26.14By analogy to the reaction in which 7-dehydrocholesterol is converted to vitamin D3, the structureof vitamin D2can be deduced from that of ergosterol.26.15(a)Fatty acid biosynthesis proceeds by the joining of acetate units.Thus, the even-numbered carbons will be labeled with 14C when palmitic acid is biosynthe-sized from 14CH3CO2H.(b)As noted in Problem 26.6, arachidonic acid (Table 26.1) is the biosynthetic precursor of PGE2.The distribution of the 14C label in PGE2biosynthesized from 14CH3CO2H refects the fattyacid origin of the prostaglandins.(c)Limonene is a monoterpene, biosynthesized from acetate by way of mevalonate and isopen-
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