Galactokinase

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Galactokinase 1
Galactokinase 1 1WUU.png
Cartoon structure of a human galactokinase 1 monomer in complex with galactose (red) and an ATP analogue (orange). A magnesium ion is visible as a green sphere. (From PDB 1WUU)
Identifiers
Symbol GALK1
Alt. symbols GALK
Entrez 2584
HUGO 4118
OMIM 604313
RefSeq NM_000154
UniProt P51570
Other data
Locus Chr. 17 q23-q25
Galactokinase 2
Identifiers
Symbol GALK2
Entrez 2585
HUGO 4119
OMIM 137028
RefSeq NM_002044
UniProt Q01415
Other data
Locus Chr. 15 [1]

Galactokinase is an enzyme (phosphotransferase) that facilitates the phosphorylation of α-D-galactose to galactose 1-phosphate at the expense of one molecule of ATP.[1] Galactokinase catalyzes the second step of the Leloir pathway, a metabolic pathway found in most organisms for the catabolism of β-D-galactose to glucose 1-phosphate.[2] First isolated from mammalian liver, galactokinase has been studied extensively in yeast,[3][4] archaea,[5] plants,[6][7] and humans.[8][9]

Structure[edit]

Galactokinase is composed of two domains separated by a large cleft. 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. The N-terminal domain is marked by five strands of mixed beta-sheet and five alpha-helices, and the C-terminal domain is characterized by two layers of anti-parallel beta-sheets and six alpha-helices.[8] Galactokinase does not belong to the sugar kinase family, but rather to a class of ATP-dependent enzymes known as the GHMP superfamily.[10] GHMP is an abbreviation referring to its original members: galactokinase, homoserine kinase, mevalonate kinase, and phosphomevalonate kinase. Members of the GHMP superfamily have great three-dimensional similarity despite only ten to 20% sequence identity. 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]

Sugar Specificity[edit]

Interestingly, galactokinases across different species display a great diversity of substrate specificities. E. coli galactokinase can also phosphorylate 2-deoxy-D-galactose, 2-amino-deoxy-D-galactose, 3-deoxy-D-galactose and D-fucose. The enzyme cannot tolerate any C-4 modifications, but changes at the C-2 position of D-galactose do not interfere with enzyme function.[12] Both human and rat galactokinases are also able to successfully phosphorylate 2-deoxy-D-galactose.[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.[3][4] Moreover, the kinetic properties of galactokinase also differ across species.[8]

Mechanism[edit]

Recently, the roles of active site residues in human galactokinase have become understood. Asp-186 abstracts a proton from C1-OH of α-D-galactose, and the resulting alkoxide nucleophile attacks the γ-phosphorus of ATP. A phosphate group is transferred to the sugar, and Asp-186 may be deprotonated by water. 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.[9] Both the aspartic acid and arginine active site residues are highly conserved among galactokinases.[8]

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)

Biological Function[edit]

The Leloir pathway catalyzes the conversion of galactose to glucose. 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. Three enzymes are required in the Leloir pathway: galactokinase, galactose-1-phosphate uridylyltransferase, and UDP-galactose 4-epimerase. Galactokinase catalyzes the first committed step of galactose catabolism, forming galactose 1-phosphate.[2][15]

Disease Relevance[edit]

Galactosemia, a rare metabolic disorder characterized by decreased ability to metabolize galactose, can be caused by a mutation in any of the three enzymes in the Leloir pathway.[2] Galactokinase deficiency, also known as galactosemia type II, is a recessive metabolic disorder caused by a mutation in human galactokinase. About 20 mutations have been identified that cause galactosemia type II, the main symptom of which is early onset cataracts. In lens cells of the human eye, aldose reductase converts galactose to galactitol. As galactose is not being catabolized to glucose due to a galactokinase mutation, galactitol accumulates. This galactitol gradient across the lens cell membrane triggers the osmotic uptake of water, and the swelling and eventual apoptosis of lens cells ensues.[16]

References[edit]

  1. ^ "galactokinase". Medical Dictionary. Retrieved 2013-01-26. 
  2. ^ a b c Frey PA, PA (Mar 1996). "The Leloir pathway: a mechanistic imperative for three enzymes to change the stereochemical configuration of a single carbon in galactose". FASEB J 10 (4): 461–70. PMID 8647345. 
  3. ^ a b Schell MA, Wilson DB, MA; Wilson, DB (May 1979). "Purification of galactokinase mRNA from Saccharomyces cerevisiae by indirect immunoprecipitation". J Biol Chem 254 (9): 3531–6. PMID 107173. 
  4. ^ a b Sellick CA, Reece RJ, C. A.; Reece, RJ (Jun 2006). "Contribution of Amino Acid Side Chains to Sugar Binding Specificity in a Galactokinase, Gal1p, and a TranscriptionalInducer, Gal3p". J Biol Chem 281 (25): 17150–5. doi:10.1074/jbc.M602086200. PMID 16603548. 
  5. ^ Hartley A, Glynn SE, Barynin V, Baker PJ, et al, Andrew; Glynn, Steven E.; Barynin, Vladimir; Baker, Patrick J.; Sedelnikova, Svetlana E.; Verhees, Corné; De Geus, Daniel; Van Der Oost, John et al. (Mar 2004). "Substrate specificity and mechanism from the structure of Pyrococcus furiosus galactokinase". J Mol Biol 337 (2): 387–98. doi:10.1016/j.jmb.2004.01.043. PMID 15003454. 
  6. ^ Foglietti MJ, Percheron F, MJ; Percheron, F (1976). "Purification and mechanism of action of a plant galactokinase". Biochimie 58 (5): 499–504. doi:10.1016/s0300-9084(76)80218-0. PMID 182286. 
  7. ^ Dey PM, PM (Oct 1983). "Galactokinase of Vicia faba seeds". Eur J Biochem 136 (1): 155–9. doi:10.1111/j.1432-1033.1983.tb07720.x. PMID 6617655. 
  8. ^ a b c d Holden HM, Thoden JB, Timson DJ, Reece RJ, H. M.; Thoden, J. B.; Timson, D. J.; Reece, R. J. (Oct 2004). "Galactokinase: structure, function and role in type II galactosemia". Cell Mol Life Sci 61 (19–20): 2471–84. doi:10.1007/s00018-004-4160-6. PMID 15526155. 
  9. ^ a b c Megarity CF, Huang M, Warnock C, Timson DJ, Clare F.; Huang, Meilan; Warnock, Claire; Timson, David J. (Mar 2011). "The role of the active site residues in human galactokinase: Implications for the mechanisms of GHMP kinases". Cell Mol Life Sci 39 (3): 120–126. doi:10.1016/j.bioorg.2011.03.001. 
  10. ^ Tang M, Wierenga K, Elsas LJ, Lai K, M.; Wierenga, K.; Elsas, L.J.; Lai, K. (Dec 2010). "Molecular and biochemical characterization of human galactokinase and its small molecule inhibitors". Chem Biol Interact 188 (3): 376–85. doi:10.1016/j.cbi.2010.07.025. PMC 2980576. PMID 20696150. 
  11. ^ a b Thoden JB, Holden HM, J. B.; Holden, HM (Aug 2003). "Molecular structure of galactokinase". J Biol Chem 278 (35): 33305–11. doi:10.1074/jbc.M304789200. PMID 12796487. 
  12. ^ Yang J, Fu X, Jia Q, Shen J, et al, Jie; Fu, Xun; Jia, Qiang; Shen, Jie; Biggins, John B.; Jiang, Jiqing; Zhao, Jingjing; Schmidt, Joshua J. et al. (Jun 2003). "Studies on the substrate specificity of Escherichia coli galactokinase". Org Lett 5 (13): 2223–6. doi:10.1021/ol034642d. PMID 12816414. 
  13. ^ Timson DJ, Reece RJ, David J; Reece, Richard J (Nov 2003). "Sugar recognition by human galactokinase". BMC Biochem 4: 16. doi:10.1186/1471-2091-4-16. PMC 280648. PMID 14596685. 
  14. ^ Walker DG, Khan HH, DG; Khan, HH (Jun 1968). "Some properties of galactokinase in developing rat liver". Biochem J 108 (2): 169–75. PMC 1198790. PMID 5665881. 
  15. ^ Holden HM, Rayment I, Thoden JB, H. M.; Rayment, I; Thoden, JB (Nov 2003). "Structure and function of enzymes of the Leloir pathway for galactose metabolism". J Biol Chem 278 (45): 43885–8. doi:10.1074/jbc.R300025200. PMID 12923184. 
  16. ^ Timson DJ, Reece RJ, David J.; Reece, Richard J. (Apr 2003). "Functional analysis of disease-causing mutations in human galactokinase". Eur J Biochem 270 (8): 1767–74. doi:10.1046/j.1432-1033.2003.03538.x. PMID 12694189. 

External links[edit]