Editing Aspartate transaminase

{{Short description|Enzyme involved in amino acid metabolism}}
{{enzyme
| Name = aspartate transaminase
| EC_number = 2.6.1.1
| CAS_number = 9000-97-9
| GO_code = 0004069
| image = Gallus gallus aspartate aminotransferase monomer.png
| width =
| caption =  [[Chicken]] aspartate aminotransferase bound with coenzyme [[pyridoxal-phosphate|pyridoxal 5-phosphate]].
{{PDB|7AAT}}
|name=}}

'''Aspartate transaminase''' ('''AST''') or '''aspartate aminotransferase''', also known as '''AspAT/ASAT/AAT''' or '''(serum) glutamic oxaloacetic transaminase''' ('''GOT''', '''SGOT'''), is a [[pyridoxal phosphate]] (PLP)-dependent [[transaminase]] enzyme ({{EC number|2.6.1.1}}) that was first described by Arthur Karmen and colleagues in 1954.<ref>{{Cite journal |vauthors=Karmen A, Wroblewski F, Ladue JS |date=January 1955 |title=Transaminase activity in human blood |journal=The Journal of Clinical Investigation |volume=34 |issue=1 |pages=126–131 |doi=10.1172/jci103055 |pmc=438594 |pmid=13221663}}</ref><ref>{{Cite journal |vauthors=Karmen A |date=January 1955 |title=A note on the spectrometric assay of glutamic-oxalacetic transaminase in human blood serum |journal=The Journal of Clinical Investigation |volume=34 |issue=1 |pages=131–133 |doi=10.1172/JCI103055 |pmc=438594 |pmid=13221664}}</ref><ref>{{Cite journal |vauthors=Ladue JS, Wroblewski F, Karmen A |date=September 1954 |title=Serum glutamic oxaloacetic transaminase activity in human acute transmural myocardial infarction |journal=Science |volume=120 |issue=3117 |pages=497–499 |bibcode=1954Sci...120..497L |doi=10.1126/science.120.3117.497 |pmid=13195683}}</ref> 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]], red blood cells and gall bladder. Serum AST level, serum ALT ([[alanine transaminase]]) level, and their ratio ([[AST/ALT ratio]]) are commonly measured clinically as [[biomarker]]s for liver health. The tests are part of [[blood test|blood panels]].

The [[biological half-life|half-life]] of total AST in the circulation approximates 17 hours and, on average, 87 hours for ''mitochondrial'' AST.<ref name="Giannini pp. 367–379">{{Cite journal |vauthors=Giannini EG, Testa R, Savarino V |date=February 2005 |title=Liver enzyme alteration: a guide for clinicians |journal=CMAJ |volume=172 |issue=3 |pages=367–379 |doi=10.1503/cmaj.1040752 |pmc=545762 |pmid=15684121 |quote=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.}}</ref> [[Aminotransferase]] is cleared by [[Liver sinusoidal endothelial cell|sinusoidal cell]]s in the liver.<ref name="Giannini pp. 367–379" />

== Function ==
Aspartate transaminase catalyzes the interconversion of [[aspartate]] (Asp) and [[α-ketoglutarate]] to [[oxaloacetate]] and [[glutamate]] (Glu):

: L-aspartate + α-ketoglutarate ↔ oxaloacetate + L-glutamate

[[File:Aspartate aminotransferase reaction.png|class=skin-invert-image|thumb|450px|Reaction catalyzed by aspartate aminotransferase]]

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.<ref name="pmid6143829">{{Cite journal |vauthors=Kirsch JF, Eichele G, Ford GC, Vincent MG, Jansonius JN, Gehring H, Christen P |date=April 1984 |title=Mechanism of action of aspartate aminotransferase proposed on the basis of its spatial structure |journal=Journal of Molecular Biology |volume=174 |issue=3 |pages=497–525 |doi=10.1016/0022-2836(84)90333-4 |pmid=6143829}}</ref> 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]].<ref name="Biochemistry">{{Cite book |title=Biochemistry |vauthors=Berg JM, Tymoczko JL, Stryer L |publisher=W.H. Freeman |year=2006 |isbn=978-0-7167-8724-2 |pages=656–660}}</ref>

==Isoenzymes==
Two isoenzymes are present in a wide variety of eukaryotes.  In humans:{{cn|date=November 2021}}
* [[GOT1]]/cAST, the [[cytosol]]ic isoenzyme derives mainly from [[red blood cell]]s and [[heart]].
* [[GOT2]]/mAST, the [[Mitochondrion|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%.<ref name="pmid2197992">{{Cite journal |vauthors=Hayashi H, Wada H, Yoshimura T, Esaki N, Soda K |year=1990 |title=Recent topics in pyridoxal 5'-phosphate enzyme studies |journal=Annual Review of Biochemistry |volume=59 |pages=87–110 |doi=10.1146/annurev.bi.59.070190.000511 |pmid=2197992}}</ref>

AST has also been found in a number of microorganisms, including ''[[E. coli]]'', ''[[Haloferax|H. mediterranei]]'',<ref name="pmid1909112">{{Cite journal |vauthors=Muriana FJ, Alvarez-Ossorio MC, Relimpio AM |date=August 1991 |title=Purification and characterization of aspartate aminotransferase from the halophile archaebacterium Haloferax mediterranei |journal=The Biochemical Journal |volume=278 |issue=1 |pages=149–154 |doi=10.1042/bj2780149 |pmc=1151461 |pmid=1909112}}</ref> and ''[[Thermus thermophilus|T. thermophilus]]''.<ref name="pmid8907187">{{Cite journal |vauthors=Okamoto A, Kato R, Masui R, Yamagishi A, Oshima T, Kuramitsu S |date=January 1996 |title=An aspartate aminotransferase from an extremely thermophilic bacterium, Thermus thermophilus HB8 |journal=Journal of Biochemistry |volume=119 |issue=1 |pages=135–144 |doi=10.1093/oxfordjournals.jbchem.a021198 |pmid=8907187}}</ref> In ''E. coli'', the enzyme is encoded by the ''aspC''gene and has also been shown to exhibit the activity of an [[aromatic-amino-acid transaminase]] ({{EC number|2.6.1.57}}).<ref name="pmid15983">{{Cite journal |vauthors=Gelfand DH, Steinberg RA |date=April 1977 |title=Escherichia coli mutants deficient in the aspartate and aromatic amino acid aminotransferases |journal=Journal of Bacteriology |volume=130 |issue=1 |pages=429–440 |doi=10.1128/JB.130.1.429-440.1977 |pmc=235221 |pmid=15983}}</ref>

== Structure ==
[[File:Gallus gallus aspartate aminotransferase dimer.png|thumb|449x449px|Structure of the aspartate transaminase dimer from chicken heart mitochondria. The large and small domains are coloured blue and red, respectively with the N-terminal residues highlighted in green. {{PDB|7AAT}}]]
[[X-ray crystallography]] studies have been performed to determine the structure of aspartate transaminase from various sources, including chicken mitochondria,<ref name="pmid1593633">{{Cite journal |vauthors=McPhalen CA, Vincent MG, Jansonius JN |date=May 1992 |title=X-ray structure refinement and comparison of three forms of mitochondrial aspartate aminotransferase |journal=Journal of Molecular Biology |volume=225 |issue=2 |pages=495–517 |doi=10.1016/0022-2836(92)90935-D |pmid=1593633}}</ref> pig heart cytosol,<ref name="pmid9211866">{{Cite journal |vauthors=Rhee S, Silva MM, Hyde CC, Rogers PH, Metzler CM, Metzler DE, Arnone A |date=July 1997 |title=Refinement and comparisons of the crystal structures of pig cytosolic aspartate aminotransferase and its complex with 2-methylaspartate |journal=The Journal of Biological Chemistry |volume=272 |issue=28 |pages=17293–17302 |doi=10.1074/jbc.272.28.17293 |pmid=9211866 |doi-access=free}}</ref> and ''E. coli''.<ref name="pmid3071527">{{Cite journal |display-authors=6 |vauthors=Kamitori S, Hirotsu K, Higuchi T, Kondo K, Inoue K, Kuramitsu S, Kagamiyama H, Higuchi Y, Yasuoka N, Kusunoki M |date=September 1988 |title=Three-dimensional structure of aspartate aminotransferase from Escherichia coli at 2.8 A resolution |journal=Journal of Biochemistry |volume=104 |issue=3 |pages=317–318 |doi=10.1093/oxfordjournals.jbchem.a122464 |pmid=3071527}}</ref><ref name="pmid1993208">{{Cite journal |vauthors=Danishefsky AT, Onnufer JJ, Petsko GA, Ringe D |date=February 1991 |title=Activity and structure of the active-site mutants R386Y and R386F of Escherichia coli aspartate aminotransferase |journal=Biochemistry |volume=30 |issue=7 |pages=1980–1985 |doi=10.1021/bi00221a035 |pmid=1993208}}</ref>  Overall, the three-dimensional polypeptide structure for all species is quite similar.  AST is [[Protein dimer|dimeric]], consisting of two identical subunits, each with approximately 400 amino acid residues and a molecular weight of approximately 45 kD.<ref name="pmid2197992" />  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.<ref name="pmid1593633" /><ref name="pmid1993208" /><ref name="pmid1522585">{{Cite journal |vauthors=McPhalen CA, Vincent MG, Picot D, Jansonius JN, Lesk AM, Chothia C |date=September 1992 |title=Domain closure in mitochondrial aspartate aminotransferase |journal=Journal of Molecular Biology |volume=227 |issue=1 |pages=197–213 |doi=10.1016/0022-2836(92)90691-C |pmid=1522585}}</ref>

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.<ref name="pmid1593633" /><ref name="pmid1993208" />

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.{{cn|date=November 2021}}

==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.<ref name="pmid15889412">{{Cite journal |vauthors=Hirotsu K, Goto M, Okamoto A, Miyahara I |year=2005 |title=Dual substrate recognition of aminotransferases |journal=Chemical Record |volume=5 |issue=3 |pages=160–172 |doi=10.1002/tcr.20042 |pmid=15889412}}</ref> 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.<ref name="pmid1735441">{{Cite journal |vauthors=Kochhar S, Christen P |date=February 1992 |title=Mechanism of racemization of amino acids by aspartate aminotransferase |journal=European Journal of Biochemistry |volume=203 |issue=3 |pages=563–569 |doi=10.1111/j.1432-1033.1992.tb16584.x |pmid=1735441 |doi-access=free}}</ref>

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.<ref name="pmid6143829" /><ref name="Biochemistry" />

[[File:Aspartate aminotransferase mechanism.png|class=skin-invert-image|thumb|center|1000px|Reaction mechanism for aspartate aminotransferase]]

# Internal [[aldimine]] formation:  First, the ε-amino group of Lys258 forms a [[Schiff base]] 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.
# [[Quinonoid zwitterion|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.
# [[Ketimine]] formation:  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 step]]s.<ref name="pmid8611515">{{Cite journal |vauthors=Goldberg JM, Kirsch JF |date=April 1996 |title=The reaction catalyzed by Escherichia coli aspartate aminotransferase has multiple partially rate-determining steps, while that catalyzed by the Y225F mutant is dominated by ketimine hydrolysis |journal=Biochemistry |volume=35 |issue=16 |pages=5280–5291 |doi=10.1021/bi952138d |pmid=8611515}}</ref>  However, it has been shown that the substrate binding step (transaldimination) drives the catalytic reaction forward.<ref name="pmid12488449">{{Cite journal |vauthors=Hayashi H, Mizuguchi H, Miyahara I, Nakajima Y, Hirotsu K, Kagamiyama H |date=March 2003 |title=Conformational change in aspartate aminotransferase on substrate binding induces strain in the catalytic group and enhances catalysis |journal=The Journal of Biological Chemistry |volume=278 |issue=11 |pages=9481–9488 |doi=10.1074/jbc.M209235200 |pmid=12488449 |doi-access=free}}</ref>

==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 ([[myocardium|cardiac muscle]]), skeletal muscle, kidneys, brain, and red blood cells.{{fact|date=March 2021}} 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, [[Renal disease|acute renal disease]], musculoskeletal diseases, and trauma.<ref>{{Cite web |title=AST/ALT |url=http://www.rnceus.com/lf/lfast.html |website=www.rnceus.com}}</ref>

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 [[Troponin test|cardiac troponins]].<ref>{{Cite journal |vauthors=Gaze DC |date=September 2007 |title=The role of existing and novel cardiac biomarkers for cardioprotection |journal=Current Opinion in Investigational Drugs |volume=8 |issue=9 |pages=711–717 |pmid=17729182}}</ref>

Laboratory tests should always be interpreted using the reference range from the laboratory that performed the test. Example reference ranges are shown below:

{| class="wikitable"
 |-
 |'''Patient type'''||[[Reference range]]s<ref name="gpnotebook-ast">[http://www.gpnotebook.co.uk/simplepage.cfm?ID=322240579 GPnotebook > reference range (AST)] Retrieved on Dec 7, 2009 {{Webarchive|url=https://web.archive.org/web/20170107125542/http://www.gpnotebook.co.uk/simplepage.cfm?ID=322240579 |date=7 January 2017 }}</ref>
 |-
 | Male || 8–40 IU/L
 |-
 | Female || 6–34 IU/L
|}

== See also ==
* [[Alanine transaminase]] (ALT/ALAT/SGPT)
* [[Transaminases]]

== References ==
{{reflist}}

== Further reading ==
{{refbegin}}
* {{Cite book |author-link=Johan Jansonius |title=Biological Macromolecules and Assemblies |vauthors=Jansonius JN, Vincent |publisher=Wiley |year=1987 |isbn=978-0-471-85142-4 |veditors=Jurnak FA, McPherson A |volume=3 |location=New York |pages=187–285 |chapter=Structural basis for catalysis by aspartate aminotransferase}}<!--| access-date = 2011-05-18 -->
* {{Cite journal |vauthors=Kuramitsu S, Okuno S, Ogawa T, Ogawa H, Kagamiyama H |date=April 1985 |title=Aspartate aminotransferase of Escherichia coli: nucleotide sequence of the aspC gene |journal=Journal of Biochemistry |volume=97 |issue=4 |pages=1259–1262 |doi=10.1093/oxfordjournals.jbchem.a135173 |pmid=3897210}}
* {{Cite journal |vauthors=Kondo K, Wakabayashi S, Yagi T, Kagamiyama H |date=July 1984 |title=The complete amino acid sequence of aspartate aminotransferase from Escherichia coli: sequence comparison with pig isoenzymes |journal=Biochemical and Biophysical Research Communications |volume=122 |issue=1 |pages=62–67 |doi=10.1016/0006-291X(84)90439-X |pmid=6378205}}
* {{Cite journal |vauthors=Inoue K, Kuramitsu S, Okamoto A, Hirotsu K, Higuchi T, Kagamiyama H |date=August 1991 |title=Site-directed mutagenesis of Escherichia coli aspartate aminotransferase: role of Tyr70 in the catalytic processes |journal=Biochemistry |volume=30 |issue=31 |pages=7796–7801 |doi=10.1021/bi00245a019 |pmid=1868057}}
{{refend}}

== External links ==
{{Commons category}}
* {{MeshName|Aspartate+Transaminase}}
* [http://labtestsonline.org/understanding/analytes/ast/tab/test AST] - Lab Tests Online
* [https://www.nlm.nih.gov/medlineplus/ency/article/003472.htm AST: MedlinePlus Medical Encyclopedia]

{{Mitochondrial enzymes}}
{{Citric acid cycle enzymes}}
{{Amino acid metabolism enzymes}}
{{Blood tests}}
{{Nitrogenous transferases}}
{{Enzymes}}
{{Glutamate metabolism and transport modulators}}
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{{Use dmy dates|date=April 2017}}

{{DEFAULTSORT:Aspartate Transaminase}}
[[Category:Liver function tests]]
[[Category:EC 2.6.1]]
[[Category:Glutamate (neurotransmitter)]]