2-succinyl-6-hydroxy-2,4-cyclohexadiene-1-carboxylate synthase | |||||||||
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Identifiers | |||||||||
EC no. | 4.2.99.20 | ||||||||
CAS no. | 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 | ||||||||
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2-Succinyl-6-hydroxy-2,4-cyclohexadiene-1-carboxylate synthase (EC 4.2.99.20), also known as SHCHC synthase is encoded by the menH gene in Escherichia 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]
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]
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 helices on both sides. A cap of five alpha helixes serves to enclose the active site. [5] The enzyme exists in an open form until it binds the substrate, when it morphs into a closed form with an active catalytic triad. [6]
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. [7] Only fifteen amino acid residues are absolutely conserved across mutations of the enzyme. [7]
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. [5] 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. [6] 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. [5]
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]
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. [8] A protein was identified in 2013 that could fit this role. [9]