Aspartate transaminase

aspartate transaminase
Aspartate aminotransferase from Escherichia coli bound with cofactor pyridoxal 5-phosphate.[1]
Identifiers
EC number	2.6.1.1
CAS number	9000-97-9
Databases
IntEnz	IntEnz view
BRENDA	BRENDA entry
ExPASy	NiceZyme view
KEGG	KEGG entry
MetaCyc	metabolic pathway
PRIAM	profile
PDB structures	RCSB PDB PDBe PDBsum
Gene Ontology	AmiGO / EGO
Search PMC articles PubMed articles NCBI proteins

Aspartate transaminase (AST) or aspartate aminotransferase, also known as AspAT/ASAT/AAT or serum glutamic oxaloacetic transaminase (SGOT), is a pyridoxal phosphate (PLP)-dependent transaminase enzyme (EC 2.6.1.1). 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. Serum AST level, serum ALT (alanine transaminase) level, and their ratio (AST/ALT ratio) are commonly measured clinically as biomarkers for liver health. The tests are part of blood panels.

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 (Vitamin B6) 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.^[2] 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 deamination 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.^[3]

Isoenzymes

Two isoenzymes are present in a wide variety of eukaryotes. In humans:

• GOT1/cAST, the cytosolic isoenzyme derives mainly from red blood cells and heart.
• GOT2/mAST, the mitochondrial 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%.^[4]

AST has also been found in a number of microorganisms, including E. coli, H. mediterranei,^[5] and T. thermophilus.^[6] 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 (EC 2.6.1.57).^[7]

Structure

X-ray crystallography studies have been performed to determine the structure of aspartate transaminase from various sources, including chicken mitochondria,^[8] pig heart cytosol,^[9] and E. coli.^[10][11] 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.^[4] 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.^[8][11][12]

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.^[8][11]

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.^[13] 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.^[14]

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.^[2][3]

Reaction mechanism for aspartate aminotransferase

1. Internal aldimine formation: First, the ε-amino group of Lys258 forms a Schiff base linkage with the aldehyde carbon to generate an internal aldimine.
2. 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.
3. Quinonoid 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.
4. Ketimine formation: The quinonoid is reprotonated, but now at the aldehyde carbon, to form the ketimine intermediate.
5. Ketimine hydrolysis: Finally, the ketimine is hydrolyzed to form PMP and oxaloacetate.

This mechanism is thought to have multiple partially rate-determining steps.^[15] However, it has been shown that the substrate binding step (transaldimination) drives the catalytic reaction forward.^[16]

Clinical significance

AST is similar to alanine transaminase (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.^[17] 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.^[18]

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.^[19]

AST is commonly measured clinically as a part of diagnostic liver function tests, to determine liver health. Laboratory tests should always be interpreted using the reference range from the laboratory that performed the test. Example reference ranges are shown below:

Patient type	Reference ranges[20]
Male	8 - 40 IU/L
Female	6 - 34 IU/L

^[21]

See also

• Alanine transaminase (ALT/ALAT/SGPT)
• Transaminases

References

Further reading

• Jansonius, JN; Vincent, MG (1987). Jurnak FA and McPherson A, ed. Structural basis for catalysis by aspartate aminotransferase (Biological Macromolecules and Assemblies, Vol. 3). New York: Wiley. pp. 187–285. ISBN 0-471-85142-6.  |accessdate= requires |url= (help)

• Kuramitsu S, Okuno S, Ogawa T, Ogawa H, Kagamiyama H (1985). "Aspartate aminotransferase of Escherichia coli: nucleotide sequence of the aspC gene". J. Biochem. 97 (4): 1259–62. PMID 3897210. 

• Kondo K, Wakabayashi S, Yagi T, Kagamiyama H (1984). "The complete amino acid sequence of aspartate aminotransferase from Escherichia coli: sequence comparison with pig isoenzymes". Biochem. Biophys. Res. Commun. 122 (1): 62–67. doi:10.1016/0006-291X(84)90439-X. PMID 6378205. 

• Inoue K, Kuramitsu S, Okamoto A, Hirotsu K, Higuchi T, Kagamiyama H (1991). "Site-directed mutagenesis of Escherichia coli aspartate aminotransferase: role of Tyr70 in the catalytic processes". Biochemistry 30 (31): 7796–7801. doi:10.1021/bi00245a019. PMID 1868057. 

External links

• Aspartate Transaminase at the US National Library of Medicine Medical Subject Headings (MeSH)
• AST - Lab Tests Online
• AST: MedlinePlus Medical Encyclopedia

Transferase: nitrogenous groups (EC 2.6) 2.6.1: Transaminases Aspartate transaminase GOT1 GOT2 Alanine transaminase GABA transaminase Tyrosine aminotransferase Ornithine aminotransferase Branched chain aminotransferase Alanine-glyoxylate transaminase AGXT 2.6.3: Oximinotransferases Oximinotransferase 2.6.99: Other Pyridoxine 5'-phosphate synthase Biochemistry overview Enzymes overview By EC number: 1.1 2 3 4 5 6 7 8 10 11 13 14 15-18 2.1 2 3 4 5 6 7 2.7.10 11-12 8 3.1 3.1.3.48 2 3 4 3.4.21 22 23 24 5 6 7 4.1 2 3 4 5 6 5.1 2 3 4 99 6.1-3 4 5-6

Mitochondrial proteins Outer membrane fatty acid degradation Carnitine palmitoyltransferase I Long-chain-fatty-acid—CoA ligase tryptophan metabolism Kynureninase monoamine neurotransmitter metabolism Monoamine oxidase Intermembrane space Adenylate kinase Creatine kinase Inner membrane oxidative phosphorylation Coenzyme Q – cytochrome c reductase Cytochrome c NADH dehydrogenase Succinate dehydrogenase pyrimidine metabolism Dihydroorotate dehydrogenase mitochondrial shuttle Malate-aspartate shuttle Glycerol phosphate shuttle other Glutamate aspartate transporter Glycerol-3-phosphate dehydrogenase ATP synthase Carnitine palmitoyltransferase II Uncoupling protein Matrix citric acid cycle Citrate synthase Aconitase Isocitrate dehydrogenase Oxoglutarate dehydrogenase complex Succinyl coenzyme A synthetase Fumarase Malate dehydrogenase anaplerotic reactions Aspartate transaminase Glutamate dehydrogenase Pyruvate dehydrogenase complex urea cycle Carbamoyl phosphate synthetase I Ornithine transcarbamylase N-Acetylglutamate synthase alcohol metabolism ALDH2 PMPCB Other/to be sorted steroidogenesis Cholesterol side-chain cleavage enzyme Steroid 11-beta-hydroxylase Aldosterone synthase Frataxin Mitochondrial membrane transport protein Mitochondrial permeability transition pore Mitochondrial carrier Mitochondrial DNA Complex I MT-ND1 MT-ND2 MT-ND3 MT-ND4 MT-ND4L MT-ND5 MT-ND6 Complex III MT-CYB Complex IV MT-CO1 MT-CO2 MT-CO3 ATP synthase MT-ATP6 MT-ATP8 tRNA MT-TA MT-TC MT-TD MT-TE MT-TF MT-TG MT-TH MT-TI MT-TK MT-TL1 MT-TL2 MT-TM MT-TN MT-TP MT-TQ MT-TR MT-TS1 MT-TS2 MT-TT MT-TV MT-TW MT-TY see also mitochondrial diseases Index of cells Description Structure nucleus chromosome genetics Organelles peroxisome cytoskeleton centrosome epithelia cilia mitochondria Membranes proteins cell adhesions Membrane transport ion channels vesicular transport solute carrier ABC transporters ATPase oxidoreduction-driven Disease Structural peroxisome cytoskeleton cilia mitochondria nucleus scleroprotein Membrane channelopathy solute carrier ATPase ABC transporters other extracellular ligands cell surface receptors intracellular signalling Vesicular transport Pore-forming toxins

Metabolism: Citric acid cycle enzymes Cycle Citrate synthase Aconitase Isocitrate dehydrogenase Oxoglutarate dehydrogenase Succinyl CoA synthetase Succinate dehydrogenase (SDHA) Fumarase Malate dehydrogenase and ETC Anaplerotic to acetyl-CoA Pyruvate dehydrogenase complex (E1, E2, E3) (regulated by Pyruvate dehydrogenase kinase and Pyruvate dehydrogenase phosphatase) to α-ketoglutaric acid Glutamate dehydrogenase to succinyl-CoA Methylmalonyl-CoA mutase to oxaloacetate Pyruvate carboxylase Aspartate transaminase Mitochondrial electron transport chain/ oxidative phosphorylation Primary Complex I/NADH dehydrogenase Complex II/Succinate dehydrogenase Coenzyme Q Complex III/Coenzyme Q - cytochrome c reductase Cytochrome c Complex IV/Cytochrome c oxidase Coenzyme Q10 synthesis: COQ2 COQ3 COQ4 COQ5 COQ6 COQ7 COQ9 COQ10A COQ10B PDSS1 PDSS2 Other Alternative oxidase Electron-transferring-flavoprotein dehydrogenase Index of inborn errors of metabolism Description Metabolism Enzymes and pathways: citric acid cycle enzymes pentose phosphate intermediates glycoproteins enzymes glycosaminoglycans proteoglycan enzymes phospholipid metabolism cholesterol and steroid intermediates sphingolipids enzymes eicosanoids enzymes amino acid intermediates urea cycle enzymes nucleotide intermediates Disorders Citric acid cycle and electron transport chain Glycoprotein Proteoglycan Fatty-acid Phospholipid Cholesterol and steroid Eicosanoid Amino acid Purine-pyrimidine Heme metabolism Symptoms and signs Treatment Drugs other

Metabolism: Protein metabolism, synthesis and catabolism enzymes Essential amino acids are in Capitals K→acetyl-CoA LYSINE→ Saccharopine dehydrogenase Glutaryl-CoA dehydrogenase LEUCINE→ Branched chain aminotransferase Branched-chain alpha-keto acid dehydrogenase complex Isovaleryl coenzyme A dehydrogenase Methylcrotonyl-CoA carboxylase Methylglutaconyl-CoA hydratase 3-hydroxy-3-methylglutaryl-CoA lyase TRYPTOPHAN→ Indoleamine 2,3-dioxygenase/Tryptophan 2,3-dioxygenase Arylformamidase Kynureninase 3-hydroxyanthranilate oxidase Aminocarboxymuconate-semialdehyde decarboxylase Aminomuconate-semialdehyde dehydrogenase PHENYLALANINE→tyrosine→ (see below) G G→pyruvate →citrate glycine→serine→ Serine hydroxymethyltransferase Serine dehydratase glycine→creatine: Guanidinoacetate N-methyltransferase Creatine kinase alanine→ Alanine transaminase cysteine→ D-cysteine desulfhydrase threonine→ L-threonine dehydrogenase G→glutamate→ α-ketoglutarate HISTIDINE→ Histidine ammonia-lyase Urocanate hydratase Formiminotransferase cyclodeaminase proline→ Proline oxidase Pyrroline-5-carboxylate reductase 1-Pyrroline-5-carboxylate dehydrogenase/ALDH4A1 PYCR1 arginine→ Ornithine aminotransferase Ornithine decarboxylase Agmatinase →alpha-ketoglutarate→TCA Glutamate dehydrogenase Other cysteine+glutamate→glutathione: Gamma-glutamylcysteine synthetase Glutathione synthetase Gamma-glutamyl transpeptidase glutamate→glutamine: Glutamine synthetase Glutaminase G→propionyl-CoA→ succinyl-CoA VALINE→ Branched chain aminotransferase Branched-chain alpha-keto acid dehydrogenase complex Enoyl-CoA hydratase 3-hydroxyisobutyryl-CoA hydrolase 3-hydroxyisobutyrate dehydrogenase Methylmalonate semialdehyde dehydrogenase ISOLEUCINE→ Branched chain aminotransferase Branched-chain alpha-keto acid dehydrogenase complex 3-hydroxy-2-methylbutyryl-CoA dehydrogenase METHIONINE→ generation of homocysteine: Methionine adenosyltransferase Adenosylhomocysteinase regeneration of methionine: Methionine synthase/Homocysteine methyltransferase Betaine-homocysteine methyltransferase conversion to cysteine: Cystathionine beta synthase Cystathionine gamma-lyase THREONINE→ Threonine aldolase →succinyl-CoA→TCA Propionyl-CoA carboxylase Methylmalonyl CoA epimerase Methylmalonyl-CoA mutase G→fumarate PHENYLALANINE→tyrosine→ Phenylalanine hydroxylase Tyrosine aminotransferase 4-Hydroxyphenylpyruvate dioxygenase Homogentisate 1,2-dioxygenase Fumarylacetoacetate hydrolase tyrosine→melanin: Tyrosinase G→oxaloacetate asparagine→aspartate→ Asparaginase/Asparagine synthetase Aspartate transaminase Index of inborn errors of metabolism Description Metabolism Enzymes and pathways: citric acid cycle enzymes pentose phosphate intermediates glycoproteins enzymes glycosaminoglycans proteoglycan enzymes phospholipid metabolism cholesterol and steroid intermediates sphingolipids enzymes eicosanoids enzymes amino acid intermediates urea cycle enzymes nucleotide intermediates Disorders Citric acid cycle and electron transport chain Glycoprotein Proteoglycan Fatty-acid Phospholipid Cholesterol and steroid Eicosanoid Amino acid Purine-pyrimidine Heme metabolism Symptoms and signs Treatment Drugs other

Medical test: Serology, Reference range: Clinical biochemistry Blood tests (including BMP, CMP) (CPT 82000–84999) Fluid/electrolytes electrolytes Na+/K+ Cl−/HCO3− renal function, BUN-to-creatinine ratio BUN/Creatinine Ca derived values: Plasma osmolality Serum osmolal gap Acid-base Arterial blood gas Base excess Anion gap CO2 content Nutrition Iron tests: Transferrin saturation = Serum iron / Total iron-binding capacity; Ferritin Transferrin Transferrin receptor Endocrine ACTH stimulation test Thyroid function tests TSH Blood sugar: Glucose test C-peptide Fructosamine Glycated hemoglobin Metabolic Blood lipids Cardiovascular Cardiac marker: Troponin test CPK-MB test LDH Myoglobin Glycogen phosphorylase isoenzyme BB Digestive Liver function tests: protein tests Human serum albumin Serum total protein ALP transaminases ALT AST AST/ALT ratio Bilirubin Unconjugated Conjugated Amylase Lipase Pancreatic lipase Index of the urinary system Description Anatomy Physiology Development Cells Disease Electrolyte and acid-base Congenital Neoplasms and cancer Other Symptoms and signs eponymous Urine tests findings Blood tests Treatment Procedures Drugs potassium and phosphate Intravenous fluids Index of hormones Description Glands Hormones thyroid intermediates metabolism mineralocorticoids Physiology Development Disease Diabetes Congenital Neoplasms and cancer Other Symptoms and signs eponymous Treatment Procedures Drugs calcium balance corticosteroids oral hypoglycemics pituitary and hypothalamic thyroid Index of the heart Description Anatomy Physiology Development Disease Injury Congenital Neoplasms and cancer Other Symptoms and signs eponymous Blood tests Treatment Procedures Drugs glycosides other stimulants antiarrhythmics vasodilators Index of digestion Description Anatomy tract glands other Physiology enzymes Development Disease Congenital Neoplasms and cancer Inflammatory bowel disease Gluten sensitivity Other Symptoms and signs eponymous Blood tests Treatment Procedures Drugs anabolic steroids antacids diarrhoea and infection bile and liver functional gastrointestinal disorders laxatives peptic ulcer and reflux nausea and vomiting other Surgery