Aspartate transaminase
pyridoxal 5-phosphate .
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Aspartate transaminase (AST) or aspartate aminotransferase, also known as AspAT/ASAT/AAT or (serum) glutamic oxaloacetic transaminase (GOT, SGOT), is a
The
Function
Aspartate transaminase catalyzes the interconversion of
L-Aspartate (Asp) + α-ketoglutarate ↔ oxaloacetate + L-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
Isoenzymes
Two isoenzymes are present in a wide variety of eukaryotes. In humans:[citation needed]
- 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%.[7]
AST has also been found in a number of microorganisms, including
Structure
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.[11][14]
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.[citation needed]
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.
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.[5][6]
- Internal aldimine formation: First, the ε-amino group of Lys258 forms a Schiff baselinkage 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 guanidiniumgroups of the enzyme's Arg386 and Arg 292 residues.
- 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.
- Ketimineformation: The quinonoid is reprotonated, but now at the aldehyde carbon, to form the ketimine intermediate.
- Ketimine hydrolysis: Finally, the ketimine is hydrolyzed to form PMP and oxaloacetate.
This mechanism is thought to have multiple partially rate-determining steps.[18] However, it has been shown that the substrate binding step (transaldimination) drives the catalytic reaction forward.[19]
Clinical significance
AST is similar to
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
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[22] |
Male | 8–40 IU/L |
Female | 6–34 IU/L |
See also
- Alanine transaminase (ALT/ALAT/SGPT)
- Transaminases
References
- PMID 13221663.
- PMID 13221664.
- PMID 13195683.
- ^ PMID 15684121.
Aminotransferase clearance is carried out within the liver by sinusoidal cells. The half-life in the circulation is about 47 hours for ALT, about 17 hours for total AST and, on average, 87 hours for mitochondrial AST.
- ^ PMID 6143829.
- ^ ISBN 978-0-7167-8724-2.
- ^ PMID 2197992.
- PMID 1909112.
- PMID 8907187.
- PMID 15983.
- ^ PMID 1593633.
- PMID 9211866.
- PMID 3071527.
- ^ PMID 1993208.
- PMID 1522585.
- PMID 15889412.
- PMID 1735441.
- PMID 8611515.
- PMID 12488449.
- ^ "AST/ALT". www.rnceus.com.
- PMID 17729182.
- ^ GPnotebook > reference range (AST) Retrieved on Dec 7, 2009 Archived 7 January 2017 at the Wayback Machine
Further reading
- ISBN 978-0-471-85142-4.
- Kuramitsu S, Okuno S, Ogawa T, Ogawa H, Kagamiyama H (April 1985). "Aspartate aminotransferase of Escherichia coli: nucleotide sequence of the aspC gene". Journal of Biochemistry. 97 (4): 1259–1262. PMID 3897210.
- Kondo K, Wakabayashi S, Yagi T, Kagamiyama H (July 1984). "The complete amino acid sequence of aspartate aminotransferase from Escherichia coli: sequence comparison with pig isoenzymes". Biochemical and Biophysical Research Communications. 122 (1): 62–67. PMID 6378205.
- Inoue K, Kuramitsu S, Okamoto A, Hirotsu K, Higuchi T, Kagamiyama H (August 1991). "Site-directed mutagenesis of Escherichia coli aspartate aminotransferase: role of Tyr70 in the catalytic processes". Biochemistry. 30 (31): 7796–7801. PMID 1868057.
External links
- Aspartate+Transaminase at the U.S. National Library of Medicine Medical Subject Headings (MeSH)
- AST - Lab Tests Online
- AST: MedlinePlus Medical Encyclopedia