Catalytic properties of enzymes from Erwinia carotovora involved in transamination of phenylpyruvate

Abstract

K _m for L-phenylalanine, L-glutamic acid, L-aspartic acid, and the corresponding keto acids were calculated, as well as V _max was measured for the following pairs of substrates: L-phenylalanine-2-ketoglutarate, L-phenylalanine-oxaloacetate, L-glutamic acid-phenylpyruvate, and L-aspartic acid-phenylpyruvate for aminotransferases PAT1, PAT2, and PAT3 from Erwinia carotovora catalyzing transamination of phenylpyruvate. The ping-pong bi-bi mechanism was shown for the studied aminotransferases. The substrate inhibition (K _s) of PAT3 with 2-ketoglutarate and oxaloacetate was 10.23 ± 3.20 and 3.73 ± 1.99 mM, respectively.

It was shown that L-β-(N-benzylamino)alanine was a competitive inhibitor with respect to L-phenylalanine for PAT1 (K _i = 0.32 ± 0.07 mM, K _m = 0.45 ± 0.1 mM, V _max = 11. 6 ± 0.4 U/mg) at 25 mM concentration of 2-ketoglutarate in the reaction medium. L-β-(N-methylamino)alanine is a noncompetitive inhibitor with respect to L-phenylalanine for PAT3 (K _I = 138.4 ± 95.4 mM, K _m = 13.7 ±3.9 mM, V _max = 18.6 ± 4.1 U/mg) at 2 mM concentration of 2-ketoglutarate in the reaction medium. L-stereo isomers of nonprotein analogues of aromatic amino acids were studied as substrates for PAT1, PAT2, and PAT3. L-β-(2-Br-phenyl)alanine, L-β-(4-Br-phenyl)alanine, L-β-(2-F-phenyl)alanine, and L-(2-F)tryptophan were good substrates for all three aminotransferases; L-α-methyl-β-(2-Br-phenyl)alanine and L-O-benzyltyrosine were substrates only for PAT3; L-β-(4-F-phenyl)alanine was a substrate for PAT1 and PAT3. Thus, these analogues of aromatic amino acids can be stereoselectively synthesized using the studied aminotransferases in the presence of the corresponding keto acids.