GK2

GK2 encodes a 458-amino-acid protein with sequence homology to galactokinases (EC 2.7.1.6) that can complement galactokinase-deficient yeast, demonstrating intrinsic galactokinase catalytic capacity (PMID:1438294). However, when expressed in mammalian COS cells, GK2 exhibits negligible galactokinase activity compared to GALK1, indicating it is not the primary galactokinase for galactose metabolism in mammalian cells, and its physiological function remains uncharacterized (PMID:7542884).

1. 1992 High

   Identification of GK2 as a human galactokinase-encoding gene resolved whether a second galactokinase locus existed in the human genome, establishing that GK2 possesses intrinsic galactokinase activity.

   Evidence Functional complementation of gal1- S. cerevisiae by human GK2 cDNA expression

   PMID:1438294

   Open questions at the time

   • Activity was demonstrated only in yeast; mammalian cell activity was untested
   • Endogenous tissue expression pattern of GK2 was not determined
   • No comparison with GALK1 activity level was performed
2. 1995 Medium

   Demonstrating that GK2 has negligible galactokinase activity in mammalian cells established that it is not the primary galactokinase and raised the question of whether it has an alternative physiological substrate or function.

   Evidence Heterologous expression in COS cells with galactokinase activity assay and Northern blot comparison to GALK1

   PMID:7542884

   Open questions at the time

   • Single-lab observation not independently replicated in additional mammalian systems
   • No alternative substrates or binding partners were tested
   • Endogenous protein expression and tissue distribution remain uncharacterized

• The physiological role of GK2 in mammalian cells remains unknown — whether it acts on an alternative sugar substrate, functions in a specific tissue context, or has a non-catalytic role has not been addressed experimentally.
• No substrate specificity profiling beyond galactose has been performed
• No loss-of-function studies in mammalian models exist
• No structural or interactome data are available