Plant Alkaloid Biosynthesis

Coniine from the hemlock plant Conium maculatum is a simple example of an alkaloid, although its biosynthesis is unusual. It was thought that coniine would be derived from the amino-acid lysine, but labelled lysine was not converted to coniine. It is unusual in being derived from a type I polyketide (Chapter 4). Both [6-14C]-5-oxo-octanoic acid and [6-14C]-5-oxo-octanal are well-incorporated into labelled coniine (Figure 9.3).

A more typical example of the biosynthesis of an alkaloid is given by the tobacco alkaloid nicotine, which has been shown to be derived from nicotinic acid and ornithine. Its biosynthesis has been studied in detail. A dihydropyridine is an intermediate because if nicotinic acid is labelled at C-6, that label is lost during condensation. The methyl group in the pyrrole ring comes from S-adenosyl methionine (Figure 9.4). In the final stage, the labelled hydrogen is lost as H" to NADP+. Anabasine (another tobacco alkaloid) is produced similarly from nicotinic acid and lysine.

hooc hooc

Q, coniine hooc nadph

5-oxo-octanoic acid nadph

-H20

Figure 9.3 The route to coniine (in plants) from a tetraketide via 5-oxo-octanoic acid, which was labelled in the 6-position

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