Galactose-1-phosphate uridylyltransferase explained

Symbol:	GalP_UDP_transf
Galactose-1-phosphate uridyl transferase, N-terminal domain
Pfam:	PF01087
Pfam Clan:	CL0265
Prosite:	PDOC00108
Scop:	1hxp

Symbol:	GalP_UDP_tr_C
Galactose-1-phosphate uridyl transferase, C-terminal domain
Pfam:	PF02744
Pfam Clan:	CL0265
Interpro:	IPR005850
Prosite:	PDOC00108
Scop:	1hxp

Galactose-1-phosphate uridyltransferase (or GALT, G1PUT) is an enzyme responsible for converting ingested galactose to glucose.^[1]

Galactose-1-phosphate uridyltransferase (GALT) catalyzes the second step of the Leloir pathway of galactose metabolism, namely:

\rightleftharpoons glucose 1-phosphate + UDP-galactoseThe expression of GALT is controlled by the actions of the FOXO3 gene. The absence of this enzyme results in classic galactosemia in humans and can be fatal in the newborn period if lactose is not removed from the diet. The pathophysiology of galactosemia has not been clearly defined.^[1]

Mechanism

GALT catalyzes the second reaction of the Leloir pathway of galactose metabolism through ping pong bi-bi kinetics with a double displacement mechanism.^[2] This means that the net reaction consists of two reactants and two products (see the reaction above) and it proceeds by the following mechanism: the enzyme reacts with one substrate to generate one product and a modified enzyme, which goes on to react with the second substrate to make the second product while regenerating the original enzyme.^[3] In the case of GALT, the His166 residue acts as a potent nucleophile to facilitate transfer of a nucleotide between UDP-hexoses and hexose-1-phosphates.^[4]

1. UDP-glucose + E-His Glucose-1-phosphate + E-His-UMP
2. Galactose-1-phosphate + E-His-UMP UDP-galactose + E-His

Structural studies

The three-dimensional structure at 180 pm resolution (x-ray crystallography) of GALT was determined by Wedekind, Frey, and Rayment, and their structural analysis found key amino acids essential for GALT function. Among these are Leu4, Phe75, Asn77, Asp78, Phe79, and Val108, which are consistent with residues that have been implicated both in point mutation experiments as well as in clinical screening that play a role in human galactosemia.^[5]

GALT also has minimal (~0.1%) GalNAc transferase activity. X-ray crystallography revealed that the side chain of Tyr289 forms a hydrogen bond with the N-acetyl group of UDP-GalNAc. Point mutation of residue Tyr289 to Leu, Ile, or Asn eliminates this interaction, enhancing GalNAc transferase activity, with the Y289L mutation showing comparable GalNAc transferase activity as the wild-type enzyme's Gal transferase activity.^[6]

Clinical significance

Deficiency of GALT causes classic galactosemia. Galactosemia is an autosomal recessive inherited disorder detectable in newborns and childhood.^[7] It occurs at approximately 1 in every 40,000-60,000 live-born infants. Classical galactosemia (G/G) is caused by a deficiency in GALT activity, whereas the more common clinical manifestations, Duarte (D/D) and the Duarte/Classical variant (D/G) are caused by the attenuation of GALT activity.^[8] Symptoms include ovarian failure, developmental coordination disorder (difficulty speaking correctly and consistently),^[9] and neurologic deficits. A single mutation in any of several base pairs can lead to deficiency in GALT activity.^[10] For example, a single mutation from A to G in exon 6 of the GALT gene changes Glu188 to an arginine and a mutation from A to G in exon 10 converts Asn314 to an aspartic acid. These two mutations also add new restriction enzyme cut sites, which enable detection by and large-scale population screening with PCR (polymerase chain reaction). Screening has mostly eliminated neonatal death by G/G galactosemia, but the disease, due to GALT’s role in the biochemical metabolism of ingested galactose (which is toxic when accumulated) to the energetically useful glucose, can certainly be fatal.^[11] However, those afflicted with galactosemia can live relatively normal lives by avoiding milk products and anything else containing galactose (because it cannot be metabolized), but there is still the potential for problems in neurological development or other complications, even in those who avoid galactose.^[12]

Disease database

Galactosemia (GALT) Mutation Database

Further reading

• Reichardt JK . Genetic basis of galactosemia . Human Mutation . 1 . 3 . 190–6 . 1993 . 1301925 . 10.1002/humu.1380010303 . 504197 . free .
• Tyfield L, Reichardt J, Fridovich-Keil J, Croke DT, Elsas LJ, Strobl W, Kozak L, Coskun T, Novelli G, Okano Y, Zekanowski C, Shin Y, Boleda MD . Classical galactosemia and mutations at the galactose-1-phosphate uridyl transferase (GALT) gene . Human Mutation . 13 . 6 . 417–30 . 1999 . 10408771 . 10.1002/(SICI)1098-1004(1999)13:6<417::AID-HUMU1>3.0.CO;2-0 . 34860932 .
• Reichardt JK, Belmont JW, Levy HL, Woo SL . Characterization of two missense mutations in human galactose-1-phosphate uridyltransferase: different molecular mechanisms for galactosemia . Genomics . 12 . 3 . 596–600 . March 1992 . 1373122 . 10.1016/0888-7543(92)90453-Y .
• Leslie ND, Immerman EB, Flach JE, Florez M, Fridovich-Keil JL, Elsas LJ . The human galactose-1-phosphate uridyltransferase gene . Genomics . 14 . 2 . 474–80 . October 1992 . 1427861 . 10.1016/S0888-7543(05)80244-7 .
• Reichardt JK, Levy HL, Woo SL . Molecular characterization of two galactosemia mutations and one polymorphism: implications for structure-function analysis of human galactose-1-phosphate uridyltransferase . Biochemistry . 31 . 24 . 5430–3 . June 1992 . 1610789 . 10.1021/bi00139a002 .
• Reichardt JK, Packman S, Woo SL . Molecular characterization of two galactosemia mutations: correlation of mutations with highly conserved domains in galactose-1-phosphate uridyl transferase . American Journal of Human Genetics . 49 . 4 . 860–7 . October 1991 . 1897530 . 1683190 .
• Reichardt JK, Woo SL . Molecular basis of galactosemia: mutations and polymorphisms in the gene encoding human galactose-1-phosphate uridylyltransferase . Proceedings of the National Academy of Sciences of the United States of America . 88 . 7 . 2633–7 . April 1991 . 2011574 . 51292 . 10.1073/pnas.88.7.2633 . 1991PNAS...88.2633R . free .
• Flach JE, Reichardt JK, Elsas LJ . Sequence of a cDNA encoding human galactose-1-phosphate uridyl transferase . Molecular Biology & Medicine . 7 . 4 . 365–9 . August 1990 . 2233247 .
• Reichardt JK, Berg P . Cloning and characterization of a cDNA encoding human galactose-1-phosphate uridyl transferase . Molecular Biology & Medicine . 5 . 2 . 107–22 . April 1988 . 2840550 .
• Bergren WG, Donnell GN . A new variant of galactose-1-phosphate uridyltransferase in man: the Los Angeles variant . Annals of Human Genetics . 37 . 1 . 1–8 . July 1973 . 4759900 . 10.1111/j.1469-1809.1973.tb01808.x . 22699183 .
• Shih LY, Suslak L, Rosin I, Searle BM, Desposito F . Gene dosage studies supporting localization of the structural gene for galactose-1-phosphate uridyl transferase (GALT) to band p13 of chromosome 9 . American Journal of Medical Genetics . 19 . 3 . 539–43 . November 1984 . 6095663 . 10.1002/ajmg.1320190316 .
• Ashino J, Okano Y, Suyama I, Yamazaki T, Yoshino M, Furuyama J, Lin HC, Reichardt JK, Isshiki G . Molecular characterization of galactosemia (type 1) mutations in Japanese . Human Mutation . 6 . 1 . 36–43 . 1995 . 7550229 . 10.1002/humu.1380060108 . 23500152 . free .
• Elsas LJ, Langley S, Paulk EM, Hjelm LN, Dembure PP . A molecular approach to galactosemia . European Journal of Pediatrics . 154 . 7 Suppl 2 . S21-7 . 1995 . 7671959 . 10.1007/BF02143798 . 11937698 .
• Elsas LJ, Langley S, Steele E, Evinger J, Fridovich-Keil JL, Brown A, Singh R, Fernhoff P, Hjelm LN, Dembure PP . Galactosemia: a strategy to identify new biochemical phenotypes and molecular genotypes . American Journal of Human Genetics . 56 . 3 . 630–9 . March 1995 . 7887416 . 1801164 .
• Fridovich-Keil JL, Langley SD, Mazur LA, Lennon JC, Dembure PP, Elsas JL . Identification and functional analysis of three distinct mutations in the human galactose-1-phosphate uridyltransferase gene associated with galactosemia in a single family . American Journal of Human Genetics . 56 . 3 . 640–6 . March 1995 . 7887417 . 1801186 .
• Davit-Spraul A, Pourci ML, Ng KH, Soni T, Lemonnier A . Regulatory effects of galactose on galactose-1-phosphate uridyltransferase activity on human hepatoblastoma HepG2 cells . FEBS Letters . 354 . 2 . 232–6 . November 1994 . 7957929 . 10.1016/0014-5793(94)01133-8 . 45242645 . free .
• Lin HC, Kirby LT, Ng WG, Reichardt JK . On the molecular nature of the Duarte variant of galactose-1-phosphate uridyl transferase (GALT) . Human Genetics . 93 . 2 . 167–9 . February 1994 . 8112740 . 10.1007/BF00210604 . 42558872 .
• Elsas LJ, Dembure PP, Langley S, Paulk EM, Hjelm LN, Fridovich-Keil J . A common mutation associated with the Duarte galactosemia allele . American Journal of Human Genetics . 54 . 6 . 1030–6 . June 1994 . 8198125 . 1918187 .
• Reichardt JK, Novelli G, Dallapiccola B . Molecular characterization of the H319Q galactosemia mutation . Human Molecular Genetics . 2 . 3 . 325–6 . March 1993 . 8499924 . 10.1093/hmg/2.3.325 .

External links

• • GeneReviews/NIH/NCBI/UW entry on Galactosemia
• Galactosemia (GALT) Mutation Database
• GALT Protein Database
• PDBe-KB provides an overview of all the structure information available in the PDB for Human Galactose-1-phosphate uridylyltransferase

Notes and References

1. Web site: Entrez Gene: GALT galactose-1-phosphate uridylyltransferase.
2. Wong LJ, Frey PA . Galactose-1-phosphate uridylyltransferase: rate studies confirming a uridylyl-enzyme intermediate on the catalytic pathway . Biochemistry . 13 . 19 . 3889–94 . September 1974 . 4606575 . 10.1021/bi00716a011 .
3. Web site: Double displacement mechanism - Definition . 2010-05-19 . https://web.archive.org/web/20160303204044/http://www.mondofacto.com/facts/dictionary?double+displacement+mechanism . 2016-03-03 . dead .
4. Wedekind JE, Frey PA, Rayment I . Three-dimensional structure of galactose-1-phosphate uridylyltransferase from Escherichia coli at 1.8 A resolution . Biochemistry . 34 . 35 . 11049–61 . September 1995 . 7669762 . 10.1021/bi00035a010 .
5. Seyrantepe V, Ozguc M, Coskun T, Ozalp I, Reichardt JK . Identification of mutations in the galactose-1-phosphate uridyltransferase (GALT) gene in 16 Turkish patients with galactosemia, including a novel mutation of F294Y. Mutation in brief no. 235. Online . Human Mutation . 13 . 4 . 339 . 1999 . 10220154 . 10.1002/(SICI)1098-1004(1999)13:4<339::AID-HUMU18>3.0.CO;2-S . free .
6. Ramakrishnan . Boopathy . Qasba . Pradman K. . 2002-06-07 . Structure-based Design of β1,4-Galactosyltransferase I (β4Gal-T1) with Equally Efficient N-Acetylgalactosaminyltransferase Activity: POINT MUTATION BROADENS β4Gal-T1 DONOR SPECIFICITY* . Journal of Biological Chemistry . en . 277 . 23 . 20833–20839 . 10.1074/jbc.M111183200 . 11916963 . 0021-9258. free .
7. Fridovich-Keil JL . Galactosemia: the good, the bad, and the unknown . Journal of Cellular Physiology . 209 . 3 . 701–5 . December 2006 . 17001680 . 10.1002/jcp.20820 . 32233614 .
8. Elsas LJ, Langley S, Paulk EM, Hjelm LN, Dembure PP . A molecular approach to galactosemia . European Journal of Pediatrics . 154 . 7 Suppl 2 . S21-7 . 1995 . 7671959 . 10.1007/BF02143798 . 11937698 .
9. Web site: Apraxia of Speech . 2010-05-19 . dead . https://web.archive.org/web/20060228083020/http://www.nidcd.nih.gov/health/voice/apraxia.htm . 2006-02-28 .
10. Dobrowolski SF, Banas RA, Suzow JG, Berkley M, Naylor EW . Analysis of common mutations in the galactose-1-phosphate uridyl transferase gene: new assays to increase the sensitivity and specificity of newborn screening for galactosemia . The Journal of Molecular Diagnostics . 5 . 1 . 42–7 . February 2003 . 12552079 . 1907369 . 10.1016/S1525-1578(10)60450-3 .
11. Lai K, Elsas LJ, Wierenga KJ . Galactose toxicity in animals . IUBMB Life . 61 . 11 . 1063–74 . November 2009 . 19859980 . 2788023 . 10.1002/iub.262 .
12. Web site: Galactosemia - Treatment. dead . https://web.archive.org/web/20020828095734/http://www.umm.edu/ency/article/000366trt.htm . 2002-08-28 .