Molecular structure of human galactokinase: Implications for type II galactosemia

Galactokinase functions in the Leloir pathway for galactose metabolism by catalyzing the MgATP-dependent phosphorylation of the C-1 hydroxyl group of α-D-galactose. The enzyme is known to belong to the GHMP superfamily of small molecule kinases and has attracted significant research attention for well over 40 years. Approximately 20 mutations have now been identified in human galactokinase, which result in the diseased state referred to as Type II galactosemia. Here we report the three-dimensional architecture of human galactokinase with bound α-D-galactose and Mg-AMPPNP. The overall fold of the molecule can be described in terms of two domains with the active site wedged between them. The N-terminal domain is dominated by a sis-stranded mixed β-sheet whereas the C-terminal motif contains six α-helices and two layers of anti-parallel β-sheet. Those residues specifically involved in sugar binding include Arg³⁷, Gl⁴³, His⁴⁴, Asp⁴⁶, Gly¹⁸³, Asp¹⁸⁶, and Tyr²³⁶. The C-1 hydroxyl group of α-D-galactose sits within 3.3 Å of the γ-phosphorus of the nucleotide and 3.4 Å of the guanidinium group of Arg³⁷. The carboxyiate side chain of Asp¹⁸⁶ lies within ∼3.2 Å of the C-2 hydroxyl group of α-D-galactose and the guanidinium group of Arg³⁷. Both Arg³⁷ and Asp ¹⁸⁶ are strictly conserved among both prokaryotic and eukaryotic galactokinases. In addition to providing molecular insight into the active site geometry of the enzyme, the model also provides a structural framework upon which to more fully understand the consequences of the those mutations known to give rise to Type II galactosemia.