BIOCHEMICAL SYSTEMATICS 



194 



Some basic considerations of 

 biosynthetic pathways involved in the 

 production of phenolics. 



The biosynthesis of phenohc compounds has been reviewed 

 by Neish (1960) and others. A central problem, that of the initial 

 aromatization, appears to have been solved through investigations 

 into the biosynthesis of aromatic amino acids in microorganisms. The 

 presently accepted biochemical pathway to tyrosine and phenyl- 

 alanine is that elucidated by Davis and coworkers (Davis, 1956; Levin 

 and Sprinson, 1960) through studies of E. coli biochemical mutants. 

 The essential features of this scheme are illustrated and the path- 

 way extended to include several classes of phenols in Fig. 11-2. 

 Although not all evidence that these pathways are operative in 

 higher plants is direct, isotope studies from several laboratories pro- 

 vide independently strongly favorable circumstantial evidence for 

 such pathways. 



There is, now, equally strong evidence from isotope studies 

 that in the flavonoids one benzenoid portion of the molecule comes 

 from a quite different pathway, namely head to tail condensation of 

 three acetyl groups (Rickards, 1961). Confirmation of the theory of 

 acetate condensation suggested by Birch and Donovan (1953) has 

 come from studies in four different laboratories in four different 

 countries, for example, Watkin, et al. in Canada (1957), Grisebach in 

 Germany (1957), Geissman and Swain in the United States (1957), 

 and Shibata and Yamazaki in Japan (1957). The acetate conden- 

 sations are involved in the formation of the benzene ring of the 

 flavonoid molecule customarily referred to as the "A ring" while the 

 general pathway to phenols provides the B ring and the three carbons 

 adjacent to the B ring (see below, formula of quercetin). 



quercetin 



In quercetin (and its anthocyanidin analog, cyanidin) ring B 

 is derived from the shikimic acid pathway and ring A from the acetate 

 pathway. This mechanism is probably generally representative of 

 flavonoid synthesis, possibly involving a chalkone (see below) inter- 

 mediate (Grisebach and Patschke, 1961). Hutchinson, et al. (1959) 



