Phenylalanine hydrolases

The condensation of 2,4,5-triamino-6-hydroxypyrimidine and 5-deoxy-L-arabinose phenylhydrazone 1042, followed by oxidation of the intermediate 1043, gave biopterin 1044. The tetrahydrobiopterin is the natural cofactor of phenylalanine hydrolase. Various stereochemical isomers were also pre-... 

The probable metabohc defect in type I tyrosine-mia (tyrosinosis) is at himarylacetoacetate hydrolase (reaction 4, Figure 30-12). Therapy employs a diet low in tyrosine and phenylalanine. Untreated acute and chronic tyrosinosis leads to death from liver failure. Alternate metabolites of tyrosine are also excreted in type II tyrosinemia (Richner-Hanhart syndrome), a defect in tyrosine aminotransferase (reaction 1, Figure 30-12), and in neonatal tyrosinemia, due to lowered y>-hydroxyphenylpyruvate hydroxylase activity (reaction 2, Figure 30-12). Therapy employs a diet low in protein. 

Figure 17.19 Rates of hydrolysis of two families of esters by a hydrolase, chymotrypsin. The esters of N-acetyl-L-phenylalanine exhibit very similar rates because the process in each case is limited by the same enzyme deacylation reaction (Zerner et al., 1964). The esters of N-benzoyl glycine exhibit rates varying by more than a factor of 3 because their hydrolyses are mostly limited by the initial enzyme acylation step (Epand and Wilson, 1963).

Cell wall preparations of Colletrichum lindemuthianum Phaseolus vulgaris phenylalanine ammonia lyase cinnamic acid 4-hydrolase chalcone synthase chalcone isomerase (40)... 

Phenylalanine catabolism, 468 chemical structure, 19 plasma conoentralion, 46o sparing by tyrosine, 467,469 Phenylketonuria, 467,469 Phenylpyruvic acid, 469 FhIP,889,B90 Phlebotomy, 759 Phlorizin hydrolase, 109-110 Phorbol esters, cancer and, 916 Phosphatases, 54, 66 Phosphate, 694 in biologLcal fluids, 696 in bore, 697... 

Biosynthesis of flavonoids starts with the conversion of phenylalanine or tyrosine to cinnamic acid by phenylalanine ammonia lyase (PAL) (Hahlbrock and Grisebach, 1975). Subsequent reactions are catalyzed by cinnamic acid 4-hydrolase to form 4-hydroxyl cinnamic acid (p-coumaric acid). The p-coumaric acid is then catalyzed by p-coumarate CoA ligase to form p-coumaroyl CoA. 

A classic example of a typical enzymatic resolution on an industrial scale is the acylase-mediated production of L-methionine. This method has also been applied for the production of L-phenylalanine and L-valine. In addition to acylases, amidases, hydantoinases, and /i-lactam hydrolases represent versatile biocatalysts for the production of optically active L-amino acids. A schematic overview of the different type of enzymatic resolutions for the synthesis of L-amino acids is given in Fig. 2. 

Fig. 20.1 Major catabolic pathway for phenylalanine and tyrosine. The loci of known enzymatic defects are indicated by dashed lines. Note that hereditary tyrosinemia is now believed to be due to fumarylacetoacetate hydrolase, the final step in the pathway. (Redrawn with modifications from Mazur A, Harrow B Textbook of Biochemistry. WB Saunders, Philadelphia, 1971)...

Tyrosinemia type I Fumarylacetoacetate hydrolase Increased blood phenylalanine and tyrosine increased alpha-fetoprotein urinary succinylacetone Liver failure rentil tubular acidosis, failure to thrive, vomiting, diarrhea, rickets, porphyria-like crises, hepatic carcinoma Phenylalanine and tyrosine restriction (diet used in conjunction with NTBC or until liver transplantation is possible) None... 

Phenylalanine is not a precursor for L-dopa. Whereas tyrosine is incorporated into excised hypocotyls of Vicia faba seedlings, attempts at isolating a tyrosine hydrolase proved unsuccessful (Griffith and Conn, 1973). 

There are numerous examples of enantioselective hydrolyses of the types described in Schemes 3.2 and 3.3, catalysed by lipases and esterases. The selective hydrolysis of amino acid derivatives has been an important part of this field of study. For example, the hydrolase enzyme a-chymotrypsin catalyses the enantioselective hydrolysis of JV-acetyl-DL-phenylalanine methyl ester (5) to give optically pure (L)-acid (6) in 40% yield (Scheme 3.4). The lipase from the fungus Candida cylindracea (ccl) has been shown to hydrolyse octyl 2-chloropropionate (7) with high stereoselectivity on a large scale, giving the (/ )-acid (8) in 46% yield (96% e.e.) and the (S)-ester (45% yield) (Scheme 3.5). 

Ferulic acid is ubiquitously present in plants, mainly as part of the cell walls esterified to polysaccharides such as arabinoxylans. It is also an intermediate in the secondary metabolism of phenylalanine and tyrosine. It can be conveniently isolated from corn hulls, rice bran, and sugar beet. Biocatalytic processes using cinnamoyl esterases and glycosyl hydrolases have been developed for this purpose [32]. Another potentially interesting source is lignin, since ferulic acid is one of the prominent byproducts of its degradation. 

Finally, several other animal tissues yield useful enzymes that have been employed in S5mthesis. Pepsin is an important digestive protease from animal stomach whose native role is hydrolyzing amide bonds involving hydrophobic, aromatic amino adds, for example, phenylalanine, tyrosine, and tryptophan. Acylase from pordne kidney sdectivdy hydrolyzes N-acetyl amino adds and is commercially available. It has long been used for kinetic resolutions of amino adds. In addition to hydrolases, other animal enzymes have found important applications in biocatalysis. Rabbit musde aldolase is commerdally awiilable and was shown to catalyze aldol condensations between dihydroxyacetone phosphate and various nonnatural aldehydes by the Whitesides group in 1989 [10]. This seminal report touched off an avalanche of new applications for this and related enzymes in asymmetric synthesis. 

Lipases and esterases typically show selectivity toward the alcohol or amine part of a carboxylic acid derivative. Favored acyl groups are simple straight chains like acetate or butyrate. In contrast, proteases show higher specificity for the acyl part of a carboxylic acid derivative. Proteases contain a specificity pocket for the acyl group. For example, subtilisins and chymotrypsin favor ester and amides of phenylalanine esters. Another difference between the enzyme classes is that lipases and esterases catalyze hydrolysis of only esters, whereas proteases catalyze hydrolysis of both amides and esters. Several good books on hydrolases in organic synthesis are available [2-4]. 

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