Phenylalanine ammonia-lyase deamination

Phenylalanine ammonia-lyase (PAL EC 4.3.1.5) is a pivotal enzyme in controlling flow of carbon from aromatic amino acids to secondary aromatic compounds (Figure 1) (28). PAL primarily deaminates phenylalanine to form t-cinnamic acid, however, in many species, it also less efficiently deaminates tyrosine to form -coumaric acid. Because PAL is restricted to plants and is an important enzyme in plant development, Jangaard (29) suggested that PAL inhibitors might make safe and effective herbicides, however, in his screen of several herbicides, he found no compound to have a specific effect on PAL. This was also the case in studies by Hoagland and Duke (30, 31.) in which 16 herbicides were screened. 

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. 

In the first enzymatic step, phenylalanine ammonia lyase (PAL) converts phenylalanine to trans cinnamate, via a deamination reaction liberating ammonia. PAL can also convert tyrosine to p-coumarate, albeit at lower efficiency (MacDonald and D Cunha 2007). PAL functions as a tetramer of identical subunits, with two subunits combining to form one active site (Stafford 1990 MacDonald and D Cunha 2007). 

Until recently (43), only E-monolignols were considered to be involved in the process of lignification. This concept of exclusivity presumably arose from the following observations stereospecific deamination of phenylalanine, by phenylalanine ammonia lyase (PAL), affords E-cinnamic acid... 

As Figure 1 depicts, phenylalanine ammonia-lyase (PAL), which occurs ubiquitously in higher plants and the wood-rotting Basidiomycetes (1-3), seems to play a common central role in the conversion of phenylalanine (by deamination) to a wide variety of secondary metabolites. These include lignins in higher plants (4), veratryl alcohol in the white-rot fungus Phanerochaete chrysosporium (4a), and methyl p-anisate in the brown-rot fungus... 

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. 

L-Phenylalanine ammonia lyase (PAL EC 4.3.1.5), an enzyme found in a variety of plants, catalyzes both the deamination of L-phenylalanine to (L )-3-phenyl-2-butenoic acid and the reverse reaction33-36. 

Various commercial routes for the production of L-phenyalanine have been developed because of the utilization of this amino acid in the dipeptide sweetener Aspartame. One route that has been actively pursued is the synthesis of L-phenylalanine from trans-cinnamic acid using the enzyme phenylalanine ammonia lyase (105,106). This enzyme catalyzes the reversible, nonoxidative deamination of L-phenylalanine and can be isolated from various plant and microbial sources (107,108). 

Oxidative deamination of phenylalanine by phenylalanine ammonia lyase (PAL) and 4-hydroxylation affords p-coumaric acid, whose derivatives are the fundamental building blocks of lignin, as well as the... 

Histidine ammonia-lyase is the first enzyme in the degradation pathway of L-histidine and catalyzes the nonoxidative deamination of histidine (12) to form w r-urocanic acid (13) plus ammonia (Equation (3)). Histidine ammonia-lyase is present in several bacteria and in animals. The mechanism for the reaction that is catalyzed by histidine ammonia-lyase is presumed to be similar to that described above for phenylalanine ammonia-lyase (see Scheme 3). 

In the phenylpropanoid pathway, the first biosynthetic step leading to SA is a deamination of phenylalanine to CA which is catalyzed by phenylalanine ammonia lyase (PAL). This enzyme is induced by a range of biotic and abiotic stress conditions and is a key regulator in the phenylpropanoid pathway, which yields a variety of phenolics among others involved in structural and defense-related functions [42]. In recent years, PAL and its corresponding genes have been subject of numerous studies in various plant species [43 7]. 

Lignin precursors p-coumaryl (p-CA), coniferyl (CA) and sinapyl (SA) alcohols, are synthesized through the shikimate and cinamic acid pathways, starting from phenylalanine which under the action of phenylalanine ammonia-lyase (PAL) is deaminated followed by hydroxylations of the aromatic ring, methyla-tions and reductions of therminal acidic group to an alcohol leading to the formation of the monolignols (Fig. 8.2), [37, 48]. 

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. 

The enzyme phenylalanine ammonia-lyase (PAL) catalyses non-oxi-dative deamination of L-phenylalanine to form /ra 5-cinnamic acid... 

In plants and microorganisms cinnamic acid is formed from L-phenylalanine by phenylalanine ammonia-lyase (PAL). This enzyme catalyzes the antiperiplanar elimination of the pro 3S-hydrogen atom and of the NHg-group to yield trans-cinnamic acid (Fig. 294). Most PAL preparations deaminate also L-tyrosine, but to a smaller extend. In some organisms a special tyrosine ammonia-lyase exists. 

The aromatic amino acid L-phenylalanine (primary metabolite) is directed into the phe-nylpropanoid pathway leading to hydroxy-cinnamic acids, lignin and flavonoids by the activity of L-phenylalanine ammonia-lyase (PAL), which brings about its nonoxidative deamination yielding ammonia and tvans-cinnamic acid (Fig. 1). PAL is one of the most studied plant enzymes, and its crystal structure has recently been solved [2]. PAL is related to the histidine and tyrosine ammonia-lyases of amino acid catabolism. A class of bifunctional PALs found in monocotyle-donous plants and yeast can also deaminate tyrosine [3]. A single His residue is responsible for this switch in substrate preference [3, 4]. All three enzymes share a unique MIO (4-methylidene-imidazole-5-one) prosthetic group at the active site. This is formed auto-catalytically from the tripeptide Ala-Ser-Gly by cyclization and dehydration during a late... 

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. 

Biosynthesis of Plant Phenoiics. Phenolic compounds in plant foods are secondary metabolites which are derived from phenylalanine, and in some plants from tyrosine via enzymatic deamination assisted by ammonia lyase (Figure 1). Phenylpropanoids, the first products of deamination of phenylalanine and/or tyrosine consist of a phenyl ring (C ) and a 3 carbon side chain (C,). These Q-C, compounds may subsequently undergo hydroxylation in the phenyl ring and possibly subsequent methylation. This would lead to the formation of a large number of products which include cinnamic acid, p-coumaric acid, caffeic acid, ferulic acid and sinapic acid (5). 

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. 

Free amino acids are further catabolized into several volatile flavor compounds. However, the pathways involved are not fully known. A detailed summary of the various studies on the role of the catabolism of amino acids in cheese flavor development was published by Curtin and McSweeney (2004). Two major pathways have been suggested (1) aminotransferase or lyase activity and (2) deamination or decarboxylation. Aminotransferase activity results in the formation of a-ketoacids and glutamic acid. The a-ketoacids are further degraded to flavor compounds such as hydroxy acids, aldehydes, and carboxylic acids. a-Ketoacids from methionine, branched-chain amino acids (leucine, isoleucine, and valine), or aromatic amino acids (phenylalanine, tyrosine, and tryptophan) serve as the precursors to volatile flavor compounds (Yvon and Rijnen, 2001). Volatile sulfur compounds are primarily formed from methionine. Methanethiol, which at low concentrations, contributes to the characteristic flavor of Cheddar cheese, is formed from the catabolism of methionine (Curtin and McSweeney, 2004 Weimer et al., 1999). Furthermore, bacterial lyases also metabolize methionine to a-ketobutyrate, methanethiol, and ammonia (Tanaka et al., 1985). On catabolism by aminotransferase, aromatic amino acids yield volatile flavor compounds such as benzalde-hyde, phenylacetate, phenylethanol, phenyllactate, etc. Deamination reactions also result in a-ketoacids and ammonia, which add to the flavor of... 

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