FLAD1 (Q8NFF5) — Gene Review Notes

Identity and summary

FLAD1 encodes human FAD synthase (FMN adenylyltransferase; FMNAT; FADS; EC 2.7.7.2),
the enzyme that catalyzes the final, committed step of FAD-cofactor biosynthesis:
adenylylation of flavin mononucleotide (FMN) with ATP to yield flavin adenine
dinucleotide (FAD) plus diphosphate. FAD is the cofactor of hundreds of flavoenzymes
(respiratory chain Complex I/II, acyl-CoA dehydrogenases, MTHFR, dimethylglycine
dehydrogenase, monoamine oxidase, etc.). The reaction requires a divalent metal,
optimally Mg2+.

[file:human/FLAD1/FLAD1-uniprot.txt "FAD synthase catalyzes the adenylation of flavin mononucleotide (FMN) to form flavin adenine dinucleotide (FAD) coenzyme"]
[file:human/FLAD1/FLAD1-uniprot.txt "Reaction=FMN + ATP + H(+) = FAD + diphosphate"]
PMID:16643857
PMID:16643857

The human enzyme is bifunctional and multidomain, distinguishing it from the
single-domain yeast Fad1p:
- C-terminal PAPS-reductase domain (FADSy; ~residues 355-587) = the FAD-forming
catalytic (FMN adenylyltransferase) domain.
- N-terminal molybdopterin-binding-resembling (MPTb) domain (with a KH domain) =
a FAD hydrolase / FAD diphosphatase (FADHy) activity (EC 3.6.1.18), and, at lower
efficiency, an NADH diphosphatase / NADH pyrophosphatase activity (EC 3.6.1.22).

[file:human/FLAD1/FLAD1-uniprot.txt "This enzyme has two activities: FAD diphosphatase activity"]
[file:human/FLAD1/FLAD1-uniprot.txt "In the N-terminal section; belongs to the MoaB/Mog family"]
[file:human/FLAD1/FLAD1-uniprot.txt "In the C-terminal section; belongs to the PAPS reductase"]
PMID:26277395
PMID:38688286

Reactions (from UniProt CATALYTIC ACTIVITY)

[file:human/FLAD1/FLAD1-uniprot.txt "Reaction=FAD + H2O = FMN + AMP + 2 H(+)"]
[file:human/FLAD1/FLAD1-uniprot.txt "PATHWAY: Cofactor biosynthesis; FAD biosynthesis; FAD from FMN: step"]

Note the direction convention: GO:0003919 FMN adenylyltransferase activity is defined
ATP + FMN = diphosphate + FAD (the synthase direction), while GO:0047884 FAD
diphosphatase activity is FAD + H2O = AMP + FMN (the hydrolase direction). Both are
distinct, well-supported activities of this bifunctional protein.

Cofactor / metal requirement

FAD synthase activity requires a divalent metal (Mg2+ optimal; Co2+ comparable; lower
with Mn/Ca/Zn). FAD diphosphatase activity instead requires Co2+ (Ni/Mn partial;
Mg/Ca/Cu cannot substitute) and is stimulated by K+.

[file:human/FLAD1/FLAD1-uniprot.txt "The FAD synthase activity requires a divalent metal"]
[file:human/FLAD1/FLAD1-uniprot.txt "Magnesium or cobalt supports the highest FAD synthase activity"]
[file:human/FLAD1/FLAD1-uniprot.txt "The FAD diphosphatase activity requires cobalt"]

Kinetics (isoform 2; from UniProt)

KM(FMN) ~0.35-1.5 uM; KM(ATP) ~15 uM; kcat(FAD synthesis) ~0.69 s-1 (E. coli-purified).
kcat is low, consistent with FAD release being rate-limiting and with a "FAD chaperone"
delivery step.

PMID:16643857
PMID:21951714

FAD chaperone / delivery function

Beyond catalysis, hFADS2 physically interacts with client apo-flavoproteins and
directly transfers newly made FAD to them (a "FAD chaperone" role). Demonstrated for
the nuclear demethylase KDM1A/LSD1 and mitochondrial dimethylglycine dehydrogenase
(DMGDH).

[PMID:25954742 "hFADS is able to operate as a FAD \"chaperone.\""]
PMID:25954742
[file:human/FLAD1/FLAD1-uniprot.txt "Interacts with KDM1A; which promotes"]
[file:human/FLAD1/FLAD1-uniprot.txt "Interacts with DMGDH; which promotes DMGDH holoenzyme formation"]

Subcellular localization (isoform-dependent)

PMID:20060505
[file:human/FLAD1/FLAD1-uniprot.txt "Isoform 1 and 2 are located in mitochondria and cytosol, respectively"]
[file:human/FLAD1/FLAD1-uniprot.txt "SUBCELLULAR LOCATION: Nucleus"]
[file:human/FLAD1/FLAD1-uniprot.txt "SUBCELLULAR LOCATION: [Isoform 1]: Mitochondrion matrix"]
[file:human/FLAD1/FLAD1-uniprot.txt "SUBCELLULAR LOCATION: [Isoform 2]: Cytoplasm, cytosol"]

Structure

Crystal structure (PDB 8ROM/8RON) of full-length hFADS2 and its C-terminal PAPS domain
in complex with FAD. The enzyme forms a stable C2-symmetric dimer; packing of one
protomer's KH domain against the other's N-terminal domain creates the adenosine-specific
hydrolytic active site.

PMID:38688286
[file:human/FLAD1/FLAD1-uniprot.txt "SUBUNIT: Dimer (PubMed:38688286)"]

Disease

Biallelic FLAD1 variants cause Lipid Storage Myopathy due to FAD Synthase Deficiency
(LSMFLAD; MIM:255100), presenting as combined respiratory-chain deficiency and multiple
acyl-CoA dehydrogenase deficiency (MADD-like). Some (frameshift, hypomorphic) cases are
riboflavin-responsive. (Primary reference PMID:27259049 not in local cache; disease
statement quoted from UniProt.)

[file:human/FLAD1/FLAD1-uniprot.txt "Lipid storage myopathy due to flavin adenine dinucleotide"]
[file:human/FLAD1/FLAD1-uniprot.txt "Some patients show significant improvement with riboflavin treatment"]

Notes on annotation review