MetaCyc Enzyme: HMG aldolase

Gene: proA Accession Number: G-2548 (MetaCyc)

Synonyms: 4-hydroxy-4-methyl-2-oxoglutarate aldolase

Species: Pseudomonas straminea

Subunit composition of HMG aldolase = [ProA]6
         4-oxalocitramalate aldolase subunit = ProA

HMG aldolase catalyzes the conversion of 4-hydroxy-4-methyl-2-oxoglutarate (HMG) to pyruvate in the presence of Mg2+. The plant enzyme is involved in the biosynthesis of substituted glutamates. However, in bacteria the physiological substrate of the enzyme is not HMG but 2-hydroxy-4-oxobutane-1,2,4-tricarboxylate, and it is involved in the catabolism of protocatechuate.

The enzyme has been purified to homogeneity from Pseudomonas straminea cells grown on phthalate and found to be a homohexamer of 160 kDa. The enzyme requires divalent metal ions such as Mg2+, Mn2+, Co2+, Zn2+, and Cd2+ for activity, and thus belongs to the Class II (metal ion-requiring) aldolases.

4-hydroxy-4-methyl-2-oxoglutarate is also a substrate, but with much lower activity. In both reactions the enzyme prefers the L-isomers. The reaction is reversible [Maruyama83, Maruyama90]. The enzyme also possesses a β-decarboxylase activity, and decarboxylates oxaloacetate to pyruvate [Maruyama90].

The proA gene, encoding the enzyme in Pseudomonas straminea, has been cloned and sequenced. The gene was overexpressed in Escherichia coli, and the recombinant protein was purified and characterized [Maruyama01]. The enzyme requires Mg2+ for activity. Mg2+ is replaceable in vitro by Mn2+ or Co2+ [Maruyama83].

Gene Citations: [Maruyama04]

Molecular Weight of Polypeptide: 24.068 kD (from nucleotide sequence), 26 kD (experimental) [Maruyama90 ]

Molecular Weight of Multimer: 160 kD (experimental) [Maruyama90]

pI: 5.0 [Maruyama90]

Unification Links: Protein Model Portal:Q9AQI0 , UniProt:Q9AQI0

Relationship Links: Entrez-Nucleotide:PART-OF:AB050935 , InterPro:IN-FAMILY:IPR005493 , InterPro:IN-FAMILY:IPR014165 , Pfam:IN-FAMILY:PF03737

Gene-Reaction Schematic: ?

Gene-Reaction Schematic

Instance reaction of [4-hydroxy-4-methyl-2-oxoglutarate = 2 pyruvate] (
i1: (R)-4-hydroxy-4-methyl-2-oxoglutarate = 2 pyruvate (

Revised 25-Sep-2009 by Caspi R , SRI International

Enzymatic reaction of: oxaloacetate β-decarboxylase (HMG aldolase)

EC Number:

oxaloacetate + H+ <=> pyruvate + CO2

The reaction direction shown, that is, A + B ↔ C + D versus C + D ↔ A + B, is in accordance with the Enzyme Commission system.

The reaction is favored in the direction shown.

In Pathways: protocatechuate degradation I (meta-cleavage pathway)

Enzymatic reaction of: 4-hydroxy-4-methyl-2-oxoglutarate aldolase (HMG aldolase)

Synonyms: HMG aldolase

EC Number:

4-hydroxy-4-methyl-2-oxoglutarate <=> 2 pyruvate

The reaction direction shown, that is, A + B ↔ C + D versus C + D ↔ A + B, is in accordance with the Enzyme Commission system.

Reversibility of this reaction is unspecified.

Enzymatic reaction of: 4-oxalocitramalate aldolase (HMG aldolase)

EC Number:

2-hydroxy-4-oxobutane-1,2,4-tricarboxylate <=> pyruvate + oxaloacetate

The reaction direction shown, that is, A + B ↔ C + D versus C + D ↔ A + B, is in accordance with the direction of enzyme catalysis.

This reaction is reversible.

In Pathways: protocatechuate degradation I (meta-cleavage pathway)

Cofactors or Prosthetic Groups: Mg2+ [Maruyama01]

Inhibitors (Competitive): oxaloacetate [Maruyama01]

Inhibitors (Unknown Mechanism): Hg2+ [Maruyama90] , p-chloromercuribenzoate [Maruyama90]

Primary Physiological Regulators of Enzyme Activity: oxaloacetate

Kinetic Parameters:

Km (μM)

pH(opt): 8 [Maruyama01]


Maruyama01: Maruyama K, Miwa M, Tsujii N, Nagai T, Tomita N, Harada T, Sobajima H, Sugisaki H (2001). "Cloning, sequencing, and expression of the gene encoding 4-hydroxy-4-methyl-2-oxoglutarate aldolase from Pseudomonas ochraceae NGJ1." Biosci Biotechnol Biochem 65(12);2701-9. PMID: 11826967

Maruyama04: Maruyama K, Shibayama T, Ichikawa A, Sakou Y, Yamada S, Sugisaki H (2004). "Cloning and characterization of the genes encoding enzymes for the protocatechuate meta-degradation pathway of Pseudomonas ochraceae NGJ1." Biosci Biotechnol Biochem 68(7);1434-41. PMID: 15277747

Maruyama83: Maruyama K (1983). "Enzymes responsible for degradation of 4-oxalmesaconic acid in Pseudomonas ochraceae." J Biochem (Tokyo) 93(2);567-74. PMID: 6841354

Maruyama90: Maruyama K (1990). "Purification and properties of 4-hydroxy-4-methyl-2-oxoglutarate aldolase from Pseudomonas ochraceae grown on phthalate." J Biochem (Tokyo) 1990;108(2);327-33. PMID: 2229032

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Please cite the following article in publications resulting from the use of MetaCyc: Caspi et al, Nucleic Acids Research 42:D459-D471 2014
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