The Enzyme Database

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EC 2.3.3.14     
Accepted name: homocitrate synthase
Reaction: acetyl-CoA + H2O + 2-oxoglutarate = (2R)-2-hydroxybutane-1,2,4-tricarboxylate + CoA
Glossary: (R)-homocitrate = (2R)-2-hydroxybutane-1,2,4-tricarboxylate
Other name(s): 2-hydroxybutane-1,2,4-tricarboxylate 2-oxoglutarate-lyase (CoA-acetylating); acetyl-coenzyme A:2-ketoglutarate C-acetyl transferase; homocitrate synthetase; HCS
Systematic name: acetyl-CoA:2-oxoglutarate C-acetyltransferase (thioester-hydrolysing, carboxymethyl forming)
Comments: Belongs in the α-aminoadipate pathway of lysine synthesis, along with EC 4.2.1.36, homoaconitate hydratase. The enzyme also acts with oxaloacetate as substrate, but more slowly [2,3].
Links to other databases: BRENDA, EXPASY, KEGG, MetaCyc, CAS registry number: 9075-60-9
References:
1.  Strassman, M. and Ceci, L.N. Enzymatic formation of homocitric acid, an intermediate in lysine biosynthesis. Biochem. Biophys. Res. Commun. 14 (1964) 262–267. [DOI] [PMID: 5836514]
2.  Wulandari, A.P., Miyazaki, J., Kobashi, N., Nishiyama, M., Hoshino, T. and Yamane, H. Characterization of bacterial homocitrate synthase involved in lysine biosynthesis. FEBS Lett. 522 (2002) 35–40. [DOI] [PMID: 12095615]
3.  Andi, B., West, A.H. and Cook, P.F. Kinetic mechanism of histidine-tagged homocitrate synthase from Saccharomyces cerevisiae. Biochemistry 43 (2004) 11790–11795. [DOI] [PMID: 15362863]
[EC 2.3.3.14 created 1972 as EC 4.1.3.21, transferred 2002 to EC 2.3.3.14]
 
 
EC 4.2.1.36     
Accepted name: homoaconitate hydratase
Reaction: (1R,2S)-1-hydroxybutane-1,2,4-tricarboxylate = (Z)-but-1-ene-1,2,4-tricarboxylate + H2O
For diagram of l-lysine synthesis, click here
Glossary: cis-homoaconitate = (Z)-but-1-ene-1,2,4-tricarboxylate
(R)-homocitrate = (2R)-2-hydroxybutane-1,2,4-tricarboxylate
homoisocitrate = (-)-threo-homoisocitrate = (1R,2S)-1-hydroxybutane-1,2,4-tricarboxylate
Other name(s): homoaconitase; cis-homoaconitase; HACN; Lys4; LysF; 2-hydroxybutane-1,2,4-tricarboxylate hydro-lyase (incorrect)
Systematic name: (1R,2S)-1-hydroxybutane-1,2,4-tricarboxylate hydro-lyase [(Z)-but-1-ene-1,2,4-tricarboxylate-forming]
Comments: Requires a [4Fe-4S] cluster for activity. The enzyme from the hyperthermophilic eubacterium Thermus thermophilus can catalyse the reaction shown above but cannot catalyse the previously described reaction, i.e. formation of (R)-homocitrate by hydration of cis-homoaconitate. The enzyme responsible for the conversion of cis-homoaconitate into (R)-homocitrate in T. thermophilus is unknown at present but the reaction can be catalysed in vitro using aconitate hydratase from pig (EC 4.2.1.3) [2].
Links to other databases: BRENDA, EXPASY, KEGG, MetaCyc, CAS registry number: 9030-68-6
References:
1.  Strassman, M. and Ceci, L.N. Enzymatic formation of cis-homoaconitic acid, an intermediate in lysine biosynthesis in yeast. J. Biol. Chem. 241 (1966) 5401–5407. [PMID: 5954805]
2.  Jia, Y., Tomita, T., Yamauchi, K., Nishiyama, M. and Palmer, D.R. Kinetics and product analysis of the reaction catalysed by recombinant homoaconitase from Thermus thermophilus. Biochem. J. 396 (2006) 479–485. [DOI] [PMID: 16524361]
3.  Zabriskie, T.M. and Jackson, M.D. Lysine biosynthesis and metabolism in fungi. Nat. Prod. Rep. 17 (2000) 85–97. [PMID: 10714900]
[EC 4.2.1.36 created 1972, modified 2007]
 
 
EC 4.2.1.114     
Accepted name: methanogen homoaconitase
Reaction: (R)-2-hydroxybutane-1,2,4-tricarboxylate = (1R,2S)-1-hydroxybutane-1,2,4-tricarboxylate (overall reaction)
(1a) (R)-2-hydroxybutane-1,2,4-tricarboxylate = (Z)-but-1-ene-1,2,4-tricarboxylate + H2O
(1b) (Z)-but-1-ene-1,2,4-tricarboxylate + H2O = (1R,2S)-1-hydroxybutane-1,2,4-tricarboxylate
For diagram of the 2-aminoadipate pathway of L-lysine synthesis, click here
Glossary: cis-homoaconitate = (Z)-but-1-ene-1,2,4-tricarboxylate
(R)-homocitrate = (R)-2-hydroxybutane-1,2,4-tricarboxylate
homoisocitrate = (-)-threo-homoisocitrate = (1R,2S)-1-hydroxybutane-1,2,4-tricarboxylate
Other name(s): methanogen HACN
Systematic name: (R)-2-hydroxybutane-1,2,4-tricarboxylate hydro-lyase [(1R,2S)-1-hydroxybutane-1,2,4-tricarboxylate-forming]
Comments: This enzyme catalyses several reactions in the pathway of coenzyme-B biosynthesis in methanogenic archaea. Requires a [4Fe-4S] cluster for activity. In contrast to EC 4.2.1.36, homoaconitate hydratase, this enzyme can catalyse both the dehydration of (R)-homocitrate to form cis-homoaconitate and the subsequent hydration reaction that forms homoisocitrate. In addition to cis-homoaconitate, the enzyme can also catalyse the hydration of the physiological substrates dihomocitrate and trihomocitrate as well as the non-physiological substrate tetrahomocitrate. cis-Aconitate and threo-DL-isocitrate cannot act as substrates, and (S)-homocitrate and trans-homoaconitate act as inhibitors of the enzyme.
Links to other databases: BRENDA, EXPASY, KEGG, MetaCyc, PDB
References:
1.  Drevland, R.M., Jia, Y., Palmer, D.R. and Graham, D.E. Methanogen homoaconitase catalyzes both hydrolyase reactions in coenzyme B biosynthesis. J. Biol. Chem. 283 (2008) 28888–28896. [DOI] [PMID: 18765671]
[EC 4.2.1.114 created 2009]
 
 


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