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In
biochemistry Biochemistry, or biological chemistry, is the study of chemical processes within and relating to living organisms. A sub-discipline of both chemistry and biology, biochemistry may be divided into three fields: structural biology, enzymology, a ...
, isomerases are a general class of
enzyme An enzyme () is a protein that acts as a biological catalyst by accelerating chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the enzyme converts the substrates into different mol ...
s that convert a molecule from one
isomer In chemistry, isomers are molecules or polyatomic ions with identical molecular formula – that is, the same number of atoms of each element (chemistry), element – but distinct arrangements of atoms in space. ''Isomerism'' refers to the exi ...
to another. Isomerases facilitate intramolecular rearrangements in which bonds are broken and formed. The general form of such a reaction is as follows: :\ce \quad \xrightarrow
text Text may refer to: Written word * Text (literary theory) In literary theory, a text is any object that can be "read", whether this object is a work of literature, a street sign, an arrangement of buildings on a city block, or styles of clothi ...
\quad \ce There is only one substrate yielding one product. This product has the same molecular formula as the substrate but differs in bond connectivity or spatial arrangement. Isomerases catalyze reactions across many biological processes, such as in
glycolysis Glycolysis is the metabolic pathway that converts glucose () into pyruvic acid, pyruvate and, in most organisms, occurs in the liquid part of cells (the cytosol). The Thermodynamic free energy, free energy released in this process is used to form ...
and
carbohydrate metabolism Carbohydrate metabolism is the whole of the biochemistry, biochemical processes responsible for the metabolic anabolism, formation, catabolism, breakdown, and interconversion of carbohydrates in life, living organisms. Carbohydrates are central t ...
.


Isomerization

Isomerases catalyze changes within one molecule. They convert one isomer to another, meaning that the end product has the same molecular formula but a different physical structure.
Isomer In chemistry, isomers are molecules or polyatomic ions with identical molecular formula – that is, the same number of atoms of each element (chemistry), element – but distinct arrangements of atoms in space. ''Isomerism'' refers to the exi ...
s themselves exist in many varieties but can generally be classified as
structural isomer In chemistry, a structural isomer (or constitutional isomer in the IUPAC nomenclature) of a compound is a compound that contains the same number and type of atoms, but with a different connectivity (i.e. arrangement of bonds) between them. The ...
s or stereoisomers. Structural isomers have a different ordering of bonds and/or different bond connectivity from one another, as in the case of
hexane Hexane () or ''n''-hexane is an organic compound, a straight-chain alkane with six carbon atoms and the molecular formula C6H14. Hexane is a colorless liquid, odorless when pure, and with a boiling point of approximately . It is widely used as ...
and its four other isomeric forms ( 2-methylpentane, 3-methylpentane, 2,2-dimethylbutane, and 2,3-dimethylbutane). Stereoisomers have the same ordering of individual bonds and the same connectivity but the three-dimensional arrangement of bonded atoms differ. For example, 2-butene exists in two isomeric forms: ''cis''-2-butene and ''trans''-2-butene. The sub-categories of isomerases containing racemases, epimerases and cis-trans isomers are examples of enzymes catalyzing the interconversion of stereoisomers. Intramolecular lyases, oxidoreductases and transferases catalyze the interconversion of structural isomers. The prevalence of each isomer in nature depends in part on the isomerization energy, the difference in energy between isomers. Isomers close in energy can interconvert easily and are often seen in comparable proportions. The isomerization energy, for example, for converting from a stable ''cis'' isomer to the less stable ''trans'' isomer is greater than for the reverse reaction, explaining why in the absence of isomerases or an outside energy source such as
ultraviolet radiation Ultraviolet radiation, also known as simply UV, is electromagnetic radiation of wavelengths of 10–400 nanometers, shorter than that of visible light, but longer than X-rays. UV radiation is present in sunlight and constitutes about 10% of t ...
a given ''cis'' isomer tends to be present in greater amounts than the ''trans'' isomer. Isomerases can increase the reaction rate by lowering the isomerization energy. Calculating isomerase kinetics from experimental data can be more difficult than for other enzymes because the use of product inhibition experiments is impractical. That is, isomerization is not an irreversible reaction since a reaction vessel will contain one substrate and one product so the typical simplified model for calculating reaction kinetics does not hold. There are also practical difficulties in determining the rate-determining step at high concentrations in a single isomerization. Instead, tracer perturbation can overcome these technical difficulties if there are two forms of the unbound enzyme. This technique uses isotope exchange to measure indirectly the interconversion of the free enzyme between its two forms. The radiolabeled substrate and product diffuse in a time-dependent manner. When the system reaches equilibrium the addition of unlabeled substrate perturbs or unbalances it. As equilibrium is established again, the radiolabeled substrate and product are tracked to determine energetic information. The earliest use of this technique elucidated the kinetics and mechanism underlying the action of phosphoglucomutase, favoring the model of indirect transfer of
phosphate Phosphates are the naturally occurring form of the element phosphorus. In chemistry, a phosphate is an anion, salt, functional group or ester derived from a phosphoric acid. It most commonly means orthophosphate, a derivative of orthop ...
with one intermediate and the direct transfer of
glucose Glucose is a sugar with the Chemical formula#Molecular formula, molecular formula , which is often abbreviated as Glc. It is overall the most abundant monosaccharide, a subcategory of carbohydrates. It is mainly made by plants and most algae d ...
. This technique was then adopted to study the profile of proline racemase and its two states: the form which isomerizes L- proline and the other for D-proline. At high concentrations it was shown that the
transition state In chemistry, the transition state of a chemical reaction is a particular configuration along the reaction coordinate. It is defined as the state corresponding to the highest potential energy along this reaction coordinate. It is often marked w ...
in this interconversion is rate-limiting and that these enzyme forms may differ just in the
protonation In chemistry, protonation (or hydronation) is the adding of a proton (or hydron, or hydrogen cation), usually denoted by H+, to an atom, molecule, or ion, forming a conjugate acid. (The complementary process, when a proton is removed from a Brø ...
at the
acid An acid is a molecule or ion capable of either donating a proton (i.e. Hydron, hydrogen cation, H+), known as a Brønsted–Lowry acid–base theory, Brønsted–Lowry acid, or forming a covalent bond with an electron pair, known as a Lewis ...
ic and
basic Basic or BASIC may refer to: Science and technology * BASIC, a computer programming language * Basic (chemistry), having the properties of a base * Basic access authentication, in HTTP Entertainment * Basic (film), ''Basic'' (film), a 2003 film ...
groups of the active site.


Nomenclature

Generally, "the names of isomerases are formed as "''substrate'' isomerase" (for example, enoyl CoA isomerase), or as "''substrate'' ''type of isomerase''" (for example, phosphoglucomutase)."


Classification

Enzyme-catalyzed reactions each have a uniquely assigned classification number. Isomerase-catalyzed reactions have their own EC category: EC 5. Isomerases are further classified into six subclasses:


Racemases, epimerases

This category (EC 5.1) includes ( racemases) and epimerases). These isomerases invert
stereochemistry Stereochemistry, a subdiscipline of chemistry, studies the spatial arrangement of atoms that form the structure of molecules and their manipulation. The study of stereochemistry focuses on the relationships between stereoisomers, which are defined ...
at the target chiral carbon. Racemases act upon molecules with one chiral carbon for inversion of stereochemistry, whereas epimerases target molecules with multiple chiral carbons and act upon one of them. A molecule with only one chiral carbon has two
enantiomer In chemistry, an enantiomer (Help:IPA/English, /ɪˈnænti.əmər, ɛ-, -oʊ-/ Help:Pronunciation respelling key, ''ih-NAN-tee-ə-mər''), also known as an optical isomer, antipode, or optical antipode, is one of a pair of molecular entities whi ...
ic forms, such as serine having the isoforms D-serine and L-serine differing only in the
absolute configuration In chemistry, absolute configuration refers to the spatial arrangement of atoms within a molecular entity (or Functional group, group) that is chirality (chemistry), chiral, and its resultant stereochemical description. Absolute configuration is ...
about the chiral carbon. A molecule with multiple chiral carbons has two forms at each chiral carbon. Isomerization at one chiral carbon of several yields epimers, which differ from one another in absolute configuration at just one chiral carbon. For example, D-
glucose Glucose is a sugar with the Chemical formula#Molecular formula, molecular formula , which is often abbreviated as Glc. It is overall the most abundant monosaccharide, a subcategory of carbohydrates. It is mainly made by plants and most algae d ...
and D- mannose differ in configuration at just one chiral carbon. This class is further broken down by the group the enzyme acts upon:


Cis-trans isomerases

This category (EC 5.2) includes enzymes that catalyze the isomerization of cis-trans isomers.
Alkenes In organic chemistry, an alkene, or olefin, is a hydrocarbon containing a carbon–carbon double bond. The double bond may be internal or at the terminal position. Terminal alkenes are also known as Alpha-olefin, α-olefins. The Internationa ...
and cycloalkanes may have cis-trans stereoisomers. These isomers are not distinguished by
absolute configuration In chemistry, absolute configuration refers to the spatial arrangement of atoms within a molecular entity (or Functional group, group) that is chirality (chemistry), chiral, and its resultant stereochemical description. Absolute configuration is ...
but rather by the position of substituent groups relative to a plane of reference, as across a double bond or relative to a ring structure. ''Cis'' isomers have substituent groups on the same side and ''trans'' isomers have groups on opposite sides. This category is not broken down any further. All entries presently include:


Intramolecular oxidoreductases

This category (EC 5.3) includes intramolecular oxidoreductases. These isomerases catalyze the transfer of
electron The electron (, or in nuclear reactions) is a subatomic particle with a negative one elementary charge, elementary electric charge. It is a fundamental particle that comprises the ordinary matter that makes up the universe, along with up qua ...
s from one part of the molecule to another. In other words, they catalyze the
oxidation Redox ( , , reduction–oxidation or oxidation–reduction) is a type of chemical reaction in which the oxidation states of the reactants change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is ...
of one part of the molecule and the concurrent reduction of another part. Sub-categories of this class are:


Intramolecular transferases

This category (EC 5.4) includes intramolecular transferases ( mutases). These isomerases catalyze the transfer of
functional group In organic chemistry, a functional group is any substituent or moiety (chemistry), moiety in a molecule that causes the molecule's characteristic chemical reactions. The same functional group will undergo the same or similar chemical reactions r ...
s from one part of a molecule to another. Phosphotransferases (EC 5.4.2) were categorized as transferases (EC 2.7.5) with regeneration of donors until 1983. This sub-class can be broken down according to the functional group the enzyme transfers:


Intramolecular lyases

This category (EC 5.5) includes intramolecular lyases. These enzymes catalyze "reactions in which a group can be regarded as eliminated from one part of a molecule, leaving a double bond, while remaining covalently attached to the molecule." Some of these catalyzed reactions involve the breaking of a ring structure. This category is not broken down any further. All entries presently include:


Mechanisms of isomerases


Ring expansion and contraction via tautomers

A classic example of ring opening and contraction is the isomerization of
glucose Glucose is a sugar with the Chemical formula#Molecular formula, molecular formula , which is often abbreviated as Glc. It is overall the most abundant monosaccharide, a subcategory of carbohydrates. It is mainly made by plants and most algae d ...
(an aldehyde with a six-membered ring) to
fructose Fructose (), or fruit sugar, is a Ketose, ketonic monosaccharide, simple sugar found in many plants, where it is often bonded to glucose to form the disaccharide sucrose. It is one of the three dietary monosaccharides, along with glucose and gal ...
(a
ketone In organic chemistry, a ketone is an organic compound with the structure , where R and R' can be a variety of carbon-containing substituents. Ketones contain a carbonyl group (a carbon-oxygen double bond C=O). The simplest ketone is acetone ( ...
with a five-membered ring). The conversion of D-glucose-6-phosphate to D-fructose-6-phosphate is catalyzed by glucose-6-phosphate isomerase, an intramolecular oxidoreductase. The overall reaction involves the opening of the ring to form an aldose via acid/base catalysis and the subsequent formation of a cis-endiol intermediate. A ketose is then formed and the ring is closed again. Glucose-6-phosphate first binds to the active site of the isomerase. The isomerase opens the ring: its His388 residue protonates the oxygen on the glucose ring (and thereby breaking the O5-C1 bond) in conjunction with Lys518 deprotonating the C1 hydroxyl oxygen. The ring opens to form a straight-chain aldose with an acidic C2 proton. The C3-C4 bond rotates and Glu357 (assisted by His388) depronates C2 to form a double bond between C1 and C2. A cis-endiol intermediate is created and the C1 oxygen is protonated by the catalytic residue, accompanied by the deprotonation of the endiol C2 oxygen. The straight-chain
ketose In organic chemistry, a ketose is a monosaccharide containing one ketone () group per molecule. The simplest ketose is dihydroxyacetone (), which has only three carbon atoms. It is the only ketose with no optical activity. All monosaccharide keto ...
is formed. To close the fructose ring, the reverse of ring opening occurs and the ketose is protonated.


Epimerization

An example of epimerization is found in the Calvin cycle when D-ribulose-5-phosphate is converted into D-xylulose-5-phosphate by ribulose-phosphate 3-epimerase. The substrate and product differ only in
stereochemistry Stereochemistry, a subdiscipline of chemistry, studies the spatial arrangement of atoms that form the structure of molecules and their manipulation. The study of stereochemistry focuses on the relationships between stereoisomers, which are defined ...
at the third carbon in the chain. The underlying mechanism involves the deprotonation of that third carbon to form a reactive enolate intermediate. The enzyme's active site contains two Asp residues. After the substrate binds to the enzyme, the first Asp deprotonates the third carbon from one side of the molecule. This leaves a planar sp2-hybridized intermediate. The second Asp is located on the opposite side of the active side and it protonates the molecule, effectively adding a proton from the back side. These coupled steps invert stereochemistry at the third carbon.


Intramolecular transfer

Chorismate mutase is an intramolecular transferase and it catalyzes the conversion of chorismate to prephenate, used as a precursor for
L-tyrosine -Tyrosine or tyrosine (symbol Tyr or Y) or 4-hydroxyphenylalanine is one of the 20 standard amino acids that are used by cells to synthesize proteins. It is a conditionally essential amino acid with a polar side group. The word "tyrosine" is ...
and L-phenylalanine in some plants and bacteria. This reaction is a Claisen rearrangement that can proceed with or without the isomerase, though the rate increases 106 fold in the presence of chorismate mutase. The reaction goes through a
chair A chair is a type of seat, typically designed for one person and consisting of one or more legs, a flat or slightly angled seat and a back-rest. It may be made of wood, metal, or synthetic materials, and may be padded or upholstered in vario ...
transition state In chemistry, the transition state of a chemical reaction is a particular configuration along the reaction coordinate. It is defined as the state corresponding to the highest potential energy along this reaction coordinate. It is often marked w ...
with the substrate in a trans-diaxial position. Experimental evidence indicates that the isomerase selectively binds the chair transition state, though the exact mechanism of
catalysis Catalysis () is the increase in rate of a chemical reaction due to an added substance known as a catalyst (). Catalysts are not consumed by the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recycles quick ...
is not known. It is thought that this binding stabilizes the transition state through electrostatic effects, accounting for the dramatic increase in the reaction rate in the presence of the mutase or upon addition of a specifically-placed cation in the active site.


Intramolecular oxidoreduction

Isopentenyl-diphosphate delta isomerase type I (also known as IPP isomerase) is seen in
cholesterol Cholesterol is the principal sterol of all higher animals, distributed in body Tissue (biology), tissues, especially the brain and spinal cord, and in Animal fat, animal fats and oils. Cholesterol is biosynthesis, biosynthesized by all anima ...
synthesis and in particular it catalyzes the conversion of isopentenyl diphosphate (IPP) to dimethylallyl diphosphate (DMAPP). In this isomerization reaction a stable carbon-carbon double bond is rearranged top create a highly electrophilic allylic isomer. IPP isomerase catalyzes this reaction by the stereoselective antarafacial transposition of a single proton. The
double bond In chemistry, a double bond is a covalent bond between two atoms involving four bonding electrons as opposed to two in a single bond. Double bonds occur most commonly between two carbon atoms, for example in alkenes. Many double bonds exist betw ...
is protonated at C4 to form a tertiary carbocation intermediate at C3. The adjacent carbon, C2, is deprotonated from the opposite face to yield a double bond. In effect, the double bond is shifted over.


The role of isomerase in human disease

Isomerase plays a role in human disease. Deficiencies of this enzyme can cause disorders in humans.


Phosphohexose isomerase deficiency

Phosphohexose Isomerase Deficiency (PHI) is also known as phosphoglucose isomerase deficiency or Glucose-6-phosphate isomerase deficiency, and is a hereditary enzyme deficiency. PHI is the second most frequent erthoenzyopathy in
glycolysis Glycolysis is the metabolic pathway that converts glucose () into pyruvic acid, pyruvate and, in most organisms, occurs in the liquid part of cells (the cytosol). The Thermodynamic free energy, free energy released in this process is used to form ...
besides pyruvate kinase deficiency, and is associated with non-spherocytic haemolytic anaemia of variable severity. This disease is centered on the glucose-6-phosphate protein. This protein can be found in the secretion of some cancer cells. PHI is the result of a dimeric enzyme that catalyses the reversible interconversion of fructose-6-phosphate and gluose-6-phosphate. PHI is a very rare disease with only 50 cases reported in literature to date. Diagnosis is made on the basis of the clinical picture in association with biochemical studies revealing erythrocyte GPI deficiency (between 7 and 60% of normal) and identification of a mutation in the GPI gene by molecular analysis. The deficiency of phosphohexose isomerase can lead to a condition referred to as hemolytic syndrome. As in humans, the hemolytic syndrome, which is characterized by a diminished erythrocyte number, lower hematocrit, lower
hemoglobin Hemoglobin (haemoglobin, Hb or Hgb) is a protein containing iron that facilitates the transportation of oxygen in red blood cells. Almost all vertebrates contain hemoglobin, with the sole exception of the fish family Channichthyidae. Hemoglobin ...
, higher number of reticulocytes and plasma bilirubin concentration, as well as increased liver- and spleen-somatic indices, was exclusively manifested in homozygous mutants.


Triosephosphate isomerase deficiency

The disease referred to as triosephosphate isomerase deficiency (TPI), is a severe autosomal recessive inherited multisystem disorder of glycolytic metabolism. It is characterized by hemolytic anemia and neurodegeneration, and is caused by anaerobic metabolic dysfunction. This dysfunction results from a missense mutation that effects the encoded TPI protein. The most common mutation is the substitution of gene, Glu104Asp, which produces the most severe
phenotype In genetics, the phenotype () is the set of observable characteristics or traits of an organism. The term covers the organism's morphology (physical form and structure), its developmental processes, its biochemical and physiological propert ...
, and is responsible for approximately 80% of clinical TPI deficiency. TPI deficiency is very rare with less than 50 cases reported in literature. Being an autosomal recessive inherited disease, TPI deficiency has a 25% recurrence risk in the case of heterozygous parents. It is a congenital disease that most often occurs with hemolytic anemia and manifests with jaundice. Most patients with TPI for Glu104Asp mutation or heterozygous for a TPI null allele and Glu104Asp have a life expectancy of infancy to early childhood. TPI patients with other mutations generally show longer life expectancy. There are only two cases of individuals with TPI living beyond the age of 6. These cases involve two brothers from Hungary, one who did not develop neurological symptoms until the age of 12, and the older brother who has no neurological symptoms and suffers from anemia only. Individuals with TPI show obvious symptoms after 6–24 months of age. These symptoms include: dystonia, tremor, dyskinesia, pyramidal tract signs, cardiomyopathy and spinal motor neuron involvement. Patients also show frequent respiratory system bacterial infections. TPI is detected through deficiency of enzymatic activity and the build-up of dihyroxyacetone phosphate(DHAP), which is a toxic substrate, in erythrocytes. This can be detected through physical examination and a series of lab work. In detection, there is generally myopathic changes seen in muscles and chronic axonal neuropathy found in the nerves. Diagnosis of TPI can be confirmed through molecular genetics. Chorionic villus DNA analysis or analysis of fetal red cells can be used to detect TPI in antenatal diagnosis. Treatment for TPI is not specific, but varies according to different cases. Because of the range of symptoms TPI causes, a team of specialist may be needed to provide treatment to a single individual. That team of specialists would consists of pediatricians, cardiologists, neurologists, and other healthcare professionals, that can develop a comprehensive plan of action. Supportive measures such as red cell transfusions in cases of severe anaemia can be taken to treat TPI as well. In some cases, spleen removal (splenectomy) may improve the anaemia. There is no treatment to prevent progressive neurological impairment of any other non-haematological clinical manifestation of the diseases.


Industrial applications

By far the most common use of isomerases in industrial applications is in
sugar Sugar is the generic name for sweet-tasting, soluble carbohydrates, many of which are used in food. Simple sugars, also called monosaccharides, include glucose Glucose is a sugar with the Chemical formula#Molecular formula, molecul ...
manufacturing. Glucose isomerase (also known as xylose isomerase) catalyzes the conversion of D- xylose and D-
glucose Glucose is a sugar with the Chemical formula#Molecular formula, molecular formula , which is often abbreviated as Glc. It is overall the most abundant monosaccharide, a subcategory of carbohydrates. It is mainly made by plants and most algae d ...
to D- xylulose and D-
fructose Fructose (), or fruit sugar, is a Ketose, ketonic monosaccharide, simple sugar found in many plants, where it is often bonded to glucose to form the disaccharide sucrose. It is one of the three dietary monosaccharides, along with glucose and gal ...
. Like most sugar isomerases, glucose isomerase catalyzes the interconversion of aldoses and
ketose In organic chemistry, a ketose is a monosaccharide containing one ketone () group per molecule. The simplest ketose is dihydroxyacetone (), which has only three carbon atoms. It is the only ketose with no optical activity. All monosaccharide keto ...
s. The conversion of glucose to fructose is a key component of high-fructose corn syrup production. Isomerization is more specific than older chemical methods of fructose production, resulting in a higher yield of fructose and no side products. The fructose produced from this isomerization reaction is purer with no residual flavors from
contaminants Contamination is the presence of a constituent, impurity, or some other undesirable element that renders something unsuitable, unfit or harmful for the physical body, natural environment, workplace, etc. Types of contamination Within the scie ...
. High-fructose corn syrup is preferred by many confectionery and soda manufacturers because of the high sweetening power of fructose (twice that of sucrose), its relatively low cost and its inability to crystallize. Fructose is also used as a sweetener for use by diabetics. Major issues of the use of glucose isomerase involve its inactivation at higher temperatures and the requirement for a high pH (between 7.0 and 9.0) in the reaction environment. Moderately high temperatures, above 70 °C, increase the yield of fructose by at least half in the isomerization step. The enzyme requires a
divalent In chemistry, the valence (US spelling) or valency (British spelling) of an atom is a measure of its combining capacity with other atoms when it forms chemical compounds or molecules. Valence is generally understood to be the number of chemica ...
cation An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by convent ...
such as Co2+ and Mg2+ for peak activity, an additional cost to manufacturers. Glucose isomerase also has a much higher affinity for xylose than for glucose, necessitating a carefully controlled environment. The isomerization of xylose to xylulose has its own commercial applications as interest in
biofuel Biofuel is a fuel that is produced over a short time span from Biomass (energy), biomass, rather than by the very slow natural processes involved in the formation of fossil fuels such as oil. Biofuel can be produced from plants or from agricu ...
s has increased. This reaction is often seen naturally in
bacteria Bacteria (; : bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of Prokaryote, prokaryotic microorganisms. Typically a few micr ...
that feed on decaying plant matter. Its most common industrial use is in the production of
ethanol Ethanol (also called ethyl alcohol, grain alcohol, drinking alcohol, or simply alcohol) is an organic compound with the chemical formula . It is an Alcohol (chemistry), alcohol, with its formula also written as , or EtOH, where Et is the ps ...
, achieved by the
fermentation Fermentation is a type of anaerobic metabolism which harnesses the redox potential of the reactants to make adenosine triphosphate (ATP) and organic end products. Organic molecules, such as glucose or other sugars, are catabolized and reduce ...
of xylulose. The use of
hemicellulose A hemicellulose (also known as polyose) is one of a number of heteropolymers (matrix polysaccharides), such as arabinoxylans, present along with cellulose in almost all embryophyte, terrestrial plant cell walls. Cellulose is crystalline, strong, an ...
as source material is very common. Hemicellulose contains xylan, which itself is composed of xylose in β(1,4) linkages. The use of glucose isomerase very efficiently converts xylose to xylulose, which can then be acted upon by fermenting
yeast Yeasts are eukaryotic, single-celled microorganisms classified as members of the fungus kingdom (biology), kingdom. The first yeast originated hundreds of millions of years ago, and at least 1,500 species are currently recognized. They are est ...
. Overall, extensive research in genetic engineering has been invested into optimizing glucose isomerase and facilitating its recovery from industrial applications for re-use. Glucose isomerase is able to catalyze the isomerization of a range of other sugars, including D-
ribose Ribose is a simple sugar and carbohydrate with molecular formula C5H10O5 and the linear-form composition H−(C=O)−(CHOH)4−H. The naturally occurring form, , is a component of the ribonucleotides from which RNA is built, and so this comp ...
, D- allose and L- arabinose. The most efficient substrates are those similar to glucose and xylose, having equatorial hydroxyl groups at the third and fourth carbons. The current model for the mechanism of glucose isomerase is that of a hydride shift based on
X-ray crystallography X-ray crystallography is the experimental science of determining the atomic and molecular structure of a crystal, in which the crystalline structure causes a beam of incident X-rays to Diffraction, diffract in specific directions. By measuring th ...
and isotope exchange studies.


Membrane-associated isomerases

Some isomerases associate with biological membranes as peripheral membrane proteins or anchored through a single transmembrane helix,Superfamilies of single-pass transmembrane lyases
in Membranome database
for example isomerases with the thioredoxin domain, and certain prolyl isomerases.


References


External links


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