Lactase: Difference between revisions

Jump to navigation Jump to search
VisualWikitext
m (Protected "Lactase": Protecting pages from unwanted edits ([edit=sysop] (indefinite) [move=sysop] (indefinite)))
 
Line 1: Line 1:
{{Protein
{{enzyme
| Name = Lactase
| EC_number =3.2.1.108
| CAS_number=9031-11-2
| IUBMB_EC_number = 3/2/1/108
| GO_code = 0000016
| image = 1jyn.jpg
| width = 270
| caption = Lactase tetramer, E.Coli
}}
{{enzyme
| Name = Glycosylceramidase<br />(Phlorizin hydrolase)
| EC_number = 3.2.1.62
| CAS_number = 9033-10-7
| IUBMB_EC_number = 3/2/1/62
| GO_code = 0017042
| image =
| width =
| caption =
}}
{{Infobox protein
   |Name=Lactase
   |Name=Lactase
   |Symbol=LCT
   |Symbol=LCT
Line 14: Line 34:
   |UniProt=P09848
   |UniProt=P09848
}}
}}
'''Lactase''' is an [[enzyme]] produced by many organisms. It is located in the [[brush border]] of the [[small intestine]] of humans and other mammals. Lactase is essential to the complete digestion of whole [[milk]]; it breaks down [[lactose]], a [[sugar]] which gives milk its sweetness.  Lacking lactase, a person consuming dairy products may experience the symptoms of [[lactose intolerance]].<ref name="pmid19639477">{{cite journal | vauthors = Järvelä I, Torniainen S, Kolho KL | title = Molecular genetics of human lactase deficiencies | journal = Annals of Medicine | volume = 41 | issue = 8 | pages = 568–75 | year = 2009 | pmid = 19639477 | doi = 10.1080/07853890903121033 }}</ref> Lactase can be purchased as a food supplement, and is added to milk to produce "lactose-free" milk products.
Lactase (also known as '''lactase-phlorizin hydrolase''', or '''LPH'''), a part of the [[β-galactosidase]] family of [[enzyme]]s, is a [[glycoside hydrolase]] involved in the [[hydrolysis]] of the [[disaccharide]] lactose into constituent [[galactose]] and [[glucose]] [[monomer]]s. Lactase is present predominantly along the [[brush border]] [[Cell membrane|membrane]] of the differentiated [[enterocyte]]s lining the [[Intestinal villus|villi]] of the [[small intestine]].<ref name="pmid6786877"/> In humans, lactase is encoded by the LCT [[gene]].<ref name="pmid2460343">{{cite journal | vauthors = Mantei N, Villa M, Enzler T, Wacker H, Boll W, James P, Hunziker W, Semenza G | title = Complete primary structure of human and rabbit lactase-phlorizin hydrolase: implications for biosynthesis, membrane anchoring and evolution of the enzyme | journal = The EMBO Journal | volume = 7 | issue = 9 | pages = 2705–13 | date = Sep 1988 | pmid = 2460343 | pmc = 457059 | doi =  }}</ref><ref name="pmid8257087">{{cite journal | vauthors = Harvey CB, Fox MF, Jeggo PA, Mantei N, Povey S, Swallow DM | title = Regional localization of the lactase-phlorizin hydrolase gene, LCT, to chromosome 2q21 | journal = Annals of Human Genetics | volume = 57 | issue = Pt 3 | pages = 179–85 | date = Jul 1993 | pmid = 8257087 | doi = 10.1111/j.1469-1809.1993.tb01593.x }}</ref>
==Uses==
===Medical use===
Lactase supplements are sometimes used to treat lactose intolerance.<ref name=NIH2014Dig>{{cite web|title=Lactose Intolerance|url=https://www.niddk.nih.gov/health-information/health-topics/digestive-diseases/lactose-intolerance/Pages/facts.aspx|website=NIDDK|accessdate=25 October 2016|date=June 2014}}</ref>
=== Industrial use ===
Lactase produced commercially can be extracted both from [[yeast]]s such as ''[[Kluyveromyces fragilis]]'' and ''[[Kluyveromyces lactis]]'' and from molds, such as ''[[Aspergillus niger]]'' and ''[[Aspergillus oryzae]]''.<ref>{{cite journal|vauthors=Seyis I, Aksoz N |year=2004|title=Production of lactase by Trichoderma sp|journal=Food Technol Biotechnol|volume=42|pages=121–124|url=http://public.carnet.hr/ftbrfd/42-121.pdf}}</ref> Its primary commercial use, in supplements such as Lacteeze and Lactaid, is to break down lactose in milk to make it suitable for people with lactose intolerance,<ref name="urlwww.fda.gov">{{cite web | url = http://www.fda.gov/downloads/Food/IngredientsPackagingLabeling/GRAS/NoticeInventory/UCM400718 | title = Re: GRAS Notification for Acid Lactase from Aspergillus oryzae Expressed in Aspergillus niger | work = United States Food and Drug Administration }}</ref><ref name="urlnaldc.nal.usda.gov">{{cite book | chapter-url = http://naldc.nal.usda.gov/naldc/download.xhtml?id=IND93048088&content=PDF | title = Innovative Products for Food Industries | chapter = The Lactaid Story | format = | work = | first = Virginia H. | last = Holsinger | name-list-format = vanc | pages = 256–8 }}</ref> However, the [[U.S. Food and Drug Administration]] <!-- (FDA) --> has not formally evaluated the effectiveness of these products.<ref>{{cite web|url=http://www.fda.gov/Food/FoodIngredientsPackaging/GenerallyRecognizedasSafeGRAS/GRASListings/ucm153949.htm|title=Agency Response Letter GRAS Notice No. GRN 000132|last=Tarantino, LM|date=2003-12-03|publisher=U.S. Food and Drug Administration|accessdate=2009-09-21}}</ref>
Lactase is also used to screen for [[blue white screen|blue white]] colonies in the [[multiple cloning site]]s of various [[Plasmid#Vectors|plasmid vectors]] in ''[[Escherichia coli]]'' or other bacteria.{{Citation needed|date=April 2010}}<!--
<ref>{{cite web|url=http://www.fermentas.com/techinfo/nucleicacids/mappbluescriptiiskks.htm|title=pBluescript II KS(+/−), pBluescript II SK(+/−): description & restriction map|publisher=Fermentas|accessdate=2009-09-21}}</ref>
--this page is dead, and the closest live page on fermentas.com (http://www.fermentas.com/en/support/technical-reference/phage-plasmid-dna/pbluescriptII) makes no mention of lactase. If the above live URL is used, please elaborate on lactase is used in the experiments.
-->
==Mechanism==
The optimum [[temperature]] for human lactase is about 37&nbsp;[[°C]] for its activity<ref name="pmid17512743">{{cite journal | vauthors = Hermida C, Corrales G, Cañada FJ, Aragón JJ, Fernández-Mayoralas A | title = Optimizing the enzymatic synthesis of beta-D-galactopyranosyl-D-xyloses for their use in the evaluation of lactase activity in vivo | journal = Bioorganic & Medicinal Chemistry | volume = 15 | issue = 14 | pages = 4836–40 | date = Jul 2007 | pmid = 17512743 | doi = 10.1016/j.bmc.2007.04.067 }}</ref> and has an optimum [[pH]] of 6.<ref name="pmid6786877">{{cite journal | vauthors = Skovbjerg H, Sjöström H, Norén O | title = Purification and characterisation of amphiphilic lactase/phlorizin hydrolase from human small intestine | journal = European Journal of Biochemistry / FEBS | volume = 114 | issue = 3 | pages = 653–61 | date = Mar 1981 | pmid = 6786877 | doi = 10.1111/j.1432-1033.1981.tb05193.x }}</ref>
In [[metabolism]], the β-glycosidic bond in ''D''-lactose is hydrolyzed to form ''D''-galactose and ''D''-glucose, which can be absorbed through the intestinal walls and into the bloodstream.  The overall reaction that lactase catalyzes is C<sub>12</sub>H<sub>22</sub>O<sub>11</sub> + H<sub>2</sub>O → C<sub>6</sub>H<sub>12</sub>O<sub>6</sub> + C<sub>6</sub>H<sub>12</sub>O<sub>6</sub> + heat.
The catalytic mechanism of ''D''-lactose hydrolysis retains the substrate anomeric configuration in the products.<ref name = "doi10.1021/bi011727i">{{cite journal | vauthors = Sinnott M | title = Catalytic mechanisms of enzymic glycosyl transfer | journal = Chem. Rev. | volume = 90 | issue = 7 | pages = 1171–1202 |date=November 1990 | doi = 10.1021/cr00105a006}}</ref>  While the details of the mechanism are uncertain, the stereochemical retention is achieved through a double displacement reaction.  Studies of ''E. coli'' lactase have proposed that hydrolysis is initiated when a glutamate nucleophile on the enzyme attacks from the axial side of the galactosyl carbon in the β-glycosidic bond.<ref name = "pmid11732897">{{cite journal | vauthors = Juers DH, Heightman TD, Vasella A, McCarter JD, Mackenzie L, Withers SG, Matthews BW | title = A structural view of the action of Escherichia coli (lacZ) beta-galactosidase | journal = Biochemistry | volume = 40 | issue = 49 | pages = 14781–94 | date = Dec 2001 | pmid = 11732897 | doi = 10.1021/bi011727i }}</ref>  The removal of the ''D''-glucose leaving group may be facilitated by Mg-dependent acid catalysis.<ref name = "pmid11732897"/>  The enzyme is liberated from the α-galactosyl moiety upon equatorial nucleophilic attack by water, which produces ''D''-galactose.<ref name = "doi10.1021/bi011727i"/>
Substrate modification studies have demonstrated that the 3′-OH and 2′-OH moieties on the galactopyranose ring are essential for enzymatic recognition and hydrolysis.<ref name = "pmid7648581">{{cite journal | vauthors = Fernandez P, Cañada FJ, Jiménez-Barbero J, Martín-Lomas M | title = Substrate specificity of small-intestinal lactase: study of the steric effects and hydrogen bonds involved in enzyme-substrate interaction | journal = Carbohydrate Research | volume = 271 | issue = 1 | pages = 31–42 | date = Jul 1995 | pmid = 7648581 | doi = 10.1016/0008-6215(95)00034-Q }}</ref>  The 3′-hydroxy group is involved in initial binding to the substrate while the 2′- group is not necessary for recognition but needed in subsequent steps.  This is demonstrated by the fact that a 2-deoxy analog is an effective competitive inhibitor (K<sub>i</sub> = 10mM).<ref name = "pmid7648581"/>  Elimination of specific hydroxyl groups on the glucopyranose moiety does not completely eliminate catalysis.<ref name = "pmid7648581"/>


<!-- TEXT STARTS HERE -->
[[File:LactaseMechanism2.png|500px|Proposed mechanism of lactose hydrolysis by Lactase enzyme]]
'''Lactase''' (LCT), a member of the [[β-galactosidase]] family of [[enzyme]]s, is a [[glycoside hydrolase]] involved in the [[hydrolysis]] of the [[disaccharide]] [[lactose]] into constituent [[galactose]] and [[glucose]] [[monomer]]s. In humans, lactase is present predominantly along the [[brush border]] [[Cell membrane|membrane]] of the differentiated [[enterocyte]]s lining the [[villi]] of the [[small intestine]].
{{clear left}}


Lactase is essential for digestive hydrolysis of lactose in milk. Deficiency of the enzyme causes [[lactose intolerance]].
Lactase also catalyzes the conversion of [[phlorizin]] to [[phloretin]] and glucose.


== Structure and biosynthesis ==


The optimum [[temperature]] for lactase is about 48 °C (118.4 °F) for its activity and has an optimum [[pH]] of 6.5.
Preprolactase, the primary translation product, has a single polypeptide primary structure consisting of 1927 amino acids.<ref name="pmid2460343"/>  It can be divided into five domains: (i) a 19-amino-acid cleaved signal sequence; (ii) a large prosequence domain that is not present in mature lactase; (iii) the mature lactase segment; (iv) a membrane-spanning hydrophobic anchor; and (v) a short hydrophilic carboxyl terminus.<ref name = "pmid2460343"/>  The signal sequence is cleaved in the endoplasmic reticulum, and the resulting 215-kDa pro-LPH is sent to the Golgi apparatus, where it is heavily glycosylated and proteolytically processed to its mature form.<ref>{{cite journal | vauthors = Naim HY, Sterchi EE, Lentze MJ | title = Biosynthesis and maturation of lactase-phlorizin hydrolase in the human small intestinal epithelial cells | journal = The Biochemical Journal | volume = 241 | issue = 2 | pages = 427–34 | date = Jan 1987 | pmid = 3109375 | pmc = 1147578 | doi=10.1042/bj2410427}}</ref>  The prodomain has been shown to act as an intramolecular chaperone in the ER, preventing trypsin cleavage and allowing LPH to adopt the necessary 3-D structure to be transported to the Golgi apparatus.<ref>{{cite journal | vauthors = Naim HY, Jacob R, Naim H, Sambrook JF, Gething MJ | title = The pro region of human intestinal lactase-phlorizin hydrolase | journal = The Journal of Biological Chemistry | volume = 269 | issue = 43 | pages = 26933–43 | date = Oct 1994 | pmid = 7523415 }}</ref>


[[Image:Lactase Processing.png|thumb|left|500px|Schematic of processing and localization of human lactase translational product]]
{{clear left}}


Mature human lactase consists of a single 160-kDa polypeptide chain that localizes to the brush border membrane of intestinal epithelial cells.  It is oriented with the N-terminus outside the cell and the C-terminus in the cytosol.<ref name = "pmid2460343"/> LPH contains two catalytic glutamic acid sites.  In the human enzyme, the lactase activity has been connected to Glu-1749, while Glu-1273 is the site of phlorizin hydrolase function.<ref>{{cite journal | vauthors = Zecca L, Mesonero JE, Stutz A, Poirée JC, Giudicelli J, Cursio R, Gloor SM, Semenza G | title = Intestinal lactase-phlorizin hydrolase (LPH): the two catalytic sites; the role of the pancreas in pro-LPH maturation | journal = FEBS Letters | volume = 435 | issue = 2-3 | pages = 225–8 | date = Sep 1998 | pmid = 9762914 | doi = 10.1016/S0014-5793(98)01076-X }}</ref>


Lactase is also used to screen for blue white colonies into the MCS of various plasmid vectors in E.Coli or other bacteria.
== Genetic expression and regulation ==


==External links==
Lactase is encoded by a single genetic locus on chromosome 2.<ref name = "pmid9148757">{{cite journal | vauthors = Troelsen JT, Mitchelmore C, Spodsberg N, Jensen AM, Norén O, Sjöström H | title = Regulation of lactase-phlorizin hydrolase gene expression by the caudal-related homoeodomain protein Cdx-2 | journal = The Biochemical Journal | volume = 322 ( Pt 3) | issue = Pt. 3 | pages = 833–8 | date = Mar 1997 | pmid = 9148757 | pmc = 1218263 }}</ref>  It is expressed exclusively by mammalian small intestine enterocytes and in very low levels in the colon during fetal development.<ref name="pmid9148757"/>  Humans are born with high levels of lactase expression. In most of the world’s population, lactase transcription is down-regulated after weaning, resulting in diminished lactase expression in the small intestine,<ref name = "pmid9148757"/>  which causes the common symptoms of adult-type hypolactasia, or lactose intolerance.<ref>https://ghr.nlm.nih.gov/gene/LCT</ref>
* ''E. coli'' &beta;-galactosidase: {{PDB|1JYY}}
 
*[http://www.lsbu.ac.uk/biology/enztech/lactase.html The use of lactases in the dairy industry]
Some population segments exhibit lactase persistence resulting from a mutation that is postulated to have occurred 5,000–10,000 years ago, coinciding with the rise of cattle domestication.<ref>{{cite journal | vauthors = Bersaglieri T, Sabeti PC, Patterson N, Vanderploeg T, Schaffner SF, Drake JA, Rhodes M, Reich DE, Hirschhorn JN | title = Genetic signatures of strong recent positive selection at the lactase gene | journal = American Journal of Human Genetics | volume = 74 | issue = 6 | pages = 1111–20 | date = Jun 2004 | pmid = 15114531 | pmc = 1182075 | doi = 10.1086/421051 }}</ref>  This mutation has allowed almost half of the world’s population to metabolize lactose without symptoms.  Studies have linked the occurrence of lactase persistence to two different single-nucleotide polymorphisms about 14 and 22 kilobases upstream of the 5’-end of the LPH gene.<ref>{{cite journal | vauthors = Kuokkanen M, Enattah NS, Oksanen A, Savilahti E, Orpana A, Järvelä I | title = Transcriptional regulation of the lactase-phlorizin hydrolase gene by polymorphisms associated with adult-type hypolactasia | journal = Gut | volume = 52 | issue = 5 | pages = 647–52 | date = May 2003 | pmid = 12692047 | pmc = 1773659 | doi = 10.1136/gut.52.5.647 }}</ref>  Both mutations, C→T at position -13910 and G→ A at position -22018, have been independently linked to lactase persistence.<ref name = "pmid15777735">{{cite journal | vauthors = Troelsen JT | title = Adult-type hypolactasia and regulation of lactase expression | journal = Biochimica et Biophysica Acta | volume = 1723 | issue = 1-3 | pages = 19–32 | date = May 2005 | pmid = 15777735 | doi = 10.1016/j.bbagen.2005.02.003 }}</ref>
*[http://www.eurekalert.org/pub_releases/2007-02/ucl-eeu022607.php The Recent Evolution of Lactose Tolerance]
 
The lactase promoter is 150 base pairs long and is located just upstream of the site of transcription initiation.<ref name = "pmid15777735"/> The sequence is highly conserved in mammals, suggesting that critical cis-transcriptional regulators are located nearby.<ref name = "pmid15777735"/>  Cdx-2, HNF-1α, and GATA have been identified as transcription factors.<ref name = "pmid15777735"/> Studies of hypolactasia onset have demonstrated that despite polymorphisms, little difference exists in lactase expression in infants, showing that the mutations become increasingly relevant during development.<ref name = "pmid9609760">{{cite journal | vauthors = Wang Y, Harvey CB, Hollox EJ, Phillips AD, Poulter M, Clay P, Walker-Smith JA, Swallow DM | title = The genetically programmed down-regulation of lactase in children | journal = Gastroenterology | volume = 114 | issue = 6 | pages = 1230–6 | date = Jun 1998 | pmid = 9609760 | doi = 10.1016/S0016-5085(98)70429-9 }}</ref> Developmentally regulated DNA-binding proteins may down-regulate transcription or destabilize mRNA transcripts, causing decreased LPH expression after weaning.<ref name = "pmid9609760"/>
 
== See also ==
{{Portal|Molecular and Cellular Biology}}
* [[MCM6]]
* [[Lactase persistence]]
 
== References ==
{{Reflist|35em}}
 
== External links ==
{{Commons category|Lactase}}
*[http://macromoleculeinsights.com/lactase.php The Lactase Protein]
* ''E. coli'' β-galactosidase: {{PDB|1JYY}}
*[http://www.ebi.ac.uk/ego/DisplayGoTerm?id=GO:0000016 Gene Ontology for Lactase]
*[http://www.ebi.ac.uk/ego/DisplayGoTerm?id=GO:0000016 Gene Ontology for Lactase]
*[http://www.hhmi.org/biointeractive/making-fittest-got-lactase-co-evolution-genes-and-culture Making of the Fittest: Got Lactase? The Co-evolution of Genes and Culture]
* [https://www.ncbi.nlm.nih.gov/pubmed/23252911 Lactase persistence shows indication of association with Obesity]


==References==
{{Fructose and galactose metabolism}}
<references/>
{{Sugar hydrolases}}
{{Enzymes}}


{{Glycoside hydrolases}}
{{Fructose and galactose metabolism}}
[[Category:Food additives]]
[[Category:Food additives]]
[[Category:Antiflatulents]]
[[Category:Antiflatulents]]
[[Category:EC 3.2.1]]
[[Category:EC 3.2.1]]
 
[[Category:Genes on human chromosome 2]]
[[da:Laktase]]
[[de:Lactase]]
[[es:Lactasa]]
[[eo:Laktazo]]
[[fr:Lactase]]
[[is:Laktasi]]
[[it:Lattasi]]
[[he:לקטאז]]
[[nl:Lactase]]
[[ja:ラクターゼ]]
[[oc:Lactasa]]
[[pl:Laktaza]]
[[pt:Lactase]]
[[fi:Laktaasi]]
[[sv:Laktas]]
[[uk:Лактаза]]
[[zh:乳糖酶]]
{{WikiDoc Sources}}

Revision as of 17:10, 23 October 2017

Lactase
File:1jyn.jpg
Lactase tetramer, E.Coli
Identifiers
EC number3.2.1.108
CAS number9031-11-2
Databases
IntEnzIntEnz view
BRENDABRENDA entry
ExPASyNiceZyme view
KEGGKEGG entry
MetaCycmetabolic pathway
PRIAMprofile
PDB structuresRCSB PDB PDBe PDBsum
Gene OntologyAmiGO / QuickGO
Glycosylceramidase
(Phlorizin hydrolase)
Identifiers
EC number3.2.1.62
CAS number9033-10-7
Databases
IntEnzIntEnz view
BRENDABRENDA entry
ExPASyNiceZyme view
KEGGKEGG entry
MetaCycmetabolic pathway
PRIAMprofile
PDB structuresRCSB PDB PDBe PDBsum
Gene OntologyAmiGO / QuickGO
Lactase
Identifiers
SymbolLCT
Alt. symbolsLAC; LPH; LPH1
Entrez3938
HUGO6530
OMIM603202
RefSeqNM_002299
UniProtP09848
Other data
EC number3.2.1.108
LocusChr. 2 q21

Lactase is an enzyme produced by many organisms. It is located in the brush border of the small intestine of humans and other mammals. Lactase is essential to the complete digestion of whole milk; it breaks down lactose, a sugar which gives milk its sweetness. Lacking lactase, a person consuming dairy products may experience the symptoms of lactose intolerance.[1] Lactase can be purchased as a food supplement, and is added to milk to produce "lactose-free" milk products.

Lactase (also known as lactase-phlorizin hydrolase, or LPH), a part of the β-galactosidase family of enzymes, is a glycoside hydrolase involved in the hydrolysis of the disaccharide lactose into constituent galactose and glucose monomers. Lactase is present predominantly along the brush border membrane of the differentiated enterocytes lining the villi of the small intestine.[2] In humans, lactase is encoded by the LCT gene.[3][4]

Uses

Medical use

Lactase supplements are sometimes used to treat lactose intolerance.[5]

Industrial use

Lactase produced commercially can be extracted both from yeasts such as Kluyveromyces fragilis and Kluyveromyces lactis and from molds, such as Aspergillus niger and Aspergillus oryzae.[6] Its primary commercial use, in supplements such as Lacteeze and Lactaid, is to break down lactose in milk to make it suitable for people with lactose intolerance,[7][8] However, the U.S. Food and Drug Administration has not formally evaluated the effectiveness of these products.[9]

Lactase is also used to screen for blue white colonies in the multiple cloning sites of various plasmid vectors in Escherichia coli or other bacteria.[citation needed]

Mechanism

The optimum temperature for human lactase is about 37 °C for its activity[10] and has an optimum pH of 6.[2]

In metabolism, the β-glycosidic bond in D-lactose is hydrolyzed to form D-galactose and D-glucose, which can be absorbed through the intestinal walls and into the bloodstream. The overall reaction that lactase catalyzes is C12H22O11 + H2O → C6H12O6 + C6H12O6 + heat.

The catalytic mechanism of D-lactose hydrolysis retains the substrate anomeric configuration in the products.[11] While the details of the mechanism are uncertain, the stereochemical retention is achieved through a double displacement reaction. Studies of E. coli lactase have proposed that hydrolysis is initiated when a glutamate nucleophile on the enzyme attacks from the axial side of the galactosyl carbon in the β-glycosidic bond.[12] The removal of the D-glucose leaving group may be facilitated by Mg-dependent acid catalysis.[12] The enzyme is liberated from the α-galactosyl moiety upon equatorial nucleophilic attack by water, which produces D-galactose.[11]

Substrate modification studies have demonstrated that the 3′-OH and 2′-OH moieties on the galactopyranose ring are essential for enzymatic recognition and hydrolysis.[13] The 3′-hydroxy group is involved in initial binding to the substrate while the 2′- group is not necessary for recognition but needed in subsequent steps. This is demonstrated by the fact that a 2-deoxy analog is an effective competitive inhibitor (Ki = 10mM).[13] Elimination of specific hydroxyl groups on the glucopyranose moiety does not completely eliminate catalysis.[13]

Proposed mechanism of lactose hydrolysis by Lactase enzyme

Lactase also catalyzes the conversion of phlorizin to phloretin and glucose.

Structure and biosynthesis

Preprolactase, the primary translation product, has a single polypeptide primary structure consisting of 1927 amino acids.[3] It can be divided into five domains: (i) a 19-amino-acid cleaved signal sequence; (ii) a large prosequence domain that is not present in mature lactase; (iii) the mature lactase segment; (iv) a membrane-spanning hydrophobic anchor; and (v) a short hydrophilic carboxyl terminus.[3] The signal sequence is cleaved in the endoplasmic reticulum, and the resulting 215-kDa pro-LPH is sent to the Golgi apparatus, where it is heavily glycosylated and proteolytically processed to its mature form.[14] The prodomain has been shown to act as an intramolecular chaperone in the ER, preventing trypsin cleavage and allowing LPH to adopt the necessary 3-D structure to be transported to the Golgi apparatus.[15]

File:Lactase Processing.png
Schematic of processing and localization of human lactase translational product

Mature human lactase consists of a single 160-kDa polypeptide chain that localizes to the brush border membrane of intestinal epithelial cells. It is oriented with the N-terminus outside the cell and the C-terminus in the cytosol.[3] LPH contains two catalytic glutamic acid sites. In the human enzyme, the lactase activity has been connected to Glu-1749, while Glu-1273 is the site of phlorizin hydrolase function.[16]

Genetic expression and regulation

Lactase is encoded by a single genetic locus on chromosome 2.[17] It is expressed exclusively by mammalian small intestine enterocytes and in very low levels in the colon during fetal development.[17] Humans are born with high levels of lactase expression. In most of the world’s population, lactase transcription is down-regulated after weaning, resulting in diminished lactase expression in the small intestine,[17] which causes the common symptoms of adult-type hypolactasia, or lactose intolerance.[18]

Some population segments exhibit lactase persistence resulting from a mutation that is postulated to have occurred 5,000–10,000 years ago, coinciding with the rise of cattle domestication.[19] This mutation has allowed almost half of the world’s population to metabolize lactose without symptoms. Studies have linked the occurrence of lactase persistence to two different single-nucleotide polymorphisms about 14 and 22 kilobases upstream of the 5’-end of the LPH gene.[20] Both mutations, C→T at position -13910 and G→ A at position -22018, have been independently linked to lactase persistence.[21]

The lactase promoter is 150 base pairs long and is located just upstream of the site of transcription initiation.[21] The sequence is highly conserved in mammals, suggesting that critical cis-transcriptional regulators are located nearby.[21] Cdx-2, HNF-1α, and GATA have been identified as transcription factors.[21] Studies of hypolactasia onset have demonstrated that despite polymorphisms, little difference exists in lactase expression in infants, showing that the mutations become increasingly relevant during development.[22] Developmentally regulated DNA-binding proteins may down-regulate transcription or destabilize mRNA transcripts, causing decreased LPH expression after weaning.[22]

See also

References

  1. ↑ Järvelä I, Torniainen S, Kolho KL (2009). "Molecular genetics of human lactase deficiencies". Annals of Medicine. 41 (8): 568–75. doi:10.1080/07853890903121033. PMID 19639477.
  2. ↑ 2.0 2.1 Skovbjerg H, Sjöström H, Norén O (Mar 1981). "Purification and characterisation of amphiphilic lactase/phlorizin hydrolase from human small intestine". European Journal of Biochemistry / FEBS. 114 (3): 653–61. doi:10.1111/j.1432-1033.1981.tb05193.x. PMID 6786877.
  3. ↑ 3.0 3.1 3.2 3.3 Mantei N, Villa M, Enzler T, Wacker H, Boll W, James P, Hunziker W, Semenza G (Sep 1988). "Complete primary structure of human and rabbit lactase-phlorizin hydrolase: implications for biosynthesis, membrane anchoring and evolution of the enzyme". The EMBO Journal. 7 (9): 2705–13. PMC 457059. PMID 2460343.
  4. ↑ Harvey CB, Fox MF, Jeggo PA, Mantei N, Povey S, Swallow DM (Jul 1993). "Regional localization of the lactase-phlorizin hydrolase gene, LCT, to chromosome 2q21". Annals of Human Genetics. 57 (Pt 3): 179–85. doi:10.1111/j.1469-1809.1993.tb01593.x. PMID 8257087.
  5. ↑ "Lactose Intolerance". NIDDK. June 2014. Retrieved 25 October 2016.
  6. ↑ Seyis I, Aksoz N (2004). "Production of lactase by Trichoderma sp" (PDF). Food Technol Biotechnol. 42: 121–124.
  7. ↑ "Re: GRAS Notification for Acid Lactase from Aspergillus oryzae Expressed in Aspergillus niger". United States Food and Drug Administration.
  8. ↑ Holsinger VH. "The Lactaid Story". Innovative Products for Food Industries. pp. 256–8.
  9. ↑ Tarantino, LM (2003-12-03). "Agency Response Letter GRAS Notice No. GRN 000132". U.S. Food and Drug Administration. Retrieved 2009-09-21.
  10. ↑ Hermida C, Corrales G, Cañada FJ, Aragón JJ, Fernández-Mayoralas A (Jul 2007). "Optimizing the enzymatic synthesis of beta-D-galactopyranosyl-D-xyloses for their use in the evaluation of lactase activity in vivo". Bioorganic & Medicinal Chemistry. 15 (14): 4836–40. doi:10.1016/j.bmc.2007.04.067. PMID 17512743.
  11. ↑ 11.0 11.1 Sinnott M (November 1990). "Catalytic mechanisms of enzymic glycosyl transfer". Chem. Rev. 90 (7): 1171–1202. doi:10.1021/cr00105a006.
  12. ↑ 12.0 12.1 Juers DH, Heightman TD, Vasella A, McCarter JD, Mackenzie L, Withers SG, Matthews BW (Dec 2001). "A structural view of the action of Escherichia coli (lacZ) beta-galactosidase". Biochemistry. 40 (49): 14781–94. doi:10.1021/bi011727i. PMID 11732897.
  13. ↑ 13.0 13.1 13.2 Fernandez P, Cañada FJ, Jiménez-Barbero J, Martín-Lomas M (Jul 1995). "Substrate specificity of small-intestinal lactase: study of the steric effects and hydrogen bonds involved in enzyme-substrate interaction". Carbohydrate Research. 271 (1): 31–42. doi:10.1016/0008-6215(95)00034-Q. PMID 7648581.
  14. ↑ Naim HY, Sterchi EE, Lentze MJ (Jan 1987). "Biosynthesis and maturation of lactase-phlorizin hydrolase in the human small intestinal epithelial cells". The Biochemical Journal. 241 (2): 427–34. doi:10.1042/bj2410427. PMC 1147578. PMID 3109375.
  15. ↑ Naim HY, Jacob R, Naim H, Sambrook JF, Gething MJ (Oct 1994). "The pro region of human intestinal lactase-phlorizin hydrolase". The Journal of Biological Chemistry. 269 (43): 26933–43. PMID 7523415.
  16. ↑ Zecca L, Mesonero JE, Stutz A, Poirée JC, Giudicelli J, Cursio R, Gloor SM, Semenza G (Sep 1998). "Intestinal lactase-phlorizin hydrolase (LPH): the two catalytic sites; the role of the pancreas in pro-LPH maturation". FEBS Letters. 435 (2–3): 225–8. doi:10.1016/S0014-5793(98)01076-X. PMID 9762914.
  17. ↑ 17.0 17.1 17.2 Troelsen JT, Mitchelmore C, Spodsberg N, Jensen AM, Norén O, Sjöström H (Mar 1997). "Regulation of lactase-phlorizin hydrolase gene expression by the caudal-related homoeodomain protein Cdx-2". The Biochemical Journal. 322 ( Pt 3) (Pt. 3): 833–8. PMC 1218263. PMID 9148757.
  18. ↑ https://ghr.nlm.nih.gov/gene/LCT
  19. ↑ Bersaglieri T, Sabeti PC, Patterson N, Vanderploeg T, Schaffner SF, Drake JA, Rhodes M, Reich DE, Hirschhorn JN (Jun 2004). "Genetic signatures of strong recent positive selection at the lactase gene". American Journal of Human Genetics. 74 (6): 1111–20. doi:10.1086/421051. PMC 1182075. PMID 15114531.
  20. ↑ Kuokkanen M, Enattah NS, Oksanen A, Savilahti E, Orpana A, Järvelä I (May 2003). "Transcriptional regulation of the lactase-phlorizin hydrolase gene by polymorphisms associated with adult-type hypolactasia". Gut. 52 (5): 647–52. doi:10.1136/gut.52.5.647. PMC 1773659. PMID 12692047.
  21. ↑ 21.0 21.1 21.2 21.3 Troelsen JT (May 2005). "Adult-type hypolactasia and regulation of lactase expression". Biochimica et Biophysica Acta. 1723 (1–3): 19–32. doi:10.1016/j.bbagen.2005.02.003. PMID 15777735.
  22. ↑ 22.0 22.1 Wang Y, Harvey CB, Hollox EJ, Phillips AD, Poulter M, Clay P, Walker-Smith JA, Swallow DM (Jun 1998). "The genetically programmed down-regulation of lactase in children". Gastroenterology. 114 (6): 1230–6. doi:10.1016/S0016-5085(98)70429-9. PMID 9609760.

External links

Retrieved from "https://www.wikidoc.org/index.php?title=Lactase&oldid=1411996"