Aspartate transaminase

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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

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 biomarkers for liver health. The tests are part of blood panels
.

The

Aminotransferase is cleared by sinusoidal cells in the liver.[4]

Function

Aspartate transaminase catalyzes the interconversion of

glutamate
.

L-Aspartate (Asp) + α-ketoglutarate ↔ oxaloacetate + L-glutamate (Glu)

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

pyridoxamine phosphate (PMP) form.[5] 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.[6]

Isoenzymes

Two isoenzymes are present in a wide variety of eukaryotes. In humans:[citation needed]

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

E. coli, H. mediterranei,[8] and T. thermophilus.[9] 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).[10]

Structure

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

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.[11][14][15]

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.

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.[17]

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]

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
    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.[18] However, it has been shown that the substrate binding step (transaldimination) drives the catalytic reaction forward.[19]

Clinical significance

AST is similar to

acute renal disease, musculoskeletal diseases, and trauma.[20]

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

References

Further reading

External links