The molecular architecture of human N-acetylgalactosamine kinase.
Galactokinase plays a key role in normal galactose metabolism by catalyzing the conversion of alpha-d-galactose to galactose 1-phosphate. Within recent years, the three-dimensional structures of human galactokinase and two bacterial forms of the enzyme have been determined. Originally, the gene encoding galactokinase in humans was mapped to chromosome 17. An additional gene, encoding a protein with sequence similarity to galactokinase, was subsequently mapped to chromosome 15. Recent reports have shown that this second gene (GALK2) encodes an enzyme with greater activity against GalNAc than galactose. This enzyme, GalNAc kinase, has been implicated in a salvage pathway for the reutilization of free GalNAc derived from the degradation of complex carbohydrates. Here we report the first structural analysis of a GalNAc kinase. The structure of the human enzyme was solved in the presence of MnAMPPNP and GalNAc or MgATP and GalNAc (which resulted in bound products in the active site). The enzyme displays a distinctly bilobal appearance with its active site wedged between the two domains. The N-terminal region is dominated by a seven-stranded mixed beta-sheet, whereas the C-terminal motif contains two layers of anti-parallel beta-sheet. The overall topology displayed by GalNAc kinase places it into the GHMP superfamily of enzymes, which generally function as small molecule kinases. From this investigation, the geometry of the GalNAc kinase active site before and after catalysis has been revealed, and the determinants of substrate specificity have been defined on a molecular level.
Thoden
James B
JB
Department of Biochemistry, University of Wisconsin, Madison, Wisconsin 53706, USA.
Holden
Hazel M
HM
D020816:Amino Acid Motifs; D001665:Binding Sites; D002384:Catalysis; D002884:Chromosomes, Human, Pair 15; D003001:Cloning, Molecular; D018360:Crystallography, X-Ray; D004926:Escherichia coli / Q000378:metabolism; D005686:Galactokinase / Q000737:chemistry* / Q000378:metabolism; D006801:Humans; D008956:Models, Chemical; D008958:Models, Molecular; D008968:Molecular Conformation; D017853:Phosphotransferases (Alcohol Group Acceptor) / Q000737:chemistry*; D011485:Protein Binding; D011487:Protein Conformation; D017433:Protein Structure, Secondary; D017434:Protein Structure, Tertiary; D012645:Selenomethionine / Q000737:chemistry; D013379:Substrate Specificity; D014965:X-Rays
D012645:Selenomethionine; D017853:Phosphotransferases (Alcohol Group Acceptor); C484715:GALK1 protein, human; C505446:GALK2 protein, human; D005686:Galactokinase