Enzymatic catalysis by Friedel-Crafts-type reactions.

Although most enzymes work in aqueous medium, at their active sites they can adjust the polarity to meet the requirements of the reactions they catalyse. Thus, a Friedel-Crafts-type electrophilic substitution which is normally conducted in water-free media, can be used to activate the substrate for chemically difficult transformations. It is shown that histidine and phenylalanine ammonia lyases which contain the rare prosthetic group dehydroalanine, make use of a Friedel-Crafts-type reaction which was formerly thought to occur only in rather abiotic conditions. While histidine ammonia-lyase catalyses the first step of histidine degradation in most cells, phenylalanine ammonia-lyase is an important plant enzyme, producing cinnamic acid which is the precursor of lignins, coumarins and flavonoids responsible for the marvelous colours of many flowers.