HOME

TheInfoList




β-galactosidase, also called lactase, beta-gal or β-gal, is a family of
glycoside hydrolase 1HNY, a glycoside hydrolase Glycoside hydrolases (also called glycosidases or glycosyl hydrolases) catalysis, catalyze the hydrolysis Hydrolysis (; ) is any chemical reaction in which a molecule of water breaks one or more chemical bonds. The ...
enzyme Enzymes () are proteins that act as biological catalysts (biocatalysts). Catalysts accelerate chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the enzyme converts the substrates in ...

enzyme
s that
catalyzes that utilizes a low-temperature oxidation catalyst to convert carbon monoxide to less toxic carbon dioxide Carbon dioxide (chemical formula ) is a colorless gas with a density about 53% higher than that of dry air. Carbon dioxide molecules ...

catalyzes
the
hydrolysis Hydrolysis (; ) is any chemical reaction in which a molecule of water breaks one or more chemical bonds. The term is used broadly for substitution Substitution may refer to: Arts and media *Chord substitution, in music, swapping one chord fo ...

hydrolysis
of β-galactosides into
monosaccharide Monosaccharides (from Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is ap ...
s through the breaking of a
glycosidic bond A glycosidic bond or glycosidic linkage is a type of covalent bond that joins a carbohydrate (sugar) molecule to another group, which may or may not be another carbohydrate. A glycosidic bond is formed between the hemiacetal or hemiketal group ...
. β-galactosides include carbohydrates containing
galactose Galactose (, '' galacto-'' + ''-ose The suffix In linguistics Linguistics is the science, scientific study of language. It encompasses the analysis of every aspect of language, as well as the methods for studying and modeling them. Th ...
where the glycosidic bond lies above the galactose molecule.
Substrate Substrate may refer to: Physical layers *Substrate (biology), the natural environment in which an organism lives, or the surface or medium on which an organism grows or is attached **Substrate (locomotion), the surface over which an organism loco ...
s of different β-galactosidases include
ganglioside A ganglioside is a molecule composed of a glycosphingolipidImage:Sphingosine structure.svg, Sphingosine Glycosphingolipids are a subtype of glycolipids containing the amino alcohol sphingosine. They may be considered as sphingolipids with an attac ...
GM1, lactosylceramides,
lactose Lactose, a disaccharide A disaccharide (also called a double sugar or ''biose'') is the sugar Sugar is the generic name for sweet-tasting, soluble carbohydrate is a disaccharide A disaccharide (also called a double sugar or ''biose ...

lactose
, and various
glycoprotein Glycoproteins are protein Proteins are large s and s that comprise one or more long chains of . Proteins perform a vast array of functions within organisms, including , , , providing and , and from one location to another. Proteins dif ...
s.


Nomenclature

The Enzyme Commission is responsible for creating an enzyme classification system based on numbers. The first number describes which class the enzyme belongs to, the second number reference sub-class, the third value specifies the nature of the substrate, and the fourth number is a serial number assigned to enzymes within a subclass.  The EC (''Enzyme Commission)'' number of β-galactosidase is 3.2.1.23  β-galactosidase belongs to class 3, which refers to the
hydrolases Hydrolase is a class of enzyme that commonly perform as biochemical catalysts that use water to break a chemical bond, which typically results in dividing a larger molecule into smaller molecules. Some common examples of hydrolase enzymes are estera ...
. β-gal belongs to a sub-class of glycosylases with an oxygen substrate nature.


Function

β-galactosidase is an
exoglycosidase A exoglycosidase is a glycoside hydrolase enzyme which breaks the glycosidic bonds at the terminal residue. See also * Endoglycosidase External links

* Enzymes {{hydrolase-stub ...
which hydrolyzes the β-
glycosidic bond A glycosidic bond or glycosidic linkage is a type of covalent bond that joins a carbohydrate (sugar) molecule to another group, which may or may not be another carbohydrate. A glycosidic bond is formed between the hemiacetal or hemiketal group ...
formed between a
galactose Galactose (, '' galacto-'' + ''-ose The suffix In linguistics Linguistics is the science, scientific study of language. It encompasses the analysis of every aspect of language, as well as the methods for studying and modeling them. Th ...
and its organic moiety. It may also cleave and but with much lower efficiency. It is an essential enzyme in the human body. Deficiencies in the protein can result in
galactosialidosis Galactosialidosis, also known as Neuraminidase deficiency with beta-galactosidase deficiency, is a genetic lysosomal storage disease. It is caused by a mutation in the CTSA gene which leads to a deficiency of enzymes Beta-galactosidase, β-galacto ...
or
Morquio B syndrome Morquio syndrome, also known as Mucopolysaccharidosis Type IV (MPS IV), is a rare metabolic disorder in which the body cannot process certain types of sugar molecules called glycosaminoglycans (AKA GAGs, or mucopolysaccharides). In Morquio syndrome ...
. In '''', the ''lacZ'' gene is the structural gene for β-galactosidase; which is present as part of the inducible system which is activated in the presence of
lactose Lactose, a disaccharide A disaccharide (also called a double sugar or ''biose'') is the sugar Sugar is the generic name for sweet-tasting, soluble carbohydrate is a disaccharide A disaccharide (also called a double sugar or ''biose ...

lactose
when
glucose Glucose is a simple with the . Glucose is the most abundant , a subcategory of s. Glucose is mainly made by and most during from water and carbon dioxide, using energy from sunlight, where it is used to make in s, the most abundant carbohydr ...

glucose
level is low. β-galactosidase synthesis stops when glucose levels are sufficient. Beta-galactosidase has many homologues based on similar sequences. A few are evolved beta-galactosidase (EBG),
beta-glucosidase Beta-glucosidase is an enzyme that catalyzes the hydrolysis of the glycosidic bonds to terminal non-reducing residues in beta-D-glucosides and oligosaccharides, with release of glucose. Structure Beta-glucosidase is composed of two polypeptide c ...
, 6-phospho-beta-galactosidase, beta-mannosidase, and lactase-phlorizin hydrolase. Although they may be structurally similar, they all have different functions. Beta-gal is inhibited by ,
iodine Iodine is a chemical element In chemistry, an element is a pure Chemical substance, substance consisting only of atoms that all have the same numbers of protons in their atomic nucleus, nuclei. Unlike chemical compounds, chemical ele ...

iodine
, and 2-phenylethyl 1-thio-beta-D-galactopyranoside (PETG), D-galactonolactone, isopropyl thio-beta-D-galactoside (IPTG), and galactose. β-galactosidase is important for organisms as it is a key provider in the production of energy and a source of carbons through the break down of lactose to galactose and glucose. It is also important for the lactose intolerant community as it is responsible for making lactose-free milk and other dairy products. Many adult humans lack the
lactase Lactase is an enzyme Enzymes () are proteins that act as biological catalysts (biocatalysts). Catalysts accelerate chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the enzyme conve ...

lactase
enzyme, which has the same function of beta-gal, so they are not able to properly digest dairy products. Beta-galactose is used in such dairy products as yogurt, sour cream, and some cheeses which are treated with the enzyme to break down any lactose before human consumption. In recent years, beta-galactosidase has been researched as a potential treatment for lactose intolerance through gene replacement therapy where it could be placed into the human DNA so individuals can break down lactose on their own.


Structure

The 1,023
amino acid Amino acids are organic compound In , organic compounds are generally any s that contain - . Due to carbon's ability to (form chains with other carbon s), millions of organic compounds are known. The study of the properties, reactions, a ...

amino acid
s of '''' β-galactosidase were accurately sequenced in 1983, and its structure determined twenty-four years later in 1994. The
protein Proteins are large biomolecule , showing alpha helices, represented by ribbons. This poten was the first to have its suckture solved by X-ray crystallography by Max Perutz and Sir John Cowdery Kendrew in 1958, for which they received a No ...

protein
is a 464-
kDa The dalton or unified atomic mass unit (symbols: Da or u) is a unit Unit may refer to: Arts and entertainment * UNIT Unit may refer to: Arts and entertainment * UNIT, a fictional military organization in the science fiction television series ' ...
homotetramer A tetrameric protein is a protein Proteins are large biomolecules or macromolecules that are comprised of one or more long chains of amino acid residue (biochemistry), residues. Proteins perform a vast array of functions within organisms, inc ...
with 2,2,2-point
symmetry Symmetry (from Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is appro ...

symmetry
. Each unit of β-galactosidase consists of five domains; domain 1 is a jelly-roll type β-
barrel A barrel or cask is a hollow cylindrical A cylinder (from ) has traditionally been a Solid geometry, three-dimensional solid, one of the most basic of curvilinear geometric shapes. Geometrically, it can be considered as a Prism (geometry) ...
, domain 2 and 4 are fibronectin type III-like barrels, domain 5 a novel β-sandwich, while the central domain 3 is a distorted TIM-type barrel, lacking the fifth helix with a distortion in the sixth strand. The third domain contains the active site. The active site is made up of elements from two subunits of the tetramer, and disassociation of the tetramer into dimers removes critical elements of the active site. The amino-terminal sequence of β-galactosidase, the α-peptide involved in α-complementation, participates in a subunit interface. Its residues 22-31 help to stabilize a four-helix bundle which forms the major part of that interface, and residue 13 and 15 also contributing to the activating interface. These structural features provide a rationale for the phenomenon of α-complementation, where the deletion of the amino-terminal segment results in the formation of an inactive dimer.


Reaction

β-galactosidase can catalyze three different reactions in organisms. In one, it can go through a process called transgalactosylation to make , creating a
positive feedback loop Positive feedback (exacerbating feedback, self-reinforcing feedback) is a process that occurs in a feedback loop which exacerbates the effects of a small disturbance. That is, the effects of a perturbation on a system include an increase in th ...

positive feedback loop
for the production of β-gal. Allolactose can also be cleaved to form monosaccharides. It can also hydrolyze lactose into
galactose Galactose (, '' galacto-'' + ''-ose The suffix In linguistics Linguistics is the science, scientific study of language. It encompasses the analysis of every aspect of language, as well as the methods for studying and modeling them. Th ...
and
glucose Glucose is a simple with the . Glucose is the most abundant , a subcategory of s. Glucose is mainly made by and most during from water and carbon dioxide, using energy from sunlight, where it is used to make in s, the most abundant carbohydr ...

glucose
which will proceed into
glycolysis Glycolysis is the metabolic pathway In biochemistry, a metabolic pathway is a linked series of chemical reaction A chemical reaction is a process that leads to the IUPAC nomenclature for organic transformations, chemical transformation of on ...

glycolysis
. The active site of β-galactosidase catalyzes the hydrolysis of its
disaccharide A disaccharide (also called a double sugar or ''biose'') is the sugar Sugar is the generic name for sweet-tasting, soluble carbohydrate is a disaccharide A disaccharide (also called a double sugar or ''biose'') is the sugar formed wh ...
substrate via "shallow" (nonproductive site) and "deep" (productive site) binding.
GalactosideA galactoside is a glycoside containing galactose. The H of the OH group on carbon-1 of galactose is replaced by an organic moiety. Depending on whether the glycosidic bond lies "above" or "below" the plane of the galactose molecule, galactosides ar ...
s such as PETG and IPTG will bind in the shallow site when the enzyme is in "open" conformation while
transition state analogTransition state analogs (transition state analogues), are chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecular entity, molecular entities) composed of atoms from more than one chemical ...
s such as L-ribose and D-galactonolactone will bind in the deep site when the conformation is "closed". The enzymatic reaction consists of two chemical steps, galactosylation (k2) and degalactosylation (k3). Galactosylation is the first chemical step in the reaction where Glu461 donates a proton to a glycosidic oxygen, resulting in galactose covalently bonding with Glu537. In the second step, degalactosylation, the covalent bond is broken when Glu461 accepts a proton, replacing the galactose with water. Two
transition state The transition state of a chemical reaction A chemical reaction is a process that leads to the chemical transformation of one set of chemical substance A chemical substance is a form of matter In classical physics and general chemistr ...

transition state
s occur in the deep site of the enzyme during the reaction, once after each step. When water participates in the reaction, galactose is formed, otherwise, when D-glucose acts as the acceptor in the second step, transgalactosylation occurs . It has been kinetically measured that single tetramers of the protein catalyze reactions at a rate of 38,500 ± 900 reactions per minute. Monovalent
potassium Potassium is a chemical element In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, properties, b ...

potassium
ion An ion () is an atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can be touched are ...
s (K+) as well as divalent
magnesium Magnesium is a chemical element upright=1.0, 500px, The chemical elements ordered by link=Periodic table In chemistry Chemistry is the science, scientific study of the properties and behavior of matter. It is a natural science ...

magnesium
ions (Mg2+) are required for the enzyme's optimal activity. The beta-linkage of the substrate is cleaved by a terminal
carboxyl A carboxylic acid is an organic acid An organic acid is an organic compound In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: th ...

carboxyl
group on the
side chain In organic chemistry Organic chemistry is a branch of that studies the structure, properties and reactions of s, which contain in .Clayden, J.; Greeves, N. and Warren, S. (2012) ''Organic Chemistry''. Oxford University Press. pp. 1–15. . ...
of a
glutamic acid Glutamic acid (symbol Glu or E; the ionic form is known as glutamate) is an α-amino acid Amino acids are organic compound , CH4; is among the simplest organic compounds. In chemistry, organic compounds are generally any chemical compound ...
. In '''', Glu-461 was thought to be the
nucleophile In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, properties, behavior and the changes they undergo during a ...

nucleophile
in the
substitution Substitution may refer to: Arts and media *Chord substitution, in music, swapping one chord for a related one within a chord progression *Substitution (poetry), a variation in poetic scansion *Substitution (song), "Substitution" (song), a 2009 so ...
reaction. However, it is now known that Glu-461 is an
acid An acid is a or capable of donating a (hydrogen ion H+) (a ), or, alternatively, capable of forming a with an (a ). The first category of acids are the proton donors, or s. In the special case of , proton donors form the H3O+ and are ...
catalyst. Instead, Glu-537 is the actual nucleophile, binding to a galactosyl intermediate. In
human Humans (''Homo sapiens'') are the most abundant and widespread species In biology, a species is the basic unit of biological classification, classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A speci ...

human
s, the
nucleophile In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, properties, behavior and the changes they undergo during a ...

nucleophile
of the
hydrolysis Hydrolysis (; ) is any chemical reaction in which a molecule of water breaks one or more chemical bonds. The term is used broadly for substitution Substitution may refer to: Arts and media *Chord substitution, in music, swapping one chord fo ...

hydrolysis
reaction is Glu-268. Gly794 is important for β-gal activity. It is responsible for putting the enzyme in a "closed", ligand bounded, conformation or "open" conformation, acting like a "hinge" for the active site loop. The different conformations ensure that only preferential binding occurs in the active site. In the presence of a slow substrate, Gly794 activity increased as well as an increase in galactosylation and decrease in degalactosylation.


Applications

The β-galactosidase assay is used frequently in
genetics Genetics is a branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, ...

genetics
,
molecular biology Molecular biology is the branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, P ...
, and other
life science Life is a characteristic that distinguishes physical entities that have biological processes, such as signaling In signal processing Signal processing is an electrical engineering subfield that focuses on analysing, modifying, a ...
s. An active enzyme may be detected using artificial chromogenic substrate 5-bromo-4-chloro-3-indolyl-β-d-galactopyranoside,
X-gal X-gal (also abbreviated BCIG for 5-bromo-4-chloro-3-indolyl-β--galactopyranoside) is an organic compound In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the comp ...

X-gal
. β-galactosidase will cleave the glycosidic bond in
X-gal X-gal (also abbreviated BCIG for 5-bromo-4-chloro-3-indolyl-β--galactopyranoside) is an organic compound In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the comp ...

X-gal
and form galactose and 5-bromo-4-chloro-3-hydroxyindole which dimerizes and oxidizes to 5,5'-dibromo-4,4'-dichloro-indigo, an intense blue product that is easy to identify and quantify. It is used for example in
blue white screen Blue is one of the three primary colours of pigments in painting and traditional colour theory In the visual arts The visual arts are art forms such as painting, drawing, printmaking, sculpture Sculpture is the branch of the vi ...
. Its production may be induced by a non-hydrolyzable analog of , , which binds and releases the lac repressor from the lac operator, thereby allowing the initiation of transcription to proceed. It is commonly used in molecular biology as a to monitor gene expression. It also exhibits a phenomenon called α-complementation which forms the basis for the blue/white screening of recombinant clones. This enzyme can be split in two peptides, LacZ α and LacZ Ω, neither of which is active by itself but when both are present together, spontaneously reassemble into a functional enzyme. This property is exploited in many
cloning vector A cloning vector is a small piece of DNA Deoxyribonucleic acid (; DNA) is a molecule File:Pentacene on Ni(111) STM.jpg, A scanning tunneling microscopy image of pentacene molecules, which consist of linear chains of five carbon rings. ...
s where the presence of the ''lacZα'' gene in a plasmid can complement in ''
trans Trans- is a Latin prefix meaning "across", "beyond", or "on the other side of". Used alone, trans may refer to: Arts, entertainment, and media * Trans (festival) trans was an annual, non-commercial arts festival held in Belfast, Northern Irel ...
'' another mutant gene encoding the LacZΩ in specific laboratory strains of ''E. coli''. However, when DNA fragments are inserted in the vector, the production of LacZα is disrupted, the cells therefore show no β-galactosidase activity. The presence or absence of an active β-galactosidase may be detected by
X-gal X-gal (also abbreviated BCIG for 5-bromo-4-chloro-3-indolyl-β--galactopyranoside) is an organic compound In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the comp ...

X-gal
, which produces a characteristic blue dye when cleaved by β-galactosidase, thereby providing an easy means of distinguishing the presence or absence of cloned product in a plasmid. In studies of leukaemia chromosomal translocations, Dobson and colleagues used a fusion protein of LacZ in mice, exploiting β-galactosidase's tendency to oligomerise to suggest a potential role for oligomericity in MLL fusion protein function. A new isoform for beta-galactosidase with optimum activity at pH 6.0 (Senescence Associated beta-gal or SA-beta-gal) which is specifically expressed in
senescence Senescence () or biological aging is the gradual deterioration of functional Functional may refer to: * Movements in architecture: ** Functionalism (architecture) In architecture File:Plan d'exécution du second étage de l'hôtel de Brio ...

senescence
(the irreversible growth arrest of cells). Specific quantitative assays were even developed for its detection. However, it is now known that this is due to an overexpression and accumulation of the lysosomal endogenous beta-galactosidase, and its expression is not required for senescence. Nevertheless, it remains the most widely used biomarker for senescent and aging cells, because it is reliable and easy to detect.


Evolution

Some species of bacteria, including ''E. coli'', have additional β-galactosidase genes. A second gene, called evolved β-galactosidase (''ebgA'') gene was discovered when strains with the ''lacZ'' gene deleted (but still containing the gene for galactoside permease, ''lacY''), were plated on medium containing lactose (or other 3-galactosides) as sole carbon source. After a time, certain colonies began to grow. However, the EbgA protein is an ineffective lactase and does not allow growth on lactose. Two classes of single point mutations dramatically improve the activity of ebg enzyme toward lactose. and, as a result, the mutant enzyme is able to replace the lacZ β-galactosidase. EbgA and LacZ are 50% identical on the DNA level and 33% identical on the amino acid level. The active ebg enzyme is an aggregate of ebgA -gene and ebgC-gene products in a 1:1 ratio with the active form of ebg enzymes being an ''α''4 β4 hetero-octamer.


Species distribution

Much of the work done on β-galactosidase is derived from ''E. coli.'' However β-gal can be found in many plants (especially fruits), mammals, yeast, bacteria, and fungi. β-galactosidase genes can differ in the length of their coding sequence and the length of proteins formed by amino acids. This separates the β-galactosidases into four families: GHF-1, GHF-2, GHF-35, and GHF- 42. ''E. Coli'' belongs to GHF-2, all plants belong to GHF-35, and '' Thermus thermophilus'' belongs to GHF-42. Various fruits can express multiple β-gal genes. There are at least 7 β-gal genes expressed in tomato fruit development, that have amino acid similarity between 33% and 79%. A study targeted at identifying fruit softening of peaches found 17 different gene expressions of β-galactosidases. The only other known crystal structure of β-gal is from '' Thermus thermophilus.''


References


External links

* {{DEFAULTSORT:Beta-Galactosidase EC 3.2.1 Hydrolases Enzymes Galactosides Aging-related enzymes Enzymes of known structure