Phenylalanine transamination

Keywords Phenylpropanoids - phenylalanine ammonia-lyase phenylalanine transamination - aminooxyacetic acid - aminooxyphenyIpropionic acid... 

Phenylalanine transamination, on the other hand, is more susceptible to inhibition by A0A(Table D.The vitro studies indicate, therefore, that AOPP displays a certain specificity for PAL deamination as compared to transamination, ... 

Fig. 25.8 (a) Normal metabolism, in which phenylalanine is converted by phenylalanine 4-mono-oxygenase to tyrosine, (b) Phenylketonuria, in which there is a transamination reaction between phenylalanine and a-ketoglutaric acid. Phenylalanine 4-mono-oxygenase is absent in about 1 in every 10000 human beings because of a recessive mutant gene. 

The anaerobic metabolism of L-phenylalanine by Thauera aromatica under denitrifying conditions involves several steps that result in the formation of benzoyl-CoA (a) conversion to the CoA-ester by a ligase, (b) transamination to phenylacetyl-CoA, (c) a-oxidation to phenylglyoxalate, and (d) decarboxylation to benzoyl-CoA (Schneider et al. 1997). 

Figure 2. Alternative enzymatic routing for L-phenylalanine biosynthesis. Dehydration followed by transamination defines the phenylpyruvate route, whereas the reverse order of reactions defines the arogenate route. Abbreviations GLU, L-glutamate aKG, 2-ketoglutarate.

These alkaloids can also be derived from non-aminoacid precursors. The N atom is inserted into the molecule at a relatively late stage, for example, in the case of steroidal or terpenoid skeletons. Certainly, the N atom can also be donated by an amino acid source across a transamination reaction, if there is a suitable aldehyde or ketone. Pseudoalkaloids can be acetate and phenylalanine-derived or terpenoid, as well as steroidal alkaloids. Examples of pseudoalkaloids include such compounds as coniine, capsaicin, ephedrine, solanidine, caffeine, theobromine and pinidine (Figure 6). More examples appear in Table 1. 

The precursors of true alkaloids and protoalkaloids are aminoacids (both their precursors and postcursors), while transamination reactions precede pseudoalkaloids (Tables 1 and 10). It is not difficult to see that from all aminoacids only a small part is known as alkaloid precursors (Table 19). Both true and proto alkaloids are synthesized mainly from the aromatic amino acids, phenylalanine, tyrosine (isoquinoline alkaloids) and tryptophan (indole alkaloids). Lysine is the... 

Although 2-phenylethanol can be synthesised by normal microbial metabolism, the final concentrations in the culture broth of selected microorganisms generally remain very low [110, 111] therefore, de novo synthesis cannot be a strategy for an economically viable bioprocesses. Nevertheless, the microbial production of 2-phenylethanol can be greatly increased by adding the amino acid L-phenylalanine to the medium. The commonly accepted route from l-phenylalanine to 2-phenylethanol in yeasts is by transamination of the amino acid to phenylpyruvate, decarboxylation to phenylacetaldehyde and reduction to the alcohol, first described by Ehrlich [112] and named after him (Scheme 23.8). 

In this transamination, the effect of para substitient groups has been studied using fluorinated phenylpyruvic acids and L-aspartic acid. From these results, the migratory preference is H > F > Cl > Br > CF3. This order has been attributed to the bulkiness of the substituted group [57]. Direct amination of p-substituted succinic acid with phenylalanine ammonialyase (EC 4.3.1.5) has suggested very high substrate specificity that the order of reaction rate is m-F o-F P-p-F >CF3. 

In individuals with PKU, a secondary, normally little-used pathway of phenylalanine metabolism comes into play. In this pathway phenylalanine undergoes transamination with pyruvate to yield phenylpyruvate (Fig. 18-25). Phenylalanine and phenylpyruvate accumulate in the blood and tissues and are excreted in the urine—hence the name phenylketonuria. Much of the phenylpyruvate, rather than being excreted as such, is either decarboxylated to phenylacetate or reduced to phenyllactate. Phenylacetate imparts a characteristic odor to the urine, which nurses have traditionally used to detect PKU in infants. The accumulation of phenylalanine or its metabolites in early life impairs normal development of the brain, causing severe mental retardation. This may be caused by excess phenylalanine competing with other amino acids for transport across the blood-brain barrier, resulting in a deficit of required metabolites. 

In plants and bacteria, phenylalanine and tyrosine are synthesized from chorismate in pathways much less complex than the tryptophan pathway. The common intermediate is prephenate (Fig. 22-19). The final step in both cases is transamination with glutamate. 

Figure 25-5 shows the principal catabolic pathways, as well as a few biosynthetic reactions, of phenylalanine and tyrosine in animals. Transamination to phenylpyruvate (reaction a) occurs readily, and the product may be oxidatively decarboxylated to phen-ylacetate. The latter may be excreted after conjugation with glycine (as in Knoop s experiments in which phenylacetate was excreted by dogs after conjugation with glycine, Box 10-A). Although it does exist, this degradative pathway for phenylalanine must be of limited importance in humans, for an excess of phenylalanine is toxic unless it can be oxidized to tyrosine (reaction b, Fig. 25-5). Formation of phenylpyruvate may have some function in animals. The enzyme phenylpyruvate tautomerase, which catalyzes interconversion of enol and oxo isomers of its substrate, is also an important immunoregulatory cytokine known as macrophage migration inhibitory factor.863...

Previously, AAT had been transformed into an L-tyrosine aminotransferase (TAT) by site-specific mutation of up to six amino acid residues lining the active site of wild-type AAT. The hextuple AAT-mutant achieved kinetic data towards the transamination of aromatic substrates such as i-phenylalanine within an order of magnitude of wild-type TAT (Onuffer, 1995). 

In cases where the natural amino acid side chains of enzymes are insufficient to carry out a desired reaction, the enzyme frequently uses coenzymes. A coenzyme is bound by the enzyme along with the substrate, and the enzyme catalyses the bimolecular reaction between the coenzyme and the substrate (cf. Section 2.6.3). A simple model for a-amino acid synthesis by transamination was developed by substituting /I-cyclodextrin with pyridoxamine. Pyridoxamine is able to carry out the transformation of a-keto acids to a-amino acids even without the presence of the cyclodextrin, but with the cyclodextrin cavity as well, the enzyme model proves to be more selective and transaminates substrates with aryl rings bound strongly by the cyclodextrin much more rapidly than those having little affinity for the cyclodextrin. Thus (p-le/f-butylphenyl) pyruvic acid and phenylpyruvic acid are transaminated respectively 15 000 and 100 times faster then pyruvic acid itself, to give (p-le/f-butylphenyl) alanine and phenylalanine (Scheme 12.5). 

Phenylalanine is first converted to tyrosine by the monooxygenase phenylalanine hydroxylase a reaction involving the coenzyme tetrahydrobiopterin. The tyrosine is then converted first by transamination and then by a dioxygenase reaction to homogentisate, which in turn is further metabolized to fumarate and acetoacetate. 

An inability to degrade amino acids causes many genetic diseases in humans. These diseases include phenylketonuria (PKU), which results from an inability to convert phenylalanine to tyrosine. The phenylalanine is instead transaminated to phenylpyruvic acid, which is excreted in the urine, although not fast enough to prevent harm. PKU was formerly a major cause of severe mental retardation. Now, however, public health laboratories screen the urine of every newborn child in the United States for the presence of phenylpyru-vate, and place children with the genetic disease on a synthetic low-phenylalanine diet to prevent neurological damage. 

They lack the enzyme phenylalanine oxidase, which converts phenylalanine to tyrosine. Thus phenylalanine accumulates in the body and it is degraded to phenylpyruvate by transamination ... 

Murakami et al. also found that the transamination reaction between hydrophobic pyridoxals (36 and 37) and a-amino acids, to produce a-keto acids, was extremely slow for neutral pyridoxals even in the presence of Cu(n) ions [24]. Detailed kinetic analysis of the reactions carried out in the vesicular system indicated that the transformation of the Cu(n) -quinonoid chelate into the Cu(n) -ketimine chelate was kinetically unfavorable compared with the competing formation of the Cu(n)-aldimine chelate from the same quinonoid species. This problem was solved to a certain extent by quaternization of the pyridyl nitrogen in pyridoxal, as Murakami et al. successfully accomplished transamination between catalyst 36 and L-phenylalanine to produce phenylpyruvic acid. 

Having successfully accelerated the reversible isomerization between the aldimine and ketimine Schiff bases, Murakami et al. then studied how to obtain turnovers in the full transamination reaction between one amino acid and one keto acid [25]. They found that the bilayer vesicle system constituted with 33, 36, and Cu(n) ions showed some turnovers for the transamination between L-phenylalanine and pyruvic add. However, such turnover behavior was not observed in a vesicular system composed of 32, 36, and Cu(n) ions, and an aqueous system involving N-methylpyridoxal and Cu(n) ions without amphiphiles. Therefore, both the hydrophobic effect and the imidazole catalysis effect were proposed as important for the turnover behavior. 

Products of Amino Acid Transamination Name and draw the structure of the a-keto acid resulting when each of the following amino acids undergoes transamination with a-ketoglutarate (a) aspartate, (b) glutamate, (c) alanine, (d) phenylalanine. 

Phenylketonuria is due to an inborn error of phenylalanine metabolism. Typically, it is due to a deficiency of phenylalanine hydroxylase. Atypically, it can be caused by a deficiency of dihydrobiopterin reductase and a resultant inability to synthesize biopterin. All these conditions cause an accumulation of phenylalanine, which can be transaminated to phenylpyruvic acid. 

We have arrived at prephenic acid, which as its name suggests is the last compound before aromatic compounds are formed, and we may call this the end of the shikimic acid pathway. The final stages of the formation of phenylalanine and tyrosine start with aromatization. Prephenic acid is unstable and loses water and CO2 to form phenylpyruvic acid. This a-keto-acid can be converted into the amino acid by the usual transamination with pyridoxal. 

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