Terpenoids are a class of organic compounds made up of 5 carbon isoprene units. There are two pathways, melvalonate and MEP/DOXP, that synthesize the terpenoid backbone components. Both of these create isopentenyl pyrophosphate, which may then react using isopentenyl diphosphate isomerase in the chloroplast to form dimethylallylprophosphate. This molecule is also produced by the MEP/DOXP pathway. Isopentenyl pyrophosphate and dimethylallylprophosphate can react with geranylphosphate synthase in the mitochondrion to form geranyl-pyrophosphate, the main compound used in monoterpenoid biosynthesis. Geranyl-pyrophosphate may also react again with isopentenyl pyrophosphate using solanesyl diphosphate synthase 2 in the chloroplast to form solanesyl pyrophosphate, a potential end product of this pathway. Alternately, they can react with (2E,6E)-farnesyl diphosphate synthase, also in the mitochondrion, to form farnesyl phosphate. Farnesyl pyrophosphate may then be used as the main precursor in the sesquiterpenoid and triterpenoid biosynthesis pathways. It may also react with geranylgeranyl pyrophosphate 6 in the mitochondrion to form geranylgeranyl pyrophosphate. Geranylgeranyl pyrophosphate can react with isopentenyl pyrophosphate, catalyzed by solanesyl diphosphate syntahse 2, again in the chloroplast, to form solanesyl pyrophosphate. Aside this reaction, it can be converted by geranylgeranyl dehydrogenase in the chloroplast to form phytyl pyrophosphate, another end product of this pathway. Farnesyl pyrophosphate can additionally react using an undecaprenyl pyrophosphate synthetase family protein as a catalyst in order to form dehydrolichol pyrophosphate, or with the protein farnesyltransferase complex, which will add a protein-cysteine to the farnesyl pyrophosphate, which in turn loses its pyrophosphate group. The S-farnesyl protein then reacts with either CAAX prenyl protease 1 or 2 in the endoplasmic reticulum membrane to form protein C-terminal S-farnesyl-L-cysteine. This complex then reacts using protein-S-isoprenylcysteine O-methyltransferase B, still in the endoplasmic reticulum membrane, to form protein-C-terminal S-farnesyl-L-cysteine methyl ester. This reaction may be reversed by isoprenylcysteine alpha-carbonyl methylesterase, yet again in the endoplasmic reticulum membrane. Alternately, through an as of yet unknown reaction, the protein may be removed, as well as several other structure changes, leaving farnesylcysteine. In the lysosome, farnesylcysteine can be catalyzed by farnesylcysteine to remove the cysteine group, leaving behind farnesal. Then, a NAD-binding Rossman-fold superfamily protein can catalyze its transformation into farnesol. Finally, within the chloroplast, farnesol can be catalyzed by farnesol kinase to form farnesyl phosphate, the final product of this pathway.

Terpenoid Backbone Biosynthesis References