Enzyme
2.6.1.1 - Aspartate transaminase
Alternative Name(s)
- Transaminase A.
- Aspartate aminotransferase.
- Glutamic--oxaloacetic transaminase.
- Glutamic--aspartic transaminase.
Catalytic Activity
2-oxoglutarate + L-aspartate = L-glutamate + oxaloacetate
Cofactors
Pyridoxal 5'-phosphate.
Reaction Mechanism
Aspartyl transferase is able to catalyse the PLP dependent transamination reaction between aspartate and 2-oxoglutarate, forming oxaloacetate and glutamate. It is part of the family of PLP dependent amino acid transferase enzymes which have high sequence homology and identical active site organisation, with the only difference being in the amino acid and ketoacid substrates. The enzymes all play key roles in the catabolism of amino acids as the products feed into the Krebs cycle and the Urea cycle.
The reaction follows a ping-pong mechanism.
In the enzyme, PLP is anchored to the protein at Lys258 with its pyridoxal moiety in the reactive bipolar ionic form. In the first step, the cofactor PLP is transferred from Lys 258 to the aspartate alpha amino group. The alpha-amino group of the aspartate attacks the PLP C4' from the front side in a direction perpendicular to the plane of the pyridine ring. The attraction of opposite charges on the substrate N atom and O3' of the coenzyme brings about a 90 degrees rotation of the 2 C-N bonds around C4-C4' which brings Lys258 behind the plane of the pyridine ring and hence Lys258 can be released. Trp 140 sterically constraining the PLP to allow the attack of aspartate.
The deprotonation of the alpha-carbon of aspartate gives rise to the quinonoid intermediate, which is stabilised by the electron link of the protonated pyridine ring. Asp223 facilitates the deprotonation by stabilising the positive charge at N1 of PLP with a salt bridge and hence enhancing the electron withdrawing capacity of the amino acid substrate. Lys 258 acts as a base here. Lys258 then protonates C4' from the si side, giving rise to keimine intermediate. It then deprotonates a water molecule to allow its nucleophilic attack on the alpha-carbon. The tetrahedral intermediate dissociates into PMP and oxo-acid product. Subsequent reversal of the reaction steps described occurs except with the carboxyl group of the alpha keto-glutarate acting as the nucleophile leading to the formation of glutamate and the completion of the reaction cycle.
The deprotonation of the alpha-carbon of aspartate gives rise to the quinonoid intermediate, which is stabilised by the electron link of the protonated pyridine ring. Asp223 facilitates the deprotonation by stabilising the positive charge at N1 of PLP with a salt bridge and hence enhancing the electron withdrawing capacity of the amino acid substrate. Lys 258 acts as a base here. Lys258 then protonates C4' from the si side, giving rise to keimine intermediate. It then deprotonates a water molecule to allow its nucleophilic attack on the alpha-carbon. The tetrahedral intermediate dissociates into PMP and oxo-acid product. Subsequent reversal of the reaction steps described occurs except with the carboxyl group of the alpha keto-glutarate acting as the nucleophile leading to the formation of glutamate and the completion of the reaction cycle.
Catalytic Residues
| AA | Uniprot | Uniprot Resid | PDB | PDB Resid |
|---|---|---|---|---|
| Asp | P00509 | 211 | 1aam | 211 |
| Lys | P00509 | 246 | 1aam | 246 |
| Trp | P00509 | 130 | 1aam | 130 |
Reaction Parameters
-
Kinetic Parameters
Organism KM Value [mM] Substrate Comment Arabidopsis thaliana 3.98 2-oxoglutarate pH 7.5, 25°C, recombinant mutant T84V Bacillus subtilis 84.38 2-oxoglutarate pH 8.0, 45°C Bubalus bubalis 251.4 2-oxoglutarate pH 7.5, 35°C Escherichia coli 0.006 L-glutamate pH 7.5, 25°C, mutant I33Q/Y214Q/R280Y Advenella mimigardefordensis 5.1 L-cysteine at pH 7.4 and 30°C -
Temperature
Organism Temperature Range Comment Bubalus bubalis 15 30% activity Saccharolobus solfataricus 25 - 95 Pseudoalteromonas haloplanktis 35 - 63 half-maximal activity at 35°C and 63°C Haloferax mediterranei 40 - 80 40°C: about 40% of activity maximum, 80°C: about 65% of activity maximum Phormidium lapideum 45 - 85 44% of maximum activity at 45°C, 80% of maximum activity at 85°C - pH
Associated Proteins
Citations
- The association of aspartate transaminase-to-alanine transaminase ratio and metabolic syndrome among HIV patients in Sidama Region, South Ethiopia.
- Diagnostic and Prognostic Value of Aspartate Transaminase-to-platelet Ratio Index, Gamma-glutamyl Transpeptidase-to-platelet Ratio, and Fibrosis-4 for Compensated Hepatitis B-related Liver Cirrhosis.
- Aspartate Transaminase AST2 Involved in Sporulation and Necrotrophic Pathogenesis in the Hemibiotrophs Magnaporthe oryzae and Colletotrichum graminicola.
- Prognostic Value of Aspartate Transaminase/Alanine Transaminase Ratio in Patients With Hepatitis B Virus-Related Hepatocellular Carcinoma Undergoing Hepatectomy.
- The prognostic significance of serum aspartate transaminase and gamma-glutamyl transferase in liver deceased donors.
- Aspartate transaminase/alanine transaminase (De Ritis ratio) predicts survival in major burn patients.
- The Role of Aspartate Transaminase to Platelet Ratio Index (APRI) for the Prediction of Non-Alcoholic Fatty Liver Disease (NAFLD) in Severely Obese Children and Adolescents.
- Correction: Rigamonti et al. The Role of Aspartate Transaminase to Platelet Ratio Index (APRI) for the Prediction of Non-Alcoholic Fatty Liver Disease (NAFLD) in Severely Obese Children and Adolescents. Metabolites 2022, 12, 155.
- A Rise in Aspartate Transaminase and Alanine Transaminase Associated With Ondansetron Administration in a Pregnant Female.
- Preoperative aspartate transaminase/alanine transaminase ratio as a prognostic biomarker in primary non-muscle-invasive bladder cancer: a propensity score-matched study.
- Effects of fingolimod treatments on alanine transaminase and aspartate transaminase levels in patients with multiple sclerosis.