In enzymology, a 2-C-methyl-D-erythritol 2,4-cyclodiphosphate synthase (EC 184.108.40.206) is an enzyme is a lyase which participates in biosynthesis of steroids. It catalyzes the chemical reaction:
- 2-phospho-4-(cytidine 5'-diphospho)-2-C-methyl-D-erythritol 2-C-methyl-D-erythritol 2,4-cyclodiphosphate + CMP
Hence, this enzyme has one substrate, 2-phospho-4-(cytidine 5'-diphospho)-2-C-methyl-D-erythritol, and two products, 2-C-methyl-D-erythritol 2,4-cyclodiphosphate and CMP.
The enzyme is considered a phosphorus-oxygen lyase. The systematic name of this enzyme class is 2-phospho-4-(cytidine 5'-diphospho)-2-C-methyl-D-erythritol CMP-lyase (cyclizing; 2-C-methyl-D-erythritol 2,4-cyclodiphosphate-forming). Other names in common use include MECDP-synthase, 2-phospho-4-(cytidine 5'-diphospho)-2-C-methyl-D-erythritol, and CMP-lyase (cyclizing).
 Structural studies
As of late 2007, 20 structures have been solved for this class of enzymes, with PDB accession codes 1GX1, 1H47, 1H48, 1IV1, 1IV2, 1IV3, 1IV4, 1T0A, 1U3L, 1U3P, 1U40, 1U43, 1VH8, 1VHA, 1W55, 1W57, 1YQN, 2AMT, 2GZL, and 2PMP.