2-succinyl-6-hydroxy-2,4-cyclohexadiene-1-carboxylate synthase

2-succinyl-6-hydroxy-2,4-cyclohexadiene-1-carboxylate synthase
Identifiers
EC number 4.2.99.20
CAS number 122007-88-9
Databases
IntEnz IntEnz view
BRENDA BRENDA entry
ExPASy NiceZyme view
KEGG KEGG entry
MetaCyc metabolic pathway
PRIAM profile
PDB structures RCSB PDB PDBe PDBsum

2-succinyl-6-hydroxy-2,4-cyclohexadiene-1-carboxylate synthase, also known as SHCHC synthase, is encoded by the menH gene in E.coli and functions in the synthesis of vitamin K.[1] The specific step in the synthetic pathway that SHCHC synthase catalyzes is the conversion of 5-enolpyruvoyl-6-hydroxy-2-succinylcyclohex-3-ene-1-carboxylate to (1R,6R)-6-hydroxy-2-succinylcyclohexa-2,4-diene-1-carboxylate and pyruvate.[2]

Background

Vitamin K is a fat soluble vitamin known to aid in blood clotting. It is recommended that all newborns receive an injection of vitamin K in order to prevent excessive bleeding of the brain after birth. There are two major forms of vitamin K that occur naturally. Phylloquinone, also known as K1, is synthesized by plants and is the major form of vitamin K in the diet. Menaquinone, K2, includes a range of forms that are synthesized by bacteria in the gut.[3]

Vitamin K is synthesized from the molecule chorismate in a nine step conversion process. SHCHC synthase catalyzes the third step in the process.[4]

Chemistry

Reaction scheme

Enzyme Structure

Monomer of MenH enzyme in E.coli (SHCHC synthase).[5] The structure is one continuous amino acid sequence, but the catalytic triad is shown as cyan spheres and the alpha helix cap is shown in red. The rest of the protein sequence is shown in green.

The crystal structure of the MenH enzyme in E.coli (SHCHC synthase) exists as a complex of three protein molecules shown in the diagram. SHCHC synthase forms an alpha/beta hydrolase fold with a central set of seven parallel beta sheets surrounded by alpha helixes on both sides. A cap of five alpha helixes serves to enclose the active site.[6] The enzyme exists in an open form until it binds the substrate, when it morphs into a closed form with an active catalytic triad.[7]

Energetic analysis shows that SHCHC synthase has a low energetic burden for catalytic activity.[1] This means the enzyme is more prone to mutation and is one of the most diverse enzymes in the vitamin K synthetic pathway.[8] Only fifteen amino acid residues are absolutely conserved across mutations of the enzyme.[8]

Catalytic Mechanism

The active site contains a catalytic triad of syrine, histine and arginine, which is conserved across all mutants and is proposed to initiate the reaction.[1] The triad residues are located at Ser86, Asp210, and His232.[6] This triad is proposed to catalyze a proton extraction which triggers a transfer of electrons leading to the elimination of pyruvate and formation of SHCHC.[7] Originally, it was proposed that the transition state was stabilized by a nontraditional oxyanion hole. Now a traditional oxyanion hole is favored, but not definitive.[6]

Reaction Mechanism

Cofactors and Alternate Reactions

SHCHC synthase is unaffected by traditional cofactors such as divalent metal ions and EDTA.[1] The enzyme is fairly specific and only acts on SEPHCHC and close derivatives.[2]

Controversy

MenH (SHCHC synthase) was previously thought to be a thioesterase involved in hydrolyzing DHNA-CoA in a later step of menaquinone synthesis. In 2008, it was determined that MenH has poor catalytic activity toward palmitoyl-CoA, casting doubt on its role as a thioesterase.[1] Direct analysis confirmed that MenH is unable to hydrolyze DHNA-CoA.[1] In 2009, it was proposed that a dedicated hotdog fold thioesterase would be needed to catalyze the hydrolysis of DHNA-CoA.[9] A protein was identified in 2013 that could fit this role.[10]

References

  1. 1 2 3 4 5 6 Jiang; et al. (2008-02-20). "Identification and Characterization of (1R,6R)-2-Succinyl-6-hydroxy-2,4-cyclohexadiene-1-carboxylate Synthase in the Menaquinone Biosynthesis of Escherichia coli". Biochemistry 47: 3426–3434. doi:10.1021/bi7023755.
  2. 1 2 "Information on EC 4.2.99.20 - 2-succinyl-6-hydroxy-2,4-cyclohexadiene-1-carboxylate synthase". Brenda: The Comprehensive Enzyme Information System. TU Braunschwieg. 2014-07-01. Retrieved 2014-11-01.
  3. ↑ "Micronutrient Information Center". Linus Pauling Institute. 2014-11-30. Retrieved 2014-11-30.
  4. ↑ van Oostende C, Widhalm JR, Furt F, Ducluzeau AL, Basset GJC (2011) Phylloquinone (Vitamin K1): function, enzymes and genes. in Advances in Botanical Research, eds Fabrice Rébeillé and Roland Douce, 59: 229-61, Academic Press (Amsterdam).
  5. ↑ Johnston; et al. (2013-01-01). "Crystal Structure of E.coli MenH". RCSB Protein Data Bank. doi:10.2210/pdb4gdm/pdb. Retrieved 2014-11-24.
  6. 1 2 3 Johnston; et al. (2013-04-18). "Crystal Structures of E. coli Native MenH and Two Active Site Mutants". PLOS ONE 8: e61325. doi:10.1371/journal.pone.0061325.
  7. 1 2 Sun; et al. (2013-11-22). "Molecular Basis of the General Base Catalysis of an α/β-Hydrolase Catalytic Triad". Journal of Biological Chemistry 289: 15867–15879. doi:10.1074/jbc.M113.535641.
  8. 1 2 Jiang; et al. (2009-06-22). "Catalytic Mechanism of SHCHC Synthase in the Menaquinone Biosynthesis of Escherichia coli: Identification and Mutational Analysis of the Active Site Residues". Biochemistry 48: 6921–6931. doi:10.1021/bi900897h.
  9. ↑ Widhalm; et al. (2009-04-07). "A dedicated thioesterase of the Hotdog-fold family is required for the biosynthesis of the naphthoquinone ring of vitamin K1". PNAS 106: 5599–603. doi:10.1073/pnas.0900738106. PMID 19321747.
  10. ↑ Chen; et al. (2013-06-01). "Identification of a Hotdog Fold Thioesterase Involved in the Biosynthesis of Menaquinone in Escherichia coli". Journal of Bacteriology 195: 2768–2775. doi:10.1128/JB.00141-13.
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