GAL1 (YBR020W, P04385) notes

Activity

• Galactokinase EC 2.7.1.6 [UniProt:P04385 "Reaction=alpha-D-galactose + ATP = alpha-D-galactose 1-phosphate + ADP"]; GHMP kinase family, GalK subfamily [UniProt:P04385 "Belongs to the GHMP kinase family. GalK subfamily."].
• Purified to homogeneity from induced cells; monomer of 58 kDa; highly galactose-specific PMID:14144.
• Kinetics: compulsory ordered ternary-complex mechanism PMID:12106903.
• First step of galactose catabolism PMID:12106903.

Second (regulatory) function: GAL3-like inducer

• Gal1 and its catalytically inactive paralog Gal3 act as ligand sensors in the GAL switch PMID:12106903.
• In the presence of galactose + ATP, Gal1 binds the repressor Gal80 (1:1) PMID:11964151.
• Multicopy GAL1 complements gal3 long-term-adaptation, independent of kinase activity [PMID:2199310 "Thus, the complementation activity provided by GAL1 is not likely due to galactokinase activity, but rather due to a distinct GAL3-like activity."; "the results indicate that GAL1 encodes a bifunctional protein"].
• Overproduction of Gal1 gives galactose-independent constitutive GAL expression; galactokinase-less alleles retain inducing ability PMID:1317007.

Assessment

• Core: galactokinase in the cytosolic Leloir pathway. Regulatory Gal80-binding inducer function is real but secondary to Gal3 in wild-type cells (gal3 cells show a 3-5 day lag, PMID:2199310); kept as non-core.
• IPI "protein binding" with Gal80 -> transcription corepressor binding (GO:0001222).
• YeastPathways reaction (EC 2.7.1.6, alpha-D-galactose + ATP) is correct.