Tyrosine-ammonium lyase

First let us mention the biosynthesis of the cinnamic acids themselves (Fig. 96). As mentioned earlier they are derived from phenylalanine and tyrosine. By oxidative deamination phenylalanine is converted to cinnamic acid and tyrosine to / -coumaric acid. Since ammonia in the form of ammonium ions is set free in this reaction the enzymes concerned are called ammonium lyases. The tyrosine-ammonium lyase seems to be particularly important in grasses but is also to be found in the rest of the plant kingdom. However, the phenylalanine-ammonium lyase (PAL) is the more important of these two enzymes. We shall come across it again as the key enzyme of phenylpropane synthesis. 

Fig. 96. The biosynthesis of cinnamic acids. 1 = Phenylalanine-ammonium-lyase, 2 = Tyrosine-ammonium-lyase.

The structures and properties of quinone methides were recently reviewed . Inter alia, the microbial tyrosine phenol lyase (TPL) catalyzes the a, /3-elimination of L-tyrosine to phenol and ammonium pyruvate. It is assumed that the process includes three steps, the second of which is tautomerization of the aromatic moiety which converts it into a good leaving group (equation 49) °. 

Applications of ammonia lyases Ammonia lyases catalyze the reversible, regio- and stereoselective addition of ammonia to alkenes. The family of lyases includes aspartate, methyl aspartate, histidine, tyrosine and phenylalanine ammonia lyases. The system consisting of phenylalanine ammonia lyase (PAL, EC 4.3.5.1), cinnamic acid, water and ammonium ions was optimized in terms of pH, temperature, reaction time, concentration and buffer for the synthesis of (2S,3S)- and (25,37 )-[3- H]phenylalanines. 

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