MetaCyc Polypeptide: acetohydroxy-acid synthase small subunit

Gene: ilvN Accession Number: G-9238 (MetaCyc)

Species: Methanococcus aeolicus

Component of: acetohydroxy-acid synthase (extended summary available)

Gene Citations: [Bowen97]

Molecular Weight of Polypeptide: 18.833 kD (from nucleotide sequence)

Unification Links: ModBase:O08354 , Protein Model Portal:O08354 , Swiss-Model:O08354 , UniProt:O08354

Relationship Links: Entrez-Nucleotide:RELATED-TO:U35458 , InterPro:IN-FAMILY:IPR002912 , InterPro:IN-FAMILY:IPR004789 , InterPro:IN-FAMILY:IPR019455 , Pfam:IN-FAMILY:PF01842 , Pfam:IN-FAMILY:PF10369 , Prosite:IN-FAMILY:PS51671

Gene-Reaction Schematic: ?

Gene-Reaction Schematic

Subunit of: acetohydroxy-acid synthase

Species: Methanococcus aeolicus

Subunit composition of acetohydroxy-acid synthase = [IlvB]2[IlvN]2
         acetohydroxy-acid synthase large subunit = IlvB
         acetohydroxy-acid synthase small subunit = IlvN

acetohydroxy-acid synthase (AHAS) catalyses a key step in the biosynthesis of the branched-chain amino acids, L-isoleucine, L-leucine, and L-valine. The enzyme catalyzes two different reactions; the condensation of pyruvate with 2-oxobutanoate to yield (S)-2-aceto-2-hydroxybutanoate, a precursor of L-isoleucine, and the condensation of two molecules of pyruvate to yield (S)-2-acetolactate, a precursor of L-leucine and L-valine.

The Methanococcus aeolicus enzyme was purified to homogeneity and characterized [Xing94]. It was initially reported to be a homodimer composed of a single type (large) subunit, although subsequent work has showed that the gene encoding the large subunit is followed by a gene encoding a small subunit, suggesting that the enzyme is actually composed of two types of subunits, similarly to the Escherichia coli enzyme [Bowen97].

The enzyme is sensitive to oxygen, requires either Mg2+ or Mn2+, thiamin diphosphate and a flavin [Xing94].

Molecular Weight: 125 kD (experimental) [Xing94]

pI: 5.6 [Xing94]

Enzymatic reaction of: pyruvate:2-oxobutanoate acetaldehydetransferase (decarboxylating) (acetohydroxy-acid synthase)

EC Number:

pyruvate + 2-oxobutanoate + H+ <=> (S)-2-aceto-2-hydroxybutanoate + 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: L-isoleucine biosynthesis II

Enzymatic reaction of: pyruvate:pyruvate acetaldehydetransferase (decarboxylating) (acetohydroxy-acid synthase)

EC Number:

2 pyruvate + H+ <=> (S)-2-acetolactate + CO2

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

This reaction is reversible.

In Pathways: L-valine biosynthesis

Citations: [Xing91]

Cofactors or Prosthetic Groups: a flavin [Xing94], thiamin diphosphate [Xing94], Mg2+ [Xing94]

Inhibitors (Competitive): L-valine [Xing94] , L-isoleucine [Xing94]

Primary Physiological Regulators of Enzyme Activity: L-valine , L-isoleucine

Kinetic Parameters:

Km (μM)

pH(opt): 7.6 [Xing94]


Bowen97: Bowen TL, Union J, Tumbula DL, Whitman WB (1997). "Cloning and phylogenetic analysis of the genes encoding acetohydroxyacid synthase from the archaeon Methanococcus aeolicus." Gene 188(1);77-84. PMID: 9099862

Xing91: Xing RY, Whitman WB (1991). "Characterization of enzymes of the branched-chain amino acid biosynthetic pathway in Methanococcus spp." J Bacteriol 173(6);2086-92. PMID: 2002010

Xing94: Xing R, Whitman WB (1994). "Purification and characterization of the oxygen-sensitive acetohydroxy acid synthase from the archaebacterium Methanococcus aeolicus." J Bacteriol 176(5);1207-13. PMID: 8113159

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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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