The alkaloid precursor is a substrate for the alkaloid molecule, as well as being the source of the alkaloid's nitrogen and skeleton. Precursors of alkaloids are very diverse in their structure, type and metabolic function. Generally, alkaloid precursors may be non-protein aminoacids (e.g., ornithine, nicotinic acid, anthranilic acid), protein aminoacids (e.g., lysine, tyrosine, tryptophan, histidine, phenylalanine) and various compounds (e.g., acetate and malonate), L-phenylalanine in the case of providing nitrogen from amination (geraniol, cholesterol, adenine, guanine in transamination reactions). In some cases there may be more that one precursor, when alkaloids can be synthesized via alternative pathways. This is typical, for example, of L-tyrosine and L-phenylalanine. Both are protein amino acids with aromatic side chains, but L-phenylalanine is not so frequently used in alkaloid synthesis. Moreover, L-phenylalanine generally contributes only carbon atom units (e.g., C6C3, C6C2, C6Q) to the alkaloid skeletons, without providing a nitrogen atom. Phenethylisoquinoline alkaloids, for example autumnaline, floramultine, kreysigine and colchicine, are derived from both these amino acids.
Although L-phenylalanine is a protein amino acid, and is known as a protein acid type of alkaloid precursor, its real role in biosynthesis (providing C and N atoms) only relates to carbon atoms. L-phenylalanine is a part of "magic 20" (a term deployed by Crick in his discussion of the genetic code) and just for this reason should also be listed as a protein amino acid type of alkaloid precursor, although its duty in alkaloid synthesis is not the same as other protein amino acids. However, in relation to "magic 20" it is necessary to observe that only part of these amino acids are well-known alkaloid precursors. They are formed from only two amino acid families: Histidine and Aromatic226 227 and the Aspartate family226.
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