Galactokinase

The two regions are known as the N- and C-terminal domains, and the adenine ring of ATP binds in a hydrophobic pocket located at their interface.

These enzymes contain three well-conserved motifs (I, II, and III), the second of which is involved in nucleotide binding and has the sequence Pro-X-X-X-Gly-Leu-X-Ser-Ser-Ala.[11] Galactokinases across different species display a great diversity of substrate specificities.

[13][14] Galactokinase from S. cerevisiae, on the other hand, is highly specific for D-galactose and cannot phosphorylate glucose, mannose, arabinose, fucose, lactose, galactitol, or 2-deoxy-D-galactose.

[8] The sugar specificity of galactokinases from different sources has been dramatically expanded through directed evolution[15] and structure-based protein engineering.

Nearby Arg-37 stabilizes Asp-186 in its anionic form and has also been proven to be essential to galactokinase function in point mutation experiments.

Galactose is found in dairy products, as well as in fruits and vegetables, and can be produced endogenously in the breakdown of glycoproteins and glycolipids.

The likely galactokinase mechanism. [ 9 ] The aspartate residue is stabilized in its anionic form by a nearby arginine residue.
Crystal structure of galactokinase active site from Lactococcus lactis. [ 11 ] Galactokinase is shown in green, phosphate in orange, and the residues responsible for binding the sugar ligand are shown in magenta: Arg-36, Glu-42, Asp-45, Asp-183, and Tyr-233. Arg-36 and Asp-183 of Lactococcus lactis galactokinase are analogous to Arg-37 and Asp-186 in human galactokinase. (From PDB : 1PIE ​)