Terpenes locating isoprene units

Locating Isoprene units in terpenes was discussed in Section 29.7. 

Show the location of the isoprene units in the following terpenes ... 

Notice (from your answer to Problem 15.8) that all of the isoprene units in farnesol and retinal are joined in a head-to-tail manner, but that they are not all arranged that way in squalene and jS-carotene. Where does the head-to-tail arrangement break down in these two terpenes Hint Count carbons to locate the terpene units.) What does this suggest about the way these two terpenes might be bio synthesized ... 

AcCoA is the building block of fatty acids, polyketides, and mevalonic acid (MVA), a cytosolic precursor of the C5 isoprene units for the biosynthesis of terpenes in the C,5 and Cj series (mind it is different from the MEP pathway, in product, and in cell location). Finally, AcCoA enters the citric acid or Krebs cycle, which leads to several precursors of amino acids. These are oxaloacetic acid, precursor of aspartic acid through transamination (thus toward lysine as a nitrogenated C N linear unit and methionine as a methyl supplier), and 2-oxoglutaric acid, precursor of glutamic acid (and subsequent derivatives such as ornithine as a nitrogenated C N linear unit). All these amino acids are key precursors in the biosynthesis of many alkaloids. 

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