A thermostable L-aminoacylase from Thermococcus litoralis: cloning, overexpression, characterization, and applications in biotransformations.

A thermostable L-aminoacylase from Thermococcus litoralis was cloned, sequenced, and overexpressed in Escherichia coli. The enzyme is a homotetramer of 43 kDa monomers and has an 82% sequence identity to an aminoacylase from Pyrococcus horikoshii and 45% sequence identity to a carboxypeptidase from Sulfolobus solfataricus. It contains one cysteine residue that is highly conserved among aminoacylases. Cell-free extracts of the recombinant enzyme were characterized and were found to have optimal activity at 85 degrees C in Tris-HCl at pH 8.0. The recombinant enzyme is thermostable, with a half-life of 25 h at 70 degrees C. Aminoacylase inhibitors, such as mono-tert-butyl malonate, had only a slight effect on activity. The enzyme was partially inhibited by EDTA and p-hydroxymercuribenzoate, suggesting that the cysteine residue and a metal ion are important, but not essential, for activity. Addition of Zn2+ and Co2+ to the apoenzyme increased the enzyme activity, whereas Sn4+ and Cu2+ almost completely abolished enzyme activity. The enzyme was most specific for substrates containing N-benzoyl- or N-chloroacetyl-amino acids. preferring substrates containing hydrophobic, uncharged, or weakly charged amino acids such as phenylalanine, methionine, and cysteine.

Metal- and Ligand-Accelerated Catalysis of the Baylis-Hillman Reaction.

The Baylis-Hillman reaction, the coupling of an unsaturated carbonyl compound/nitrile with aldehydes, is a valuable reaction but is limited in its practicality by poor reaction rates. We have endeavored to accelerate the reaction using Lewis acids and found that while conventional Lewis acids gave reduced rates group III, and lanthanide triflates (5 mol %) gave increased rates. The optimum metal salts were La(OTf)(3) and Sm(OTf)(3), which gave rate accelerations (k(rel)) of approximately 4.7 and 4.9, respectively, in reactions between tert-butyl acrylate and benzaldehyde when using stoichiometric amounts of DABCO. At low loadings of DABCO (up to 10 mol %), no reaction occurred due to association of DABCO with the metal. Use of additional ligands to displace the DABCO from the metal was studied, and the rate of reaction was found to increase further in most cases. Of the ligands tested, at 5 mol %, (+)-binol gave one of the largest rate accelerations (3.4-fold) and was studied in more detail. It was found that reactions occurred even at low DABCO concentration so that here the Lewis base and Lewis acid were able to promote the reaction without interference from each other. While the (+)-binol (and other chiral ligands) failed to provide any significant asymmetric induction, a substantial nonlinear effect was observed with binol. Thus, use of racemic binol gave no effect on the rate. In seeking to maximize the rate attainable, more soluble (liquid) ligands were studied. Diethyl tartrate and triethanolamine gave rate enhancements of 5.2x and 3.5x at 50 mol %, respectively, versus 1.5x and 2.3x at 5 mol %. The best protocol was to use 100 mol % DABCO, 50 mol % triethanolamine, and 5 mol % La(OTf)(3). This gave overall rate accelerations of between 23-fold and 40-fold depending on the acrylate and approximately 5-fold for acrylonitrile. A simple acid wash removed the reagents, leaving the product in the organic phase. While triethanolamine accelerated the reaction without the lanthanum triflate (18-22-fold at 80 mol %), the reaction in the presence of the metal salt was faster. The system was tested synthetically on various substrates and found to give good rate accelerations with both activated (benzaldehyde and p-nitrobenzaldehyde) and less activated aldehydes (anisaldehyde and cyclohexanecarboxaldehyde) with acrylates. The limited amount of dimerized acrylate in the latter reactions is noteworthy and should extend the range of substrates that can be made by the Baylis-Hillman reaction using our optimum conditions.