model phenotype (PMID:29563254). Reason: This is the core molecular function of Mcad, the fly ortholog of human MCAD. An independent OpenScientist analysis (run blinded to this review action as a neutral function-assignment hypothesis, focused on the substrate cavity) strongly supports medium-chain specificity: 20 of 21 active-site/substrate-binding residues are identical to human MCAD/ACADM, the single substitution (Drosophila F372 vs human L376) is a conservative hydrophobic swap that preserves the pocket, and AlphaFold active-site geometry is near-identical (CA-CA distances from the catalytic glutamate correlate r=1.0 with human MCAD) - with no evidence for a short-, long-, or very-long-chain preference. Supporting Evidence: PMID:29563254 MCAD's canonical function is to break down medium-chain fatty acids (C6-12) file:DROME/Mcad/Mcad-hypotheses/function-hypothesis-go-0070991/openscientist.md The single substitution (F372 in Drosophila vs. L376 in human) is a conservative hydrophobic replacement (both are bulky hydrophobic residues) that maintains the character of the substrate-binding pocket. |
| GO:0003995 acyl-CoA dehydrogenase activity | IEA GO_REF:0000120 | KEEP AS NON CORE | Summary: Automated annotation of the general acyl-CoA dehydrogenase activity. Correct but a broad parent of the specific medium-chain activity (GO:0070991). Retained as accurate but non-core because the specific child term is the informative annotation. Reason: Mcad is an acyl-CoA dehydrogenase; this general term is correct but subsumed by the more specific medium-chain activity that captures Mcad's actual function. Supporting Evidence: PMID:29563254 MCAD is still enzymatically active as an acyl-CoA dehydrogenase in PINK1 null flies |
| GO:0005739 mitochondrion | IEA GO_REF:0000120 | KEEP AS NON CORE | Summary: Automated mitochondrial localization, consistent with the fly experimental data. Correct but less precise than mitochondrial matrix. Reason: Mitochondrial localization is correct; matrix is the precise core location. Supporting Evidence: PMID:29563254 both endogenous and transgenic MCAD protein localized to the mitochondria as well as the cytosol, consistent with a previous report in mammals |
| GO:0005759 mitochondrial matrix | IEA GO_REF:0000044 | ACCEPT | Summary: Automated subcellular-location-based annotation to mitochondrial matrix, matching the experimentally supported localization (EXP, PMID:29563254) of this mitochondrially targeted FAO flavoenzyme. This is the core location annotation for Mcad. Reason: Mcad is a mitochondrial matrix enzyme; matrix is the correct, precise compartment, independently supported by experimental evidence in fly. |
| GO:0005829 cytosol | IEA GO_REF:0000044 | KEEP AS NON CORE | Summary: Automated subcellular-location annotation to cytosol. A cytosolic pool of Mcad was in fact directly observed in fly by subcellular fractionation (PMID:29563254), so the call is not spurious; however the primary, function-defining compartment for this FAO enzyme is the mitochondrial matrix. Reason: A secondary cytosolic pool is experimentally observed in fly, but the core catalytic compartment is the mitochondrial matrix. Supporting Evidence: PMID:29563254 both endogenous and transgenic MCAD protein localized to the mitochondria as well as the cytosol, consistent with a previous report in mammals |
| GO:0006635 fatty acid beta-oxidation | IEA GO_REF:0000002 | ACCEPT | Summary: InterPro-based annotation to fatty acid beta-oxidation, the pathway Mcad initiates. This is a core biological process for Mcad, redundant with the more specific acyl-CoA dehydrogenase step (GO:0033539) and directly supported in fly. Reason: Mcad catalyzes the first step of mitochondrial fatty acid beta-oxidation; this is a core process annotation. Supporting Evidence: PMID:29563254 an enzyme critical to fatty acid Ξ²-oxidation, medium-chain acyl-coenzyme A dehydrogenase (MCAD) |
| GO:0016627 oxidoreductase activity, acting on the CH-CH group of donors | IEA GO_REF:0000002 | KEEP AS NON CORE | Summary: InterPro-based annotation of the general oxidoreductase class. Mcad's reaction oxidizes the C2-C3 (alpha-beta) CH-CH bond, so this term is accurate but a broad parent of the specific acyl-CoA dehydrogenase activity. Reason: Correct intermediate-level term; subsumed by the specific medium-chain acyl-CoA dehydrogenase activity that is the informative MF annotation. |
| GO:0050660 flavin adenine dinucleotide binding | IEA GO_REF:0000002 | ACCEPT | Summary: InterPro-based annotation of FAD binding. Mcad is an acyl-CoA dehydrogenase family flavoprotein carrying a non-covalently bound FAD cofactor (annotated in UniProt by similarity to human MCAD), essential for the dehydrogenation reaction. A correct and supported cofactor-binding function. Reason: FAD binding is a required cofactor function for Mcad catalysis, supported by family membership and the FAD-dependent EC 1.3.8.7 reaction. Supporting Evidence: file:DROME/Mcad/Mcad-uniprot.txt Name=FAD; Xref=ChEBI:CHEBI:57692 |
| GO:0070991 medium-chain fatty acyl-CoA dehydrogenase activity | IEA GO_REF:0000120 | ACCEPT | Summary: Automated (Rhea/EC 1.3.8.7) annotation of the core medium-chain acyl-CoA dehydrogenase activity, fully consistent with the IBA and the experimental IMP annotations. Correct and at the right specificity. Reason: Core molecular function of Mcad, redundantly supported by EC/Rhea mapping and experiment. |
| GO:0033539 fatty acid beta-oxidation using acyl-CoA dehydrogenase | IMP PMID:29563254 Phosphorylation of MCAD selectively rescues PINK1 deficienci... | ACCEPT | Summary: Direct fly IMP annotation. Course et al. generated a CRISPR deletion (MCADmut), a transheterozygous MCADmut-over-deficiency, and an RNAi knockdown, all of which elevated medium-chain acylcarnitines (C6, C8, C10:1) as in human MCAD deficiency, with full rescue by wild-type Mcad. This directly demonstrates Mcad's in vivo role in the acyl-CoA dehydrogenase step of medium-chain beta-oxidation. Core process. Reason: Loss-of-function fly models establish Mcad's requirement for medium-chain fatty acid beta-oxidation in vivo. Supporting Evidence: PMID:29563254 significant elevations in medium-chain acylcarnitines characteristic of human MCAD deficiency: C6, C8, and C10:1 acylcarnitines |
| GO:0005759 mitochondrial matrix | EXP PMID:29563254 Phosphorylation of MCAD selectively rescues PINK1 deficienci... | ACCEPT | Summary: Experimental localization to the mitochondrial matrix (the paper describes Mcad as a mitochondrial matrix protein and confirms mitochondrial fractionation of endogenous fly Mcad). This is the core, precise location annotation for Mcad. Reason: Mitochondrial matrix is the experimentally supported, correct subcellular location of Mcad. Supporting Evidence: PMID:29563254 MCAD, a mitochondrial matrix protein critical to fatty acid metabolism |
| GO:0005739 mitochondrion | IDA PMID:29563254 Phosphorylation of MCAD selectively rescues PINK1 deficienci... | KEEP AS NON CORE | Summary: Direct (fractionation) annotation of mitochondrial localization of endogenous and transgenic fly Mcad. Consistent with all evidence; less specific than mitochondrial matrix. Reason: Mitochondrial localization is correct; matrix is the precise core compartment. Supporting Evidence: PMID:29563254 both endogenous and transgenic MCAD protein localized to the mitochondria as well as the cytosol, consistent with a previous report in mammals |
| GO:0005829 cytosol | IDA PMID:29563254 Phosphorylation of MCAD selectively rescues PINK1 deficienci... | KEEP AS NON CORE | Summary: Direct annotation of a cytosolic pool of Mcad, observed by subcellular fractionation of fly lysates for both endogenous and transgenic protein (mirroring a mammalian report). This is a genuine but secondary observation; the primary catalytic compartment is the mitochondrial matrix. Reason: A cytosolic pool is directly observed in fly, but the function-defining location is the mitochondrial matrix. Supporting Evidence: PMID:29563254 both endogenous and transgenic MCAD protein localized to the mitochondria as well as the cytosol, consistent with a previous report in mammals |
| GO:0070991 medium-chain fatty acyl-CoA dehydrogenase activity | IMP PMID:29563254 Phosphorylation of MCAD selectively rescues PINK1 deficienci... | ACCEPT | Summary: Direct fly IMP annotation of Mcad's core medium-chain acyl-CoA dehydrogenase activity. Loss-of-function models (deletion, deficiency, RNAi) reproduce the C6/C8/C10:1 acylcarnitine signature of MCAD deficiency, and PINK1-null flies retain acyl-CoA dehydrogenase activity, together establishing that fly Mcad enables this medium-chain dehydrogenation. This is the strongest support for the defining molecular function in fly. Reason: Experimentally supported core molecular function of Mcad; the fly deficiency phenotype is diagnostic of medium-chain acyl-CoA dehydrogenase loss. Supporting Evidence: PMID:29563254 significant elevations in medium-chain acylcarnitines characteristic of human MCAD deficiency: C6, C8, and C10:1 acylcarnitines |
| GO:0005739 mitochondrion | HDA PMID:19317464 Mapping organelle proteins and protein complexes in Drosophi... | KEEP AS NON CORE | Summary: High-throughput organelle-mapping (LOPIT) assignment to mitochondrion in Drosophila embryos. Consistent with the well-supported mitochondrial localization. The cached record is abstract-only, so protein-level supporting text is not extractable; treated as confirmatory but non-core (less specific than matrix). Reason: High-throughput mitochondrial localization is consistent with experimental evidence; matrix is the precise core compartment. Supporting Evidence: PMID:29563254 both endogenous and transgenic MCAD protein localized to the mitochondria as well as the cytosol, consistent with a previous report in mammals |
| GO:0005739 mitochondrion | HDA PMID:16212416 Characterization of the Drosophila melanogaster mitochondria... | KEEP AS NON CORE | Summary: High-throughput 2-D gel / MALDI Drosophila mitochondrial proteome assignment to mitochondrion. Consistent with the well-supported mitochondrial localization. Cached record is abstract-only; treated as confirmatory but non-core (less specific than matrix). Reason: High-throughput mitochondrial proteome localization is consistent with experimental evidence; matrix is the precise core compartment. Supporting Evidence: PMID:29563254 both endogenous and transgenic MCAD protein localized to the mitochondria as well as the cytosol, consistent with a previous report in mammals |
| GO:0003995 acyl-CoA dehydrogenase activity | ISS GO_REF:0000024 | KEEP AS NON CORE | Summary: Sequence-similarity (ISS from human MCAD, UniProtKB:P11310) annotation of the general acyl-CoA dehydrogenase activity. Correct but a broad parent of the specific medium-chain activity (GO:0070991); retained as non-core. Reason: Correct general activity inferred from the human ortholog; subsumed by the specific medium-chain dehydrogenase MF that is the informative annotation. Supporting Evidence: PMID:29563254 MCAD is still enzymatically active as an acyl-CoA dehydrogenase in PINK1 null flies |
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Download this section (compressed HTML)Q: What is the molecular mechanism by which PINK1-dependent phosphorylation of Mcad at Ser347 contributes to rescue of PINK1-null climbing, flight, and thoracic/wing phenotypes, given that this rescue is independent of Mcad's acyl-CoA dehydrogenase activity and of ATP production? Does phosphorylated Mcad acquire a moonlighting (non-catalytic) function or protein interaction?
Q: Does fly Mcad share the same medium-chain substrate optimum (hexanoyl-/octanoyl-CoA) and chain-length boundaries as mammalian MCAD, and how does its activity partition with other Drosophila acyl-CoA dehydrogenases across the fly FAO chain-length range?
Experiment: Purify recombinant fly Mcad and determine its substrate-specificity profile across C4-C16 saturated acyl-CoAs using an ETF- or artificial-acceptor-coupled dehydrogenase assay, to confirm the medium-chain optimum and directly ground the GO:0070991 molecular-function annotation in fly biochemistry.
Experiment: Use phosphosite-specific (S347A/S347D) Mcad knock-in flies combined with proximity-labeling interactomics and metabolomics in wild-type and PINK1-null backgrounds to define the catalysis-independent, phosphorylation-dependent function of Mcad.
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