Aliphatic amidase enzymes

Aliphatic amidase enzymes demonstrate sequence similarity to the nitrilase superfamily thus indicating some form of evolutionary relationship. These amidases contain a Glu-Lys-Cys catalytic triad and exist as homotetrameric or homohexameric sttuctures that function via a ping-pong (bi-bi) reaction mechanism [60, 61]. 

Novo, C., Tata, R., Clemente, A., et al. 1995. Pseudomonas aeruginosa aliphatic amidase is related to the nitrilase/cyanide hydratase enzyme family and Cys is predicted to be the active site nucleophile of the catalytic mechanism. EEBS letters, 367 275-9. 

Aliphatic nitriles are often metabolized in two stages. First they are converted to the corresponding carboxamide by a nitrile hydratase and then to the carboxylic acid by an amidase enzyme (a protease) [626]. 

The types of enzymes that bring about hydrolysis are hydrolase enzymes. Like most enzymes involved in the metabolism of xenobiotic compounds, hydrolase enzymes occur prominently in the liver. They also occur in tissue lining the intestines, nervous tissue, blood plasma, the kidney, and muscle tissue. Enzymes that enable the hydrolysis of esters are called esterases, and those that hydrolyze amides are amidases. Aromatic esters are hydrolyzed by the action of aryl esterases and alkyl esters by aliphatic esterases. Hydrolysis products of xenobiotic compounds may be either more or less toxic than the parent compounds. 

Aliphatic nitrile hydratases, that catalyzed the hydrataiion of nitriles to amides, were purified and characterized in Arthrobacter sp. J1 [128], Brevibacterium R312 [129], and Rhodococcus sp. N774 [130]. In tixe first strain, the activity of an amidase, which forms acetic add and ammonia stoichiometrically from acetamide, was also detected [131]. Bioconversion of dinitrile to mononitrile catalyzed by nitrile hydratase and amidase was obtained from Corynebacterium sp. C5. The two enzymes were constitutively formed in cells [132]. 

Recently, the potential of bacterial enzymes for the synthesis of aromatic, optically active amides, and carboxylic acids firom racemic nitriles was evaluated. An enantiomer-selective amidase, active on several 2-aryl and 2-aryloxy propionamides, was identifided and purified from Brevibacterium sp. strain R312 [145]. A nitrilase, found in Acinetobacter sp. strain AK226 and able to hydrolyze efihciently both aromatic and aliphatic nitriles, was reported to hydrolyze racemic nitriles to optically active 2-aryl propionic acids [146]. Enzyme system of Rhodococcus butanica could be successfully adapted for the kinetic resolution of a-arylpropionitriles resulting in the formation of (R)-... 

The product of a NHase/amidase cascade reaction is an acid, which is the same as the single enzymatic reaction performed by a nitrilase. However, the NHases usually have different substrate specificities than nitrilases, making them more suitable for the production of certain compounds. Although most organisms have both NHase and amidase activity (see earlier text), it is sometimes preferable, in a synthetic application, to combine enzymes from different organisms. The reasons for this are the enantioselectivity of the amidase or specific activity or substrate specificity of either of the enzymes. In this way, products with different enantiomeric purity can be obtained. Recently, a coupling of a NHase with two different amidases with opposite enantiopreference together with an -amino-a-caprolactam racemase that allows the formation of small aliphatic almost enantiopure (R)- or (S)-amino acids via dynamic kinetic resolution processes has been described [52]. 

Investigation of the structure-function relationships in nitrilases was largely based on sequence analyses, homology modeling, and mutational studies, as the crystal structures of nitrile-hydrolyzing enzymes have not been available except for an aliphatic nitrilase from Pyrococcus abyssi [13]. Other crystallized members of the nitrilase superfamily (amidases, N-carbamoyl-D-amino acid amidohydrolases, etc. [9]) shared only low levels of identity with experimentally confirmed nitrilases. 

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