NANS (Q9NR45) — review notes

Identity

UniProt: Q9NR45, SIAS_HUMAN, 359 aa, gene symbol NANS (HGNC:19237), synonym SAS.
RecName: N-acetylneuraminate-9-phosphate synthase (EC 2.5.1.57); AltName 3-deoxy-D-glycero-D-galacto-nononate 9-phosphate synthase (EC 2.5.1.132); AltName Sialic acid synthase / Sialic acid phosphate synthase.
[file:human/NANS/NANS-uniprot.txt "RecName: Full=N-acetylneuraminate-9-phosphate synthase"]
[file:human/NANS/NANS-uniprot.txt "EC=2.5.1.57"] and [file:human/NANS/NANS-uniprot.txt "EC=2.5.1.132"]

Core molecular function

NANS catalyzes the committed condensation step of de novo sialic acid (Neu5Ac) biosynthesis:
condensation of phosphoenolpyruvate (PEP) with N-acetylmannosamine 6-phosphate (ManNAc-6-P) to
form N-acetylneuraminate 9-phosphate (Neu5Ac-9-P).
- UniProt FUNCTION: [file:human/NANS/NANS-uniprot.txt "Catalyzes the condensation of phosphoenolpyruvate (PEP) and"] / [file:human/NANS/NANS-uniprot.txt "acetylneuraminate-9-phosphate (Neu5Ac-9-P) (PubMed:10749855)."]
- It also condenses PEP with D-mannose 6-phosphate (Man-6-P) to make KDN-9-P (deaminoneuraminic acid 9-phosphate), but with much lower activity toward the non-acetyl product.
- [file:human/NANS/NANS-uniprot.txt "catalyzes the condensation of PEP and D-mannose 6-phosphate (Man-6-P)"]
- Primary characterization: PMID:10749855
- The enzyme complements a neuB-negative E. coli mutant (homolog of bacterial sialic acid synthase NeuB).
- PMID:10749855

Important CAUTION (do not over-annotate)

UniProt explicitly notes NANS does NOT have "sialic acid synthase activity" acting on the
non-phosphorylated substrates ManNAc / mannose (i.e. it is a phosphate synthase, not the
ManNAc→Neu5Ac reaction, EC of GO:0050462).
- [file:human/NANS/NANS-uniprot.txt "Does not exhibit sialic acid synthase activity, which"] / [file:human/NANS/NANS-uniprot.txt "acetylmannosamine (ManNAc) and mannose, respectively."]
- Therefore the correct MF term is GO:0047444 N-acylneuraminate-9-phosphate synthase activity (EC 2.5.1.57, reaction: PEP + ManNAc-6-P + H2O = Neu5Ac-9-P + phosphate), NOT GO:0050462 N-acetylneuraminate synthase activity (the ManNAc→Neu5Ac reaction). GO:0047444 def (go.db): "Catalysis of the reaction: H2O + phosphoenolpyruvate + N-acyl-D-mannosamine 6-phosphate = phosphate + N-acylneuraminate 9-phosphate." N-acyl generalizes over N-acetyl, and also accommodates the KDN-9-P (EC 2.5.1.132) side activity.

Biological process

De novo sialic acid biosynthesis (5 steps: GNE → NANS → NANP → CMAS → …). NANS is the sialic-acid-synthase step.
- NANS KO in human cell lines reduces cell-surface sialylation and dramatically reduces CMP-sialic acid.
- PMID:31121216
- PMID:31121216
- Pathway context: PMID:31121216
- Core BP term: GO:0046380 N-acetylneuraminate biosynthetic process (direct product path). GO:0006054 (N-acetylneuraminate metabolic process) is the more general parent (metabolic ⊃ biosynthetic; confirmed via go.db entailed_edge: GO:0046380 is-a GO:0006054).
- CMP-N-acetylneuraminate biosynthetic process (GO:0006055): NANS acts upstream of the cytidylyltransferase (CMAS) step; its loss abolishes CMP-Neu5Ac, so involvement is defensible but is the pathway product rather than NANS's own catalytic step. Kept as non-core.

Location

Cytosolic enzyme. UniProt keyword set + Reactome place NANS in cytosol.
GO:0005829 cytosol (Reactome TAS; ARBA IEA) — accept; consistent with a soluble cytosolic metabolic enzyme.
GO:0005737 cytoplasm (NAS, PMID:10749855) — accept as non-core (less specific than cytosol).
GO:0005654 nucleoplasm (IDA, HPA immunofluorescence, GO_REF:0000052) — over-annotation; NANS is a cytosolic sialic-acid-synthesis enzyme with no known nuclear role. HPA IF localizations frequently over-call nucleoplasm. Mark as over-annotated (IDA → keep in record, not REMOVE).
GO:0070062 extracellular exosome (HDA, PMID:23533145) — high-throughput mass-spec detection in urinary prostatic-secretion exosomes; a common HTP contaminant/co-purification for abundant cytosolic enzymes. Not a functional location. Mark as over-annotated.

Disease

NANS-CDG / Spondyloepimetaphyseal dysplasia, Genevieve type (SEMDG, MIM:610442): autosomal recessive, infantile-onset global developmental delay, intellectual disability, skeletal dysplasia, short stature.
[file:human/NANS/NANS-uniprot.txt "An autosomal recessive disorder characterized by global"] / [file:human/NANS/NANS-uniprot.txt "developmental delay with infantile onset, intellectual disability,"]
[file:human/NANS/NANS-uniprot.txt "Required for brain and skeletal development (PubMed:27213289)."]
Biallelic loss-of-function/missense variants (e.g. p.His29Asn, p.Gly133Val, p.Arg151His, p.Tyr188His, p.Pro189Leu, p.Arg237Cys) documented in UniProt VARIANT lines (PubMed:27213289). Patients accumulate N-acetylmannosamine (upstream substrate).

Structure / domains

Two-domain architecture: N-terminal (β/α)8 TIM-barrel catalytic domain (Aldolase class I / NeuB Pfam PF03102) + C-terminal antifreeze-like (AFP-like) / SAF domain (Pfam PF08666), residues 294–353.
NMR structure of the C-terminal AFP-like domain: PDB 1WVO (PMID:16597820, "Solution structure of the antifreeze-like domain of human sialic acid synthase").
PANTHER: PTHR42966:SF1 "SIALIC ACID SYNTHASE".

Annotation action summary (14 GOA rows)

Term	Evidence	Action	Rationale
GO:0047444 MF	IBA (GO_REF:33)	ACCEPT	phylogenetic; correct core MF
GO:0006054 N-acetylneuraminate metabolic process	IEA (ARBA)	KEEP_AS_NON_CORE	general parent of core BP
GO:0016051 carbohydrate biosynthetic process	IEA (InterPro)	KEEP_AS_NON_CORE	high-level, correct but generic
GO:0047444 MF	IEA (ARBA/RHEA/EC:2.5.1.57)	ACCEPT	matches EC 2.5.1.57
GO:0005829 cytosol (is_active_in)	IEA (ARBA)	ACCEPT	correct location
GO:0047444 MF	TAS (Reactome)	ACCEPT	correct core MF
GO:0005654 nucleoplasm	IDA (HPA)	MARK_AS_OVER_ANNOTATED	HPA IF over-call; no nuclear role (never REMOVE IDA)
GO:0006055 CMP-Neu5Ac biosynthetic process	IMP (PMID:31121216)	KEEP_AS_NON_CORE	downstream pathway product
GO:0046380 N-acetylneuraminate biosynthetic process	IMP (PMID:31121216)	ACCEPT	core BP
GO:0047444 MF	IDA (PMID:10749855)	ACCEPT	primary experimental characterization
GO:0006055 CMP-Neu5Ac biosynthetic process	NAS (PMID:10749855)	KEEP_AS_NON_CORE	author statement, pathway-level
GO:0070062 extracellular exosome	HDA (PMID:23533145)	MARK_AS_OVER_ANNOTATED	HTP MS co-purification (never REMOVE HDA)
GO:0005829 cytosol (located_in)	TAS (Reactome)	ACCEPT	correct location
GO:0005737 cytoplasm	NAS (PMID:10749855)	KEEP_AS_NON_CORE	less specific than cytosol

Core functions (for review)

MF: GO:0047444 N-acylneuraminate-9-phosphate synthase activity (with inputs ManNAc-6-P + PEP → Neu5Ac-9-P).
BP: GO:0046380 N-acetylneuraminate biosynthetic process.
Location: GO:0005829 cytosol.