Phenylalanine oxidative deamination

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... 

In the oxidative deamination reaction, the enzyme was active toward N-[l-D-(carboxyl)ethyl]-L-methionine, N-[l-D-(carboxyl)ethyl]-L-phenylalanine, etc. The substrate specificity for amino donors of ODH in the reductive secondary amine-forming reaction was examined with pyruvate as a fixed amino acceptor [15,24]. The enzyme utilized L-norvaline, L-2-aminobutyric acid, L-norleucine, P-chloro-L-alanine, o-acetyl-L-serine, L-methionine, L-isoleucine, L-valine, L-phenylalanine, L-homophenylalanine, L-leucine, L-alanine, etc. 3-Aminobutyric acid and L-phenylalaninol also acted as substrates for the enzyme. Other amino compounds, such as P-amino acids, amino acid esters and amides, amino alcohols, organic amines, hydroxylamines, and hydrazines, were inactive as substrates. Pyruvate, oxaloacetate, glyoxylate, and a-ketobutyrate were good amino acceptors. We named the enzyme as opine... 

The amino acids of animal tissue are involved in other reactions (1) oxidative deamination (2) non-oxidative decarboxylation (3) transamination (4) protein synthesis. Oxidative deamination is important only with respect to L-glutamate, which can be converted to 2-oxogJutarate and ammonia by glutamate dehydrogenase. Decarboxylation is confined to a few amino acids in animal tissue, notably glutamate, histidine, and (after hydroxylation) tryptophan and phenylalanine. In all cases, the products are potent pharmacological agents discussed under autocoid metabolism. Serine is also decarboxylated to ethanolamine, an important reaction which is referred to later in connection with transamination. 

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. 

After the branching point at prephenic acid (58), phenylalanine and tyrosine as well as the amines (59) are not interconvertible. Finally, deamination and oxidative cleavage of the presumed (and in some circumstances isolated) resulting alkenes yields the equivalent of benzaldehyde (60, R = H), C7H60, and />- hy dr o xyb en2 aldehyde (60, R = OH), ie, aromatics with one aliphatic carbon attached C6—C2 fragments). 

There are several examples of d to l inversion of amino acids in the literature. D-Phenylalanine may have therapeutic properties in endogenous depression and is converted to L-phenylalanine in humans [145]. o-Leucine is inverted to the L-enantiomer in rats. When o-enantiomer is administered, about 30% of the enantiomer is converted to the L-enantiomer with a measurable inversion from l to o-enantiomer. As indicated in Fig. 13, D-leucine is inverted to the L-enantiomer by two steps. It is first oxidized to a-ketoisocarproate (KIC) by o-amino acid oxidase. This a-keto acid is then asymmetrically reaminated by transaminase to form L-leucine. In addition, KIC may be decarboxylated by branched-chain a-keto acid dehydrogenase, resulting in an irreversible loss of leucine (Fig. 13) [146]. D-Valine undergoes a similar two-step inversion process, and this can be antagonized by other amino acids such as o-leucine. The primary factor appears to be interference with the deamination process [147]. 

The phenylalanine is oxidized and deaminized to generate phenylpyruvic acid, under the presence of surplus hydrogen peroxide. Phenylpyruvic acid is further oxidized and decarboxylated to generate phenylacetic add. If such a reaction system is under the condition of weak base environment of dimethyl formamide (DMF), the phenylalanine will be oxidized into tyrosine [24]. 

A brief discussion of the chemical reactivity of the products of these enzymes is central to our proposed use of these enz)nnes as antinutritive bases of resistance. Polyphenol oxidase (PPO) and peroxidase (POD) oxidize phenolics to quinones, which are strong electrophiles that alkylate nucleophilic functional groups of protein, peptides, and amino acids (e.g., -SH, -NHof -HN-, and -OH)(Figure 1)(53,63-65). This alkylation renders the derivatized amino acids nutritionally inert, often reduces the digestibility of protein by tryptic and chymotryptic enzymes, and furthermore can lead to loss of nutritional value of protein via polymerization and subsequent denaturation and precipitation (63,66-69). POD is also capable of decarboxylating and deaminating free and bound amino acids to aldehydes (e.g., lysine, valine, phenylalanine. 

Scheme 12.22. A representation of the deamination of phenylalanine (Phe,F) to (F)-cinnamic acid and the conversion of the latter into more highly oxidized derivatives. The caffeic acid and ferulic acid serve as introductory compounds to phenylpropanoids. EC numbers and some graphic materials provided in this scheme have been taken from appropriate links in a URL starting with http //www.chem.qmul.ac.uk/iubmb/enzyme/.

The second branch leads from chorismic acid first to prephenic acid. After this substance the pathway forks again via phenylpyruvate to phenylalanine and via p-hydroxyphenylpyruvate to tyrosine. These two aromatic amino acids are closely related to each other since phenylalanine can be oxidized to tyrosine. However, this last reaction does not seem to be very important in higher plants. On deamination, phenylalanine yields cinnamic acid and tyrosine p-coumaric acid, a derivative of cinnamic acid. 

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