Amino acid acylase specificity

Resolution of Racemic Amines and Amino Acids. Acylases (EC3.5.1.14) are the most commonly used enzymes for the resolution of amino acids. Porcine kidney acylase (PKA) and the fungaly3.spet i//us acylase (AA) are commercially available, inexpensive, and stable. They have broad substrate specificity and hydrolyze a wide spectmm of natural and unnatural A/-acyl amino acids, with exceptionally high enantioselectivity in almost all cases. Moreover, theU enantioselectivity is exceptionally good with most substrates. A general paper on this subject has been pubUshed (106) in which the resolution of over 50 A/-acyl amino acids and analogues is described. Also reported are the stabiUties of the enzymes and the effect of different acyl groups on the rate and selectivity of enzymatic hydrolysis. Some of the substrates that are easily resolved on 10—100 g scale are presented in Figure 4 (106). Lipases are also used for the resolution of A/-acylated amino acids but the rates and optical purities are usually low (107). 

Hsu et have cloned two enzymes from Deimcoccus radiodurans for overexpression in E. coli in order to carry out a dynamic kinetic resolution to obtain L-homophenylalanine, frequently required for pharmaceutical synthesis. The starting material is the racemic mixture of A acetylated homophenylalanine, and the two enzymes are an amino acid A -acylase, which specifically removes the acetyl group from the L-enantiomer, and a racemase, which interconverts the D- and L-forms of the A acyl amino acids. The resolution was carried out successfully using whole-cell biocatalysts, with the two enzymes either expressed in separate E. coli strains or coexpressed in the same cells. 

This enzyme [EC 3.5.1.14] (also referred to as histozyme, hippuricase, benzamidase, dehydropeptidase II, amino-acylase I, and acylase I) catalyzes the hydrolysis of an A-acyl-L-amino acid to yield a fatty acid anion and an L-amino acid. The enzyme has a wide specificity for the amino acid derivative. It will also catalyze the hydrolysis of dehydropeptides. 

The acylase-catalyzed resolution of N-acetyl-D,L-amino acids to obtain enantiomerically pure i-amino acids (see Chapter 7, Section 7.2.1) has been scaled up to the multi-hundred ton level. For the immobilized-enzyme reactor (Takeda, 1969) as well as the enzyme membrane reactor technology (Degussa, 1980) the acylase process was the first to be scaled up to industrial levels. Commercially acylase has broad substrate specificity and sufficient stability during both storage and operation. The process is fully developed and allowed major market penetration for its products, mainly pharmaceutical-grade L-methionine and L-valine. 

Acylase (acylase I aminoacylase N-acetyl amino acid amidohydrolase E.C. 3.5.1.14), is one of the best-known enzymes as far as substrate specificity (Chenault, 1989) or use in immobilized (Takahashi, 1989) or membrane reactors (Wandrey, 1977, 1979 Leuchtenberger, 1984 Bommarius, 1992a) is concerned however, its exact mechanism or 3D structure is still not known (Gentzen, 1979 1980). Acylase is available in large, process-scale quantities from two sources, porcine kidney and the mold Aspergillus oryzae. 

Enzymes are chiral molecules with specific catalytic activities. For example, when an acylated amino acid is treated with an enzyme like hog kidney acylase or car-boxypeptidase, the enzyme cleaves the acyl group from just the molecules having the natural (l) configuration. The enzyme does not recognize D-amino acids, so they are unaffected. The resulting mixture of acylated D-amino acid and deacylated L-amino acid is easily separated. Figure 24-5 shows how this selective enzymatic deacylation is accomplished. 

A particularly elegant solution for the assay of proteases and acylases is offered by the fluorogenic detection of free amino acids by decomplexation of copper from calcein, which removes its quenching effect This principle has been used for assays of acylases, amidases, and proteases (Scheme 1.13) [52, 53]. For the case of proteases combinatorial assays are particularly in demand for testing multiple peptides in parallel and determining the cleavage specificity [54]. New solutions... 

Enzymatic Kinetic Resolution of N-Acyl Amino Acids Coupled with Racemization by N-Acyl Amino Acid Racemase Acylases are enzymes hydrolysing the N-acetyl derivatives of amino acids. They require the free carboxylate for activity and have long been used for the kinetic resolution of amino acids. The unreacted enantiomer is usually racemized in a separate step by treatment with acetic anhydride. While acylases from hog kidney have an L-specificity, bacterial acylases with L- and D-specificity of various origins have been isolated and used for the kinetic resolution of N-acetyl amino acids. An industrial process for the production of L-Met and other proteinogenic and non-proteinogenic L-amino acids such as L-Val, L-Phe, L-Norval, or L-aminobutyric acid has been established. Currently, several hundred tons per year of L-methionine are produced by this enzymatic conversion using an enzyme membrane reactor [46]. 

The starting material for the acylase process is a racemic mixture of N-acetyl-amino acids 20 which are chemically synthesized by acetylation of D, L-amino acids with acetyl chloride or acetic anhydride in alkaU via the Schotten-Baumann reaction. The kinetic resolution of N-acetyl-D, L-amino acids is achieved by a specific L-acylase from Aspergillus oryzae, which only hydrolyzes the L-enantiomer and produces a mixture of the corresponding L-amino acid, acetate, and N-acetyl-D-amino acid. After separation of the L-amino acid by a crystallization step, the remaining N-acetyl-D-amino acid is recycled by thermal racemization under drastic conditions (Scheme 13.18) [47]. In a similar process racemic amino acid amides are resolved with an L-spedfic amidase and the remaining enantiomer is racemized separately. Although the final yields of the L-form are beyond 50% of the starting material in these multistep processes, the effidency of the whole transformation is much lower than a DKR process with in situ racemization. On the other hand, the structural requirements for the free carboxylate do not allow the identification of derivatives racemizable in situ therefore, the racemization requires... 

The substrate specificity of acylase is very broad, and a wide range of pro-teinogenic and nonproteinogenic Af-acetyl and A-chloroacetyl amino acids are transformed by the enzyme. The enzyme membrane reactor (Figure 4) is operated continuously as a recycle reactor, and the enzyme is retained by a UF hollow-fiber membrane (MWCO 10000). 

However, the range of types of amino acids that can be resolved in this way is much greater than just the natural substrates (i.e. peptides made up of the twenty coded amino acids), because methods to relax the specificity of the enzymes have been found, in some cases by using organic solvents for the reactions. Penicillin acylase from Escherichia coli and an aminoacylase from Streptovercillium olivoreti-... 

As with the D-aminoacylases from Streptomyces sp. the enzymes from Alcaligenes strains have a preference for hydrophobic N-acetyl-amino acids. In this respect, they are similar to the L-specific acylase I from kidney preparations and Aspergillus sp. The Alcaligenesfaecalis enzyme prefers the N-acyl-D-amino acid derivatives from Met, Phe and Leu[951. If a high-affinity substrate residue occupies the hydrophobic side-chain pocket the enzyme even deacylates D-Met methyl esters or N-Ac-D-Met-Xaa dipeptide derivatives. 

The acylase-catalyzed resolution of /V-acetyl-DL-amino acids is a key commercial process. The racemization of the remaining /V-acetyl-D-amino acid after separation of the L-amino acid must be performed, but adds complexity and cost. Therefore, in situ racemization with a racemase possessing specificity for N-acylamino acids without affecting the stereochemistry of the product L-amino acids is very desirable. The pentameric enzyme from Streptomyces sp. Y-53 specifically catalyzes the racemization of N-acylamino acids without acting on amino acids [182],... 

Optical resolution of racemic amino acids (methionine, phenylalanine, tryptophan, valine) by the action of L-specific amino acylase from Aspergillus oryzae (Tanabe Seiyaku Co., Ltd.). The theoretical productivity of 1000 liter immobilized amino acylase columns ranges from 214 kg per day for L-Ala to 715 kg per day for L-Met. 

The second task, resolution of synthetic D,L-amino acids has been solved by conversion of the neutral amino acids into real carboxyUc acids by acylation of the amino group, either by the benzoyl or the formyl residue. The D,L-acyl amino acids then formed diastereomeric salts with optically active bases, mostly alkaloids, which differed in their solubihty in various solvents, and so could be separated by recrystallization. This method is still in use, although enzymatic procedures, specific oxidation of the D-antipode in the presence of D-amino acid oxidase or enzymatic, stereospecific removal of iV-acyl residues from d,l-AT-acetyl-amino acids by acylase, are more convenient. Certainly, L-amino acids became accessible from nature by the ester method, but without synthetic material, extended experiments of peptide couphng would have been impossible. 

D-Amino acids and L-amino acids have also been prepared by D-specific or L-sj ific acylases derived from microbial sources. In this process, DL-iV-acetyl amino acid 102 (Fig. 33) is resolved by hydrolytic reaction to yield the d- or L-amino acid depending on d- or L-selective acylase used in the reaction [176-178]. 

Penicillin G acylase (PGA) has pivotal role in industry for the synthesis of penicillin antibiotics. PGA catalyzes the hydrolysis of peniciUm and other P-lactam antibiotics to produce 6-amino penicillinic acid [53, 54]. Stability of PGA was investigated by assaying the enzyme activity at different time points after incubation of PGA in various ILs [53]. In the absence of substrate, about 2,000-fold increase in t, was observed in a hydrophobic IL, [EMIM][Tf2N] with respect to twpropanol. Whereas in the presence of substrate, PGA showed less stability in [Tf N]" containing ILs. The reverse trend was found for PGA in a water miscible IL [BMIM][PFg], i.e., PGA showed 9 times increase in tj in [BMIM][PF ] in the presence of substrate even at an elevated temperature of 40°C [54]. These altered t, values in hydrophobic ILs in the presence of substrates were explained on the basis of cumulative outcome of specific interaction of substrates with the active site of enzyme and inhibitory effect of the hydrolytic products formed in the reaction medium. It can be speculated that in a more hydrophilic IL like [BMIM][PF ], substrates shield the enzyme active site from direct interaction with the ionic matrix and thus imparts a stabilizing effect whereas in hydrophobic IL [EMIM][TfjN], the inhibitory effect of hydrolytic products predominates and destabilizes the enzyme. 

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