Phenolic compound metabolism regulations

The general phenylpropanoid pathway begins with the deamination of L-phenylalanine to cinnamic acid catalyzed by phenylalanine ammonia lyase (PAL), Fig. (1), the branch-point enzyme between primary (shikimate pathway) and secondary (phenylpropanoid) metabolism [5-7]. Due to the position of PAL at the entry point of phenylpropanoid metabolism, this enzyme has the potential to play a regulatory role in phenolic-compound production. The importance of this is illustrated by the high degree of regulation both during development as well as in response to environmental stimuli. 

Given the scope of the processes in which phenolic compounds are involved (e.g. interactions with plant hormones, resistance to pathogen attack, wounds, UV rays, ozone and thermal stress, implications for agriculture), knowledge of the factors that regulate the metabolism of these compounds could enable the manipulation of their synthesis or degradation, depending on the conditions chosen or the results desired. 

Xenoblotics can alter various metabolic pathways In plants. Chemical alteration of phenolic compounds for agricultural purposes has been suggested to have potential (130. 131). Since several major plant enzymes of secondary metabolism are Involved In resistance to pathogens, alteration of secondary metabolism could Influence disease development. Herbicides, plant growth regulators (synthetic and natural), and enzyme Inhibitors such as those presented for PAL (Fig. 

Boutwell, R.K., 1967, Phenolic compounds as tumor-promoting agents, in "Phenolic Compounds and Metabolic Regulation", B.J. 

The structure of thyroxine, a thyroid hormone that helps to regulate metabolic rate, was determined in part by comparison with a synthetic compound believed to have the same structure as natural thyroxine. The final step in the laboratory synthesis of thyroxine by Harington and Barger, shown helow, involves an electrophilic aromatic substitution. Draw a detailed mechanism for this step and explain why the iodine substitutions occur ortho to the phenolic hydroxyl and not ortho to the oxygen of the aryl ether. [One reason iodine is required in our diet (e.g., in iodized salt) is for the biosynthesis of thyroxine.]... 

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