Tyrosine ammonia-lyase TAL

The key reaction that links primary and secondary metabolism is provided by the enzyme phenylalanine ammonia lyase (PAL) which catalyzes the deamination of l-phenylalanine to form iran.v-cinnamic acid with the release of NH3 (see Fig. 3.3). Tyrosine is similarly deaminated by tyrosine ammonia lyase (TAL) to produce 4-hydroxycinnamic acid and NH3. The released NH3 is probably fixed by the glutamine synthetase reaction. These deaminations initiate the main phenylpropanoid pathway. 

Enzymatic preparations for PAL from monocotyledonous species (monocots) can show a similar activity against tyrosine (tyrosine ammonia lyase, TAL), and TAL enzymatic preparations also show PAL activity. That a single enzyme may account for the observed cooccurring TAL and PAL activities was confirmed by Rosier et al., who showed the recombinant Zea mays (maize) PAL converted tyrosine to 4-coumarate directly, thus removing the requirement for the usual 4-hydroxylation step in phenylpropanoid biosynthesis. 

PAL activity in soybean seedlings was increased by several herbicides such as DPX-4189, glyphosate and acifluorfen [112,113]. Also, the activities of PAL and tyrosine ammonia lyase (TAL) in both maize and soybean seedlings were also increased by alachlor [110] and metolachlor [111]. 

Oxidation of Phenolic Compounds. Phenolic compounds are widespread throughout the plant kingdom and are prevalent in fruits where they are important contributors to color and flavor (46). Phenolic compounds, particularly flavonoids and derivatives of chlorogenic acid, play a crucial role in the development of a number of postharvest disorders through their oxidation to brown compounds that discolor many fruits and vegetables and substantially reduce their quality. A number of enzymes catalyze the biosynthesis or oxidation of phenolic compounds, among them phenylalanine ammonia lyase (PAL), tyrosine ammonia lyase (TAL), cinnamic acid-4-hydroxylase (CA4H), polyphenol oxidase (PPO), and catechol oxidase (CAO). The chemistry... 

The first step of flavonone biosynthesis begins with the deamination of the amino acid phenylalanine or tyrosine by a phenylalanine ammonia-lyase (PAL) or a tyrosine ammonia-lyase (TAL), which affords cinnamic acid and p-coumaric acid, respectively (Figure 6.36). The formed cinnamic acid is first hydroxylated to p-coumaric acid by a membrane-bound P450 monooxygenase, cinnamate 4-hydroxylase (C4H), and then activated to p-coumaroyl-CoA by a 4-coumarate-CoA ligase (4CL). 4CL catalyzes also the conversion of caffeic acid, feruhc acid, and cinnamic acid to caffeoyl-CoA, feruloyl-CoA, and cinnamoyl-CoA, respectively. 

Some grasses (Poaceae or Gramineae) have the corresponding enzyme, L-tyrosine ammonia lyase (TAL), capable of converting L-tyrosine (5) into p-coumarate (2), but this enzyme is not widely distributed. TAL also acts in a highly stereospecific manner. The reaction also proceeds with removal of the pro-3S-hydrogen from the -methylene atom of L-tyrosine. It is not completely resolved that this enzyme is distinct from PAL (Hanson and Havir, 1981). 

Phenylpropanoid metabolism is initiated via deamination of the amino acids, Phe 1 and (in some instances) Tyr 2 (7), these conversions being catalyzed by the enzymes, phenylalanine ammonia-lyase (PAL) and tyrosine ammonia-lyase (TAL), respectively. With one apparent exception, Dunaliella (8), this pathway is absent in algae. Nevertheless, the essential absence of lignins (and related phenylpropanoids) in algae strongly implies that only those acquiring the padiway were able to make the transition to a terrestrial environment. 

PAL enzyme has been found to be widely distributed whereas the enzyme performing an analagous reaction with tyrosine as substrate, tyrosine ammonia lyase (TAL), has so far only been detected in grasses. 

---