Human N-acetyl-D-glucosamine kinase (GlcNAc kinase), HGNC:17174, gene ID 55577, chromosome 2.
344 aa; homodimer; sugar kinase / Hsp70 / actin (ASKHA) superfamily,
"eukaryotic-type N-acetylglucosamine kinase family"
[file:human/NAGK/NAGK-uniprot.txt "Belongs to the eukaryotic-type N-acetylglucosamine kinase"].
NAGK phosphorylates free N-acetyl-D-glucosamine (GlcNAc) using ATP to make
GlcNAc 6-phosphate + ADP + H+ (Rhea:RHEA:17417; EC 2.7.1.59). This is the salvage
entry point of amino-sugar metabolism: GlcNAc released by endogenous glycoconjugate
turnover (lysosomal degradation, O-GlcNAc removal) or from diet is recycled back into
the hexosamine / UDP-GlcNAc pool.
NAGK also phosphorylates N-acetyl-D-mannosamine (ManNAc). In UniProt the ManNAc
kinase reaction is By-similarity (ECO:0000250|UniProtKB:Q9QZ08 mouse), but the
original murine/human cloning paper reported both activities biochemically
PMID:10824116. GO:0009384 "N-acylmannosamine kinase activity" is a reasonable
(if imprecise) MF; N-acetylmannosamine metabolic process GO:0006051 is the BP.
Note: ManNAc-6-P from GNE/MNK is the canonical route to sialic acid biosynthesis;
NAGK's ManNAc activity is a secondary/salvage capacity.
Campbell et al. (eLife 2021) show glutamine deprivation in pancreatic cancer cells
raises free GlcNAc and engages NAGK to salvage it into the UDP-GlcNAc pool:
"GlcNAc salvage via N-acetylglucosamine kinase (NAGK) is engaged to feed UDP-GlcNAc pools"
and "NAGK deletion from PDA cells impairs tumor growth in mice" PMID:34844667.
"GlcNAc salvage feeds UDP-GlcNAc pools" PMID:34844667. The paper is annotated by
FlyBase as IMP for GO:0006046 (GlcNAc catabolic), GO:0006048 (UDP-GlcNAc biosynthetic)
and GO:0045127 (kinase activity). NAGK-6-P is the product feeding the hexosamine
biosynthesis pathway at the GlcNAc-6-P node (downstream of / parallel to GNPNAT1).
The paper itself notes salvage "has been little studied, and the proportion of
UDP-GlcNAc generated via the NAGK-dependent salvage pathway is unknown" PMID:34844667.
Reactome R-HSA-6803771: "dimeric GlcNAc kinase (NAGK) to phosphorylate
N-acetylglucosamine (GlcNAc) to GlcNAc-6-phosphate and N-glycolylglucosamine (GlcNGc)
to GlcNGc-6-P". Humans lack Neu5Gc synthesis (inactive CMAHP) but ingest it and must
degrade it; NAGK acts in this degradation branch [reactome R-HSA-6803771;
PMID:22692205 abstract]. This is why GO:0019262 "N-acetylneuraminate catabolic process"
is annotated (TAS to PMID:22692205). This is a real but relatively narrow, context-specific
role — the direct catalytic step is GlcNAc/GlcNGc phosphorylation.
Stafford et al. (Nature 2022) identified NAGK in a forward genetic screen as essential
for MDP sensing. NAGK directly phosphorylates the muramyl dipeptide (MDP) MurNAc C6
hydroxyl to make 6-O-phospho-MDP, which is the actual NOD2 agonist:
- "NAGK functions upstream of NOD2 by directly" ... phosphorylating MurNAc of MDP PMID:36002575.
- "constitutes an agonist for NOD2" (NAGK-phosphorylated, not unmodified, MDP) PMID:36002575.
- In vitro: "we expressed recombinant human NAGK in E. coli and used it in an in vitro kinase assay with various substrates"; "NAGK readily phosphorylated MDP and the related molecule N-acetylmuramic acid" PMID:36002575.
- Catalytic dependence: "a mutant NAGK(D107V) construct, which is predicted to disrupt enzyme activity13, did not rescue NOD2 signalling in NAGK-deficient cells" PMID:36002575; matches UniProt MUTAGEN D107V "Abolished ability to phosphorylate muramyl dipeptide."
- Knockout: "completely deficient in MDP sensing" PMID:36002575.
This supports GO:0160047 muramyl dipeptide kinase activity (IDA), GO:0032495 response to
muramyl dipeptide (IDA), GO:0042742 defense response to bacterium (IDA), and
GO:0070434 positive regulation of NOD2 signaling pathway (IDA). UniProt captures this as
EC 2.7.1.- and additional AltName "Muramyl dipeptide kinase". This is a genuine, distinct
biological role but is a moonlighting/immunity function; the enzymatic step (sugar-6-OH
phosphorylation) is mechanistically the same kinase chemistry.
Timalsina et al. (2023) report NAGK acts as a non-canonical structural/anchor protein
that, via interaction with SNRPN and the dynein light chain DYNLRB1, promotes
axodendritic branching through dynein-mediated microtubule transport
PMID:37511433. This is the basis
for the SNRPN (P63164) IntAct annotation. This role is developmental / neuron-specific and
non-core; the associated GO:0005515 IPI is a bare protein-binding capture.
Cytosolic (TAS Reactome, GO:0005829). Also detected in urinary extracellular exosomes by
mass spec PMID:19056867 — an HDA proteomics capture (GO:0070062),
non-core; exosome/secretome MS commonly detects abundant cytosolic enzymes.
GO:0005515 (protein binding) annotations come from large interactome / Y2H / MS screens:
PMID:16189514 (CCSB HI-1), PMID:21516116 (NGS interactome), PMID:25416956 (HI-II-14),
PMID:29892012, PMID:31515488, PMID:37511433 (SNRPN). These are uninformative bare
"protein binding" captures. Per policy, IPI protein-binding is not REMOVEd; mark as
over-annotated (KEEP context in notes). None of these interactors define NAGK's core
molecular function.
Two isoforms: Q9UJ70-1 (canonical, MANE) and Q9UJ70-2 (VSP_044586, N-terminal extension
from alternative splicing). No isoform-specific function established; annotations are on the
gene/canonical enzyme.