In enzymology, SEPHCHC synthase (EC EC 2.2.1.9), encoded by menD gene in E. coli, is an enzyme that catalyzes the second step of menaquinone (vitamin K2) biosynthesis.

Common names for the enzyme are: In the biosynthesis of vitamin K, SEPHCHC is involved in the second step of the pathway.

[1] The type of reaction is decarboxylating, and to have maximum activity, this enzymes uses the cofactor Mg2+, a magnesium ion.

[2] In previous years, it was thought that this reaction led to SHCHC, the product MenH.

[3] The structures reveal a stable dimer-of-dimers association in agreement with gel filtration and analytical studies confirm the classification of MenD in the pyruvate oxidase family.