Catalyzes the thiamine diphosphate-dependent decarboxylation of 2-oxoglutarate and the subsequent addition of the resulting succinic semialdehyde-thiamine pyrophosphate anion to isochorismate to yield 2-succinyl-5-enolpyruvyl-6-hydroxy-3-cyclohexene-1-carboxylate (SEPHCHC). 2-oxoglutarate + H(+) + isochorismate = 5-enolpyruvoyl-6-hydroxy-2-succinyl-cyclohex-3-ene-1-carboxylate + CO2 Binds 1 thiamine pyrophosphate per subunit. Optimum pH is 7-8. Quinol/quinone metabolism; 1,4-dihydroxy-2-naphthoate biosynthesis; 1,4-dihydroxy-2-naphthoate from chorismate: step 2/7. Quinol/quinone metabolism; menaquinone biosynthesis. Homodimer. By isopropyl-beta-D-thiogalactoside (IPTG). Belongs to the TPP enzyme family. MenD subfamily. Was originally thought (PubMed:1459959, PubMed:14621995 and PubMed:16511076) to be a 2-succinyl-6-hydroxy-2,4-cyclohexadiene-1-carboxylate (SHCHC) synthase but further protein analysis clearly suggests that it is a 2-succinyl-5-enolpyruvyl-6-hydroxy-3-cyclohexene-1-carboxylate (SEPHCHC) synthase.