The terpenoids, sometimes referred to as isoprenoids, are a class of naturally occurring chemicals similar to terpenes, derived from five-carbon isoprene units assembled and modified in thousands of ways. Most are multicyclic structures which differ from one another not only in functional groups, but also in their basic carbon skeletons.
Terpenoids of different sizes and composition are found in all classes of living things, and are the largest group of natural products. Plant terpenoids are extensively used for their aromatic qualities. They play a role in traditional herbal remedies and are under investigation for antibacterial, antineoplastic and other pharmaceutical effects.
Terpenoids contribute to the scent of eucalyptus, the flavors of cinnamon, cloves and ginger and the color of yellow flowers. The cannabinoids found in the Cannabis plant are also terpenoids.
Biosynthesis
There are two pathways of creating terpenoids:
Mevalonic acid pathway
Many organisms manifacture terpenoids/isoprenoids through the HMG-CoA reductase pathway,
the metabolic pathway that also produces cholesterol.
Isoprenylation is the addition of isoprenoids to proteins,
e.g. to enhance their attachment to the cell membrane. It was discovered in the 1950s.
The reactions take place in the cytosol.
MEP/DOXP pathway
The MEP/DOXP pathway, also known as 2-C-methyl-D-erythritol
4-phosphate/deoxy-xylulose phosphate pathway or Mevalonic acid
independent pathway is located in the plastids of plants. It was
discovered in the late 1980s.
Pyruvate is converted by DOXP synthase (DXS) to
1-Deoxy-D-xylulose-5-P and by DOXP reductase (DXR) to
2-C-Methyl-D-erythritol-4-P (MEP) and in multiple steps to Isopentenyl
disphosphate (IPP) and DMAPP.
DMAPP is a common metabolite in both pathways and and exchange of
DMAPP would be possible.
Out of this intermediate, isoprene, monoterpenes (10-carbon)
diterpenes (15-carbon), carotenoids (40-carbon), chlorophylls and
Plastoquinone-9 (45-carbon) are formed.
Intermediates are geranyl pyrophosphate (GPP) and [[geranylgeranyl
pyrophosphate]] (GGPP). Both intermediates can also be found in the MVA
pathway.
Although both pathways, MVA and MEP, operate independently under
normal conditions, interactions between them have been reported
repeatedly.
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