GYS2 (Glycogen synthase 2, liver; UniProtKB:P54840) — review notes

Provenance note

Falcon deep-research provider was out of credits (HTTP 402) at review time, so no
GYS2-deep-research-falcon.md was generated. This review is grounded in the UniProt
record (GYS2-uniprot.txt), the seeded GOA (GYS2-goa.tsv), cached publications
(publications/PMID_1731614.md, publications/PMID_9691087.md), and cached Reactome
entries (reactome/R-HSA-*.md). Both cached PMIDs are abstract-only
(full_text_available: false).

Core biology

GYS2 is the liver isoform of glycogen synthase (EC 2.4.1.11), the committed,
rate-limiting enzyme of hepatic glycogen chain elongation. It uses UDP-glucose as the
glucosyl donor and adds glucose units in alpha-1,4 linkage to the non-reducing ends of
a growing glycogen chain that has been primed by glycogenin (GYG1/GYG2); the branching
enzyme GBE1 introduces the alpha-1,6 branch points [file:human/GYS2/GYS2-uniprot.txt
"Extends the primer composed of a few glucose units formed by glycogenin by adding new
glucose units to it"; "transfers the glycosyl residue from UDP-Glc to the non-reducing end
of alpha-1,4-glucan"].

Catalytic reaction (Rhea:RHEA:18549):
[(1->4)-alpha-D-glucosyl](n) + UDP-alpha-D-glucose = [(1->4)-alpha-D-glucosyl](n+1) + UDP + H(+)
[file:human/GYS2/GYS2-uniprot.txt].

Regulation

• Allosterically activated by glucose-6-phosphate; phosphorylation is inhibitory
  (reduces activity toward UDP-glucose). When non-phosphorylated the enzyme does not
  require G6P; when phosphorylated it does [file:human/GYS2/GYS2-uniprot.txt "Allosteric
  activation by glucose-6-phosphate"; "Phosphorylation reduces the activity towards
  UDP-glucose"].
• Multisite inhibitory phosphorylation by GSK3A/GSK3B (Ser-641/645/649/653), primed by
  CK2 at Ser-657; dephosphorylation by PP1 activates the enzyme (by similarity to muscle
  GYS1/P13807, P13834).

Localization / complex

• Cytosolic; part of the glycogen synthase (GS)–glycogenin heterooctamer (GS tetramer +
  2 glycogenin dimers); ComplexPortal CPX-26492 (GYG1-GYS2) and CPX-26494 (GYG2-GYS2)
  [file:human/GYS2/GYS2-uniprot.txt].
• Tissue specificity: liver (at protein level) [PMID:1731614; file:human/GYS2/GYS2-uniprot.txt].

Minor phosphoglucose-incorporation activity (GO:0061547)

GYS2 (like muscle GYS1) can occasionally transfer the glucosyl moiety of a
glucose-phosphate, incorporating phosphomonoesters into glycogen at a very low rate (~1 per
10,000 glucose residues); laforin (EPM2A) normally removes these phosphate groups. This is
the basis of the GO:0061547 "glycogen synthase activity, transferring glucose-1-phosphate"
annotation and of Reactome R-HSA-3780994 [reactome/R-HSA-3780994.md]. Real but minor;
kept as non-core would be defensible, but it is a genuine molecular capability so ACCEPT.

Disease

Biallelic loss-of-function mutations in GYS2 cause glycogen storage disease type 0, liver
(GSD 0a; MIM:240600) — fasting ketotic hypoglycemia with low lactate/alanine, and
postprandial hyperglycemia/hyperlactatemia; hepatic glycogen is deficient. Seven missense
variants characterized in COS7 cells showed severely impaired GS activity
PMID:9691087.

Annotation decisions summary

• GO:0004373 alpha-1,4-glucan glucosyltransferase (UDP-glucose donor) activity — ACCEPT
  across all evidence (IBA, IEA, EXP, IMP, ISS, IDA). Core MF.
• GO:0005978 glycogen biosynthetic process — ACCEPT across all evidence. Core BP.
• GO:0061547 glycogen synthase activity, transferring glucose-1-phosphate — ACCEPT
  (IEA/ARBA; TAS/Reactome). Real minor side activity (phosphoglucose incorporation).
• GO:0005829 cytosol — ACCEPT (IEA, TAS×10, ISS). Canonical location.
• GO:0005737 cytoplasm — ACCEPT (IBA, ISS). Broader parent, correct.
• GO:0009749 response to glucose — KEEP_AS_NON_CORE (ISS from yeast Gsy2 P17625; plausible
  but peripheral to the core catalytic role and not experimentally verified in human).
• GO:0005856 cytoskeleton, GO:0005938 cell cortex, GO:0030864 cortical actin
  cytoskeleton — MARK_AS_OVER_ANNOTATED. All three are ISS transfers from S. cerevisiae
  glycogen synthase Gsy2 (P17625), reflecting the yeast enzyme's association with cortical
  actin patches. Human liver GYS2 is a soluble cytosolic enzyme in a GS–glycogenin complex;
  there is no evidence it localizes to the (cortical actin) cytoskeleton or cell cortex.
  Over-propagated from a distant fungal ortholog.