IPP is formed from
acetyl-CoA via the mevalonate pathway (the "upstream" part), and then is
isomerized to
dimethylallyl pyrophosphate by the enzyme
isopentenyl pyrophosphate isomerase. (DMAPP),
geranyl pyrophosphate (GPP) and
squalene. Some intermediates are omitted. The color scheme does not correctly represent the origins of the isoprene units of GPP. IPP can be synthesised via an alternative non-mevalonate pathway of
isoprenoid precursor biosynthesis, the
MEP pathway, where it is formed from
(E)-4-hydroxy-3-methyl-but-2-enyl pyrophosphate (HMB-PP) by the enzyme
HMB-PP reductase (LytB, IspH). The MEP pathway is present in many
bacteria,
apicomplexan
protozoa such as
malaria parasites, and in the
plastids of higher
plants. ==See also==