L-Ornithine (Figure 6.1) is a non-protein amino acid forming part of the urea cycle in animals, where it is produced from L-arginine in a reaction catalysed by the enzyme arginase. In plants it is formed mainly from L-glutamate (Figure 6.2). Ornithine contains both S- and a-amino groups, and it is the nitrogen from the former group which is incorporated into alkaloid structures along with the carbon chain, except for the carboxyl group.
2H L-Orn pyrrolidine C4N
Thus ornithine supplies a C4N building block to the alkaloid, principally as a pyrrolidine ring system, but also as part of the tropane alkaloids (Figure 6.1). Most of the other amino acid alkaloid precursors typically supply nitrogen from their solitary a-amino group. However, the reactions of ornithine are almost exactly paralleled by those of L-lysine, which incorporates a C5N unit containing its e-amino group (see page 307).
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