Aspartate transaminase
Encyclopedia
Aspartate transaminase (AST), also called aspartate aminotransferase (AspAT/ASAT/AAT) or serum glutamic oxaloacetic transaminase (SGOT), is a pyridoxal phosphate (PLP)-dependent transaminase
enzyme . AST catalyzes the reversible transfer of an α-amino group between aspartate and glutamate and, as such, is an important enzyme in amino acid metabolism. AST is found in the liver
, heart
, skeletal muscle
, kidneys, brain
, and red blood cells, and it is commonly measured clinically as a marker for liver health.
Aspartate (Asp) + α-ketoglutarate ⇌ oxaloacetate + glutamate (Glu)
As a prototypical transaminase, AST relies on PLP as a cofactor to transfer the amino group from aspartate or glutamate to the corresponding ketoacid. In the process, the cofactor shuttles between PLP and the pyridoxamine phosphate (PMP) form. The amino group transfer catalyzed by this enzyme is crucial in both amino acid degradation and biosynthesis. In amino acid degradation, following the conversion of α-ketoglutarate to glutamate, glutamate subsequently undergoes oxidative deamidation to form ammonium
ions, which are excreted as urea
. In the reverse reaction, aspartate may be synthesized from oxaloacetate, which is a key intermediate in the citric acid cycle
.
These isoenzymes are thought to have evolved from a common ancestral AST via gene duplication, and they share a sequence homology of approximately 45%.
AST has also been found in a number of microorganisms, including E. coli, H. mediterranei
, and T. thermophilus
. In E. coli, the enzyme is encoded by the aspCgene and has also been shown to exhibit the activity of an aromatic-amino-acid transaminase
.
studies have been performed to determine the structure of aspartate transaminase from various sources, including chicken mitochondria, pig heart cytosol, and E. coli. Overall, the three-dimensional polypeptide structure for all species is quite similar. AST is dimeric
, consisting of two identical subunits, each with approximately 400 amino acid residues and a molecular weight of approximately 45 kD. Each subunit is composed of a large and a small domain, as well as a third domain consisting of the N-terminal residues 3-14; these few residues form a strand, which links and stabilizes the two subunits of the dimer. The large domain, which includes residues 48-325, binds the PLP cofactor via an aldimine
linkage to the ε-amino group of Lys258. Other residues in this domain – Asp 222 and Tyr 225 – also interact with PLP via hydrogen bonding. The small domain consists of residues 15-47 and 326-410 and represents a flexible region that shifts the enzyme from an "open" to a "closed" conformation upon substrate binding.
The two independent active sites are positioned near the interface between the two domains. Within each active site, a couple arginine residues are responsible for the enzyme’s specificity for dicarboxylic acid
substrates: Arg386 interacts with the substrate’s proximal (α-)carboxylate group, while Arg292 complexes with the distal (side-chain) carboxylate.
In terms of secondary structure, AST contains both α and β elements. Each domain has a central sheet of β-strands with α-helices packed on either side.
The specific steps for the half-reaction of Enzyme-PLP + aspartate ⇌ Enzyme-PMP + oxaloacetate are as follows (see figure); the other half-reaction (not shown) proceeds in the reverse manner, with α-ketoglutarate as the substrate.
This mechanism is thought to have multiple partially rate-determining step
s. However, it has been shown that the substrate binding step (transaldimination) drives the catalytic reaction forward.
(ALT) in that both enzymes are associated with liver parenchymal cells. The difference is that ALT is found predominantly in the liver, with clinically negligible quantities found in the kidneys, heart, and skeletal muscle, while AST is found in the liver, heart (cardiac muscle), skeletal muscle, kidneys, brain, and red blood cells. As a result, ALT is a more specific indicator of liver inflammation
than AST, as AST may be elevated also in diseases affecting other organs, such as myocardial infarction
, acute pancreatitis
, acute hemolytic anemia
, severe burns, acute renal disease, musculoskeletal diseases, and trauma.
AST was defined as a biochemical marker for the diagnosis of acute myocardial infarction in 1954. However, the use of AST for such a diagnosis is now redundant and has been superseded by the cardiac troponins
.
AST (SGOT) is commonly measured clinically as a part of diagnostic liver function tests
, to determine liver health.
|-
|Patient type>Reference range
s
>-
| Female
| Male 8 - 40 IU/L
It has also been shown to be a marker for chronic alcoholism.
Transaminase
In biochemistry, a transaminase or an aminotransferase is an enzyme that catalyzes a type of reaction between an amino acid and an α-keto acid. To be specific, this reaction involves removing the amino group from the amino acid, leaving behind an α-keto acid, and transferring it to the...
enzyme . AST catalyzes the reversible transfer of an α-amino group between aspartate and glutamate and, as such, is an important enzyme in amino acid metabolism. AST is found in the liver
Liver
The liver is a vital organ present in vertebrates and some other animals. It has a wide range of functions, including detoxification, protein synthesis, and production of biochemicals necessary for digestion...
, heart
Heart
The heart is a myogenic muscular organ found in all animals with a circulatory system , that is responsible for pumping blood throughout the blood vessels by repeated, rhythmic contractions...
, skeletal muscle
Skeletal muscle
Skeletal muscle is a form of striated muscle tissue existing under control of the somatic nervous system- i.e. it is voluntarily controlled. It is one of three major muscle types, the others being cardiac and smooth muscle...
, kidneys, brain
Brain
The brain is the center of the nervous system in all vertebrate and most invertebrate animals—only a few primitive invertebrates such as sponges, jellyfish, sea squirts and starfishes do not have one. It is located in the head, usually close to primary sensory apparatus such as vision, hearing,...
, and red blood cells, and it is commonly measured clinically as a marker for liver health.
Function
Aspartate transaminase catalyzes the interconversion of aspartate and α-ketoglutarate to oxaloacetate and glutamate.Aspartate (Asp) + α-ketoglutarate ⇌ oxaloacetate + glutamate (Glu)
As a prototypical transaminase, AST relies on PLP as a cofactor to transfer the amino group from aspartate or glutamate to the corresponding ketoacid. In the process, the cofactor shuttles between PLP and the pyridoxamine phosphate (PMP) form. The amino group transfer catalyzed by this enzyme is crucial in both amino acid degradation and biosynthesis. In amino acid degradation, following the conversion of α-ketoglutarate to glutamate, glutamate subsequently undergoes oxidative deamidation to form ammonium
Ammonium
The ammonium cation is a positively charged polyatomic cation with the chemical formula NH. It is formed by the protonation of ammonia...
ions, which are excreted as urea
Urea
Urea or carbamide is an organic compound with the chemical formula CO2. The molecule has two —NH2 groups joined by a carbonyl functional group....
. In the reverse reaction, aspartate may be synthesized from oxaloacetate, which is a key intermediate in the citric acid cycle
Citric acid cycle
The citric acid cycle — also known as the tricarboxylic acid cycle , the Krebs cycle, or the Szent-Györgyi-Krebs cycle — is a series of chemical reactions which is used by all aerobic living organisms to generate energy through the oxidization of acetate derived from carbohydrates, fats and...
.
Isozymes
Two isoenzymes are present in a wide variety of eukaryotes. In humans:- GOT1GOT1Aspartate aminotransferase, cytoplasmic is an enzyme that in humans is encoded by the GOT1 gene.-Further reading:...
/cAST, the cytosolCytosolThe cytosol or intracellular fluid is the liquid found inside cells, that is separated into compartments by membranes. For example, the mitochondrial matrix separates the mitochondrion into compartments....
ic isoenzyme derives mainly from red blood cellRed blood cellRed blood cells are the most common type of blood cell and the vertebrate organism's principal means of delivering oxygen to the body tissues via the blood flow through the circulatory system...
s and heartHeartThe heart is a myogenic muscular organ found in all animals with a circulatory system , that is responsible for pumping blood throughout the blood vessels by repeated, rhythmic contractions...
. - GOT2GOT2Aspartate aminotransferase, mitochondrial is an enzyme that in humans is encoded by the GOT2 gene.-Further reading:...
/mAST, the mitochondrialMitochondrionIn cell biology, a mitochondrion is a membrane-enclosed organelle found in most eukaryotic cells. These organelles range from 0.5 to 1.0 micrometers in diameter...
isoenzyme is present predominantly in liver.
These isoenzymes are thought to have evolved from a common ancestral AST via gene duplication, and they share a sequence homology of approximately 45%.
AST has also been found in a number of microorganisms, including E. coli, H. mediterranei
Haloferax
In taxonomy, Haloferax is a genus of the Halobacteriaceae.- Description and Significance :Haloferax volcanii is a species of halophile which exists in extreme saline environments. Recently an isolate of this species was studied by researchers at University of California-Berkeley as part of a...
, and T. thermophilus
Thermus thermophilus
Thermus thermophilus is a Gram negative eubacterium used in a range of biotechnological applications, including as a model organism for genetic manipulation, structural genomics, and systems biology. The bacterium is extremely thermophilic, with an optimal growth temperature of about...
. In E. coli, the enzyme is encoded by the aspCgene and has also been shown to exhibit the activity of an aromatic-amino-acid transaminase
Aromatic-amino-acid transaminase
In enzymology, an aromatic-amino-acid transaminase is an enzyme that catalyzes the chemical reactionThus, the two substrates of this enzyme are aromatic amino acid and 2-oxoglutarate, whereas its two products are aromatic oxo acid and L-glutamate....
.
Structure
X-ray crystallographyX-ray crystallography
X-ray crystallography is a method of determining the arrangement of atoms within a crystal, in which a beam of X-rays strikes a crystal and causes the beam of light to spread into many specific directions. From the angles and intensities of these diffracted beams, a crystallographer can produce a...
studies have been performed to determine the structure of aspartate transaminase from various sources, including chicken mitochondria, pig heart cytosol, and E. coli. Overall, the three-dimensional polypeptide structure for all species is quite similar. AST is dimeric
Protein dimer
In biochemistry, a dimer is a macromolecular complex formed by two, usually non-covalently bound, macromolecules like proteins or nucleic acids...
, consisting of two identical subunits, each with approximately 400 amino acid residues and a molecular weight of approximately 45 kD. Each subunit is composed of a large and a small domain, as well as a third domain consisting of the N-terminal residues 3-14; these few residues form a strand, which links and stabilizes the two subunits of the dimer. The large domain, which includes residues 48-325, binds the PLP cofactor via an aldimine
Aldimine
In organic chemistry, an aldimine is an imine that is an analog of an aldehyde.As such, aldimines have the general formula R–CH=N–R. Aldimines are similar to ketimines, which are analogs of ketones.An important subset of aldimines are the Schiff bases,...
linkage to the ε-amino group of Lys258. Other residues in this domain – Asp 222 and Tyr 225 – also interact with PLP via hydrogen bonding. The small domain consists of residues 15-47 and 326-410 and represents a flexible region that shifts the enzyme from an "open" to a "closed" conformation upon substrate binding.
The two independent active sites are positioned near the interface between the two domains. Within each active site, a couple arginine residues are responsible for the enzyme’s specificity for dicarboxylic acid
Dicarboxylic acid
Dicarboxylic acids are organic compounds that contain two carboxylic acid functional groups. In molecular formulae for dicarboxylic acids, these groups are often written as HOOC-R-COOH, where R may be an alkyl, alkenyl, alkynyl, or aryl group...
substrates: Arg386 interacts with the substrate’s proximal (α-)carboxylate group, while Arg292 complexes with the distal (side-chain) carboxylate.
In terms of secondary structure, AST contains both α and β elements. Each domain has a central sheet of β-strands with α-helices packed on either side.
Mechanism
Aspartate transaminase, as with all transaminases, operates via dual substrate recognition; that is, it is able to recognize and selectively bind two amino acids (Asp and Glu) with different side-chains. In either case, the transaminase reaction consists of two similar half-reactions that constitute what is referred to as a ping-pong mechanism. In the first half-reaction, amino acid 1 (e.g., L-Asp) reacts with the enzyme-PLP complex to generate ketoacid 1 (oxaloacetate) and the modified enzyme-PMP. In the second half-reaction, ketoacid 2 (α-ketoglutarate) reacts with enzyme-PMP to produce amino acid 2 (L-Glu), regenerating the original enzyme-PLP in the process. Formation of a racemic product (D-Glu) is very rare.The specific steps for the half-reaction of Enzyme-PLP + aspartate ⇌ Enzyme-PMP + oxaloacetate are as follows (see figure); the other half-reaction (not shown) proceeds in the reverse manner, with α-ketoglutarate as the substrate.
- Internal aldimineAldimineIn organic chemistry, an aldimine is an imine that is an analog of an aldehyde.As such, aldimines have the general formula R–CH=N–R. Aldimines are similar to ketimines, which are analogs of ketones.An important subset of aldimines are the Schiff bases,...
formation: First, the ε-amino group of Lys258 forms a Schiff baseSchiff baseA Schiff base, named after Hugo Schiff, is a compound with a functional group that contains a carbon-nitrogen double bond with the nitrogen atom connected to an aryl or alkyl group, not hydrogen....
linkage with the aldehyde carbon to generate an internal aldimine. - Transaldimination: The internal aldimine then becomes an external aldimine when the ε-amino group of Lys258 is displaced by the amino group of aspartate. This transaldimination reaction occurs via a nucleophilic attack by the deprotonated amino group of Asp and proceeds through a tetrahedral intermediate. As this point, the carboxylate groups of Asp are stabilized by the guanidinium groups of the enzyme’s Arg386 and Arg 292 residues.
- QuinonoidQuinonoid zwitterionA Quinonoid zwitterion is a special type of zwitterion based on quinone related chemical compounds. The benzene derivate 1,3-dihydroxy-4,6-diaminobenzene is easily oxidized by air in water or methanol to the quinonoid. This compound was first prepared in 1883 and the quinonoid structure first...
formation: The hydrogen attached to the a-carbon of Asp is then abstracted (Lys258 is thought to be the proton acceptor) to form a quinonoid intermediate. - Ketimine formation: The quinonoid is reprotonated, but now at the aldehyde carbon, to form the ketimine intermediate.
- Ketimine hydrolysisHydrolysisHydrolysis is a chemical reaction during which molecules of water are split into hydrogen cations and hydroxide anions in the process of a chemical mechanism. It is the type of reaction that is used to break down certain polymers, especially those made by condensation polymerization...
: Finally, the ketimine is hydrolyzed to form PMP and oxaloacetate.
This mechanism is thought to have multiple partially rate-determining step
Rate-determining step
The rate-determining step is a chemistry term for the slowest step in a chemical reaction. The rate-determining step is often compared to the neck of a funnel; the rate at which water flows through the funnel is determined by the width of the neck, not by the speed at which water is poured in. In...
s. However, it has been shown that the substrate binding step (transaldimination) drives the catalytic reaction forward.
Clinical significance
AST is similar to alanine transaminaseAlanine transaminase
Alanine transaminase or ALT is a transaminase enzyme . It is also called serum glutamic pyruvic transaminase or alanine aminotransferase ....
(ALT) in that both enzymes are associated with liver parenchymal cells. The difference is that ALT is found predominantly in the liver, with clinically negligible quantities found in the kidneys, heart, and skeletal muscle, while AST is found in the liver, heart (cardiac muscle), skeletal muscle, kidneys, brain, and red blood cells. As a result, ALT is a more specific indicator of liver inflammation
Inflammation
Inflammation is part of the complex biological response of vascular tissues to harmful stimuli, such as pathogens, damaged cells, or irritants. Inflammation is a protective attempt by the organism to remove the injurious stimuli and to initiate the healing process...
than AST, as AST may be elevated also in diseases affecting other organs, such as myocardial infarction
Myocardial infarction
Myocardial infarction or acute myocardial infarction , commonly known as a heart attack, results from the interruption of blood supply to a part of the heart, causing heart cells to die...
, acute pancreatitis
Acute pancreatitis
Acute pancreatitis or acute pancreatic necrosis is a sudden inflammation of the pancreas. It can have severe complications and high mortality despite treatment...
, acute hemolytic anemia
Hemolytic anemia
Hemolytic anemia is a form of anemia due to hemolysis, the abnormal breakdown of red blood cells , either in the blood vessels or elsewhere in the human body . It has numerous possible causes, ranging from relatively harmless to life-threatening...
, severe burns, acute renal disease, musculoskeletal diseases, and trauma.
AST was defined as a biochemical marker for the diagnosis of acute myocardial infarction in 1954. However, the use of AST for such a diagnosis is now redundant and has been superseded by the cardiac troponins
Troponin test
The troponin test can be used as a test of several different heart disorders, including myocardial infarction.-Cardiac conditions:Certain subtypes of troponin are very sensitive and specific indicators of damage to the heart muscle...
.
AST (SGOT) is commonly measured clinically as a part of diagnostic liver function tests
Liver function tests
Liver function tests , are groups of clinical biochemistry laboratory blood assays designed to give information about the state of a patient's liver. The parameters measured include PT/INR, aPTT, albumin, billirubin and others...
, to determine liver health.
|Patient type>
Reference range
In health-related fields, a reference range or reference interval usually describes the variations of a measurement or value in healthy individuals...
s
>-
| Female
| Male
It has also been shown to be a marker for chronic alcoholism.