Curation Notes: `ureC` (`C5CY00` / `Vapar_4243`) in Variovorax paradoxus S110

1. Gene & Protein Identity

UniProt Accession: C5CY00 (URE1_VARPS, reviewed Swiss-Prot, 572 aa, EC 3.5.1.5)
Gene Symbol / Locus Tag: ureC / Vapar_4243
Organism: Variovorax paradoxus strain S110 (NCBITaxon:543728) [PMID:21183664, "Complete genome sequence of the metabolically versatile plant growth-promoting endophyte, Variovorax paradoxus S110."].
Sequence & Active-Site Architecture:
Matches HAMAP family rule MF_01953 (Urease_alpha), Pfam:PF00449 (Urease_alpha), Pfam:PF01979 (Amidohydro_1), TIGR01792 (urease_alph).
Dinuclear $\text{Ni}^{2+}$ metallocenter ligands (ChEBI:CHEBI:49786):
- $\text{Ni}_1$: His139, His141, and bridging N6-carboxylysine Lys222 (MOD_RES 222) + His365 (BINDING 365, Asp365)
- $\text{Ni}_2$: bridging N6-carboxylysine Lys222, His251, His277
- Substrate binding: His224; General acid proton donor: His325 (ACT_SITE 325) [UniProtKB:C5CY00, "Note=Binds 2 nickel ions per subunit. {ECO:0000255|HAMAP-Rule:MF_01953};"].

Existing Reusable Module: modules/urease_biogenesis_urea_hydrolysis.yaml (MODULE:urease_biogenesis_urea_hydrolysis).
Mechanism: UreC (alpha) combines with UreA (gamma) and UreB (beta) to form the $(\text{UreABC})_3$ apoenzyme (GO:0035550 urease complex), which undergoes $\text{CO}_2$-dependent Lys222 carbamylation and GTP-dependent $\text{Ni}^{2+}$ insertion catalyzed by the UreDFG–UreE metallochaperone machinery [PMID:20046957, "The active site of urease contains two nickel atoms bridged by a carboxylated lysine and a solvent molecule, as shown for the K. aerogenes enzyme (Fig. 2).19 In addition, Ni1 is coordinated by two His residues plus a terminal water molecule while Ni2 is coordinated by two His, an Asp, and a terminal solvent."].
Rhizosphere & Biomineralization Significance: In Variovorax paradoxus S110, urease-catalyzed urea hydrolysis ($\text{urea} + 2\text{H}_2\text{O} + \text{H}^+ \rightarrow \text{HCO}_3^- + 2\text{NH}_4^+$) both supplies bioavailable ammonium nitrogen to host plants and generates carbonate/hydroxide alkalinity that drives microbially induced carbonate precipitation (MICP) of toxic divalent heavy metals ($\text{Cd}^{2+}, \text{Ni}^{2+}, \text{Pb}^{2+}$) and rare earth elements, requiring intracellular $\text{Ni}^{2+}$ homeostasis coordinated with CzcA/CnrA RND efflux systems (czcA3, cnrA).

GO:0009039 (urease activity, IEA) -> ACCEPT: Core catalytic activity (EC 3.5.1.5, RHEA:20557).
GO:0016151 (nickel cation binding, IEA) -> ACCEPT: Core cofactor binding (2 $\text{Ni}^{2+}$ ions per subunit).
GO:0043419 (urea catabolic process, IEA) -> ACCEPT: Core biological process.
GO:0016787 (hydrolase activity, IEA) -> TOO_GENERAL: Subsumed by GO:0009039.
GO:0016810 (hydrolase activity, acting on carbon-nitrogen (but not peptide) bonds, IEA) -> TOO_GENERAL: Subsumed by GO:0009039.
GO:0005737 (cytoplasm, IEA) -> MODIFY to GO:0005829 (cytosol) + NEW (IC): Soluble cytosolic enzyme complex.
Added NEW (IC) GO:0035550 (urease complex, part_of): Captures assembly into the $(\text{UreABC})_3$ holoenzyme complex.