Enzyme - Aspartate transaminase

Alternative Name(s)
  • Transaminase A.
  • Aspartate aminotransferase.
  • Glutamic--oxaloacetic transaminase.
  • Glutamic--aspartic transaminase.

Catalytic Activity

2-oxoglutarate + L-aspartate = L-glutamate + oxaloacetate


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.
    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
    Organism pH Range Comment
    Bubalus bubalis 5 40% of maximal activity
    Phormidium lapideum 5 - 8.8
    Leptosphaeria michotii 5.5 - 9 isozyme A
    Saccharolobus solfataricus 5.5 - 9.5 no definite pH optimum
    Geobacillus sp. MAS1 6 - 8.5 more than 50% activity at pH 6.0-8.5

Associated Proteins

Protein name Organism
Aspartate aminotransferase P2, mitochondrial Narrow-leaved blue lupine
Aspartate aminotransferase, cytoplasmic isozyme 2 Mouse-ear cress
Aspartate aminotransferase 1 Alfalfa
Aspartate aminotransferase, chloroplastic Mouse-ear cress
Aspartate aminotransferase Escherichia coli (strain K12)