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2-succinyl-6-hydroxy-2,4-cyclohexadiene-1-carboxylate synthase (SHCHC Synthase)
[edit]| Identifiers | |
|---|---|
| EC Number | 4.2.99.20 |
| CAS Number | 122007-88-9 |
| Databases | |
| IntEnz | |
| BRENDA | BRENDA Entry |
| ExPASy | NiceZyme View |
| KEGG | KEGG Entry |
| MetaCyc | MetaCyc Entry |
| PRIAM | Profile |
| PDB | Structure Results |
2-succinyl-6-hydroxy-2,4-cyclohexadiene-1-carboxylate synthase[4.2.99.20], also known as SHCHC synthase, functions in the synthesis of vitamin K [1] which is important for blood clotting[2]. 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[3].
Background
[edit]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.[4]
Vitamin K is synthesized from the molecule chorismate in a nine step conversion process. SHCHC synthase catalyzes the third step in the process[5].
Chemistry
[edit]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[3].
Reaction Mechanism
[edit]
Enzyme Structure
[edit]
The crystal structure of 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[7]. The enzyme exists in an open form until it binds the substrate, when it morphs into a closed form with an active catalytic triad[8].
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[9]. Only fifteen amino acid residues are absolutely conserved across mutations of the enzyme[9].
Catalytic Mechanism
[edit]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[7]. 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[8]. 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[7].
Cofactors and Alternate Reactions
[edit]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[3].
Controversy
[edit]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[10]. A protein was identified in 2013 that could fit this role[11].
- ^ a b c d e f Jiang, Ming; et al. (02/20/2008). "Identification and Characterization of
(1R,6R)-2-Succinyl-6-hydroxy-2,4-cyclohexadiene-1-carboxylate Synthase in the Menaquinone Biosynthesis of Escherichia coli". Biochemistry. doi:10.1021/bi7023755.
{{cite journal}}:|access-date=requires|url=(help); Check date values in:|accessdate=and|date=(help); Explicit use of et al. in:|last=(help); line feed character in|title=at position 39 (help) - ^ "NIH". MedlinePlus. National Institute of Health. 11/07/2014. Retrieved 11/30/2014.
{{cite web}}: Check date values in:|accessdate=and|date=(help) - ^ a b c "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. 07/01/2014. Retrieved 11/01/2014.
{{cite web}}: Check date values in:|accessdate=and|date=(help) - ^ "Micronutrient Information Center". Linus Pauling Institute. 11/30/2014. Retrieved 11/30/2014.
{{cite web}}: Check date values in:|accessdate=and|date=(help) - ^ 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).
- ^ Johnston, J.M.; et al. (2013). "Crystal Structure of E.coli MenH". RCSB Protein Data Bank. doi:10.2210/pdb4gdm/pdb. Retrieved 11/24/14.
{{cite web}}: Check date values in:|accessdate=(help); Explicit use of et al. in:|last=(help) - ^ a b c Johnston, Jodi; et al. (April 18, 2013). "Crystal Structures of E. coli Native MenH and Two Active
Site Mutants". Plos One. doi:10.1371/journal.pone.0061325.
{{cite journal}}:|access-date=requires|url=(help); Check date values in:|accessdate=(help); Explicit use of et al. in:|last=(help); line feed character in|title=at position 57 (help)CS1 maint: unflagged free DOI (link) - ^ a b Sun, Yueru; et al. (November 22, 2013). "Molecular Basis of the General Base Catalysis of an
alpha�/beta�-Hydrolase Catalytic Triad". JOURNAL OF BIOLOGICAL CHEMISTRY. doi:10.1074/jbc.M113.535641.
{{cite journal}}: Explicit use of et al. in:|last=(help); replacement character in|title=at position 58 (help)CS1 maint: unflagged free DOI (link) - ^ a b Jiang, Ming; et al. (June 22, 2009). "Catalytic Mechanism of SHCHC Synthase in the Menaquinone Biosynthesis of Escherichia coli: Identification and Mutational Analysis of the Active Site Residues". Biochemistry. doi:10.1021/bi900897h.
{{cite journal}}:|access-date=requires|url=(help); Check date values in:|accessdate=(help); Explicit use of et al. in:|last=(help) - ^ Widhalm, Joshua; et al. (April 7, 2009). "A dedicated thioesterase of the Hotdog-fold family is
required for the biosynthesis of the naphthoquinone
ring of vitamin K1". PNAS. doi:[https://doi.org/10.1073%EF%BF%BDpnas.0900738106 10.1073�pnas.0900738106].
{{cite journal}}:|access-date=requires|url=(help); Check|doi=value (help); Check date values in:|accessdate=(help); Explicit use of et al. in:|last=(help); line feed character in|title=at position 54 (help); replacement character in|doi=at position 8 (help) - ^ Chen, Minjiao; et al. (June 2013). "Identification of a Hotdog Fold Thioesterase Involved in the
Biosynthesis of Menaquinone in Escherichia coli". Journal of Bacteriology. doi:10.1128/JB.00141-13.
{{cite journal}}:|access-date=requires|url=(help); Check date values in:|accessdate=(help); Explicit use of et al. in:|last=(help); line feed character in|title=at position 61 (help)