(E)-4-hydroxy-3-methyl-but-2-enyl pyrophosphate (HMB-PP) is an intermediate of the non-mevalonate pathway of isoprenoid biosynthesis. The enzyme HMB-PP synthase (GcpE, IspG) catalyzes the conversion of 2-C-methyl-D-erythritol 2,4-cyclopyrophosphate (MEcPP) into HMB-PP, and HMB-PP is then converted further to isopentenyl pyrophosphate (IPP) and dimethylallyl pyrophosphate (DMAPP) by HMB-PP reductase (LytB, IspH).