Synthesis of a cationic pyridoxamine conjugation reagent and application to the mechanistic analysis of an artificial transaminase

Reduction of benzaldehyde catalyzed by papain-based semisynthetic enzymes

Some features of native enzyme's active site were used to conjunction with a chemical reagent or modifying group, which would generate new functionality different from the natural enzyme. In order to obtain an efficient catalyst, we have designed four different molecular size N-derivatives of modifiers and introduced them into the active site of papain to obtain new semisynthetic enzymes, which were used as catalyst in reduction of benzaldehyde to yield benzyl alcohol respectively, and the reactions carried out with recycling agent in 0.1 M phosphate buffer pH 6.5 at 37 degrees C. The results had shown that a longer N-derivative of semisynthetic enzyme had higher catalytic activity. Furthermore, we propose a plausible model for the catalytic mechanism in the semisynthetic enzymes system.

A Molecularly Imprinted Polymer-Based Synthetic Transaminase

The design, synthesis, and evaluation of a molecularly imprinted polymer transaminase mimic is described. Methacrylic acid-ethylene glycol dimethacrylate copolymers were synthesized using, as a template, a transition state analogue (TSA) for the reaction of phenylpyruvic acid and pyridoxamine to yield phenylalanine and pyridoxal. Polymer suitability was established on the basis of (1)H NMR studies of template-functional monomer interactions. Polymer recognition characteristics were examined in a series of HPLC studies using the polymers as chromatographic stationary phases. Selectivity for the TSA, relative to substrates and products, was observed in both aqueous and nonpolar media. In the latter case (chloroform/AcOH, 96:4), an enantioseparation factor (alpha) of 2.1 was obtained, and frontal chromatographic studies revealed the presence of 11.9 +/- 0.2 micromol g(-1) (dry weight) of enantioselective sites. Polymers imprinted with the l-form of the oxazine-based TSA induced a 15-fold enhancement of the apparent reaction rate (app. V(max) 2.5 x 10(-7) mol s(-1); app. K(m) 8.2 x 10(-3) M) and enantioselective production of phenylalanine (32 +/- 4% ee) for reactions conducted in an aqueous buffer system. Substrate selectivity was evident, and a turnover number (k(cat)) of 0.1 s(-)(1) was determined. This is the first example of the catalysis of sigmatropic shifts in aqueous media by molecularly imprinted polymers.

Exploring routes to stabilize a cationic pyridoxamine in an artificial transaminase: site-directed mutagenesis versus synthetic cofactors

Two artificial transaminases were assembled by linking a pyridoxamine derivative within an engineered fatty acid binding protein. The goal of mimicking a native transamination site by stabilizing a cationic pyridoxamine ring system was approached using two different strategies. First, the scaffold of intestinal fatty acid binding protein (IFABP) was tailored by molecular modeling and site-directed mutagenesis to position a carboxylate group close to the pyridine nitrogen of the cofactor. When these IFABP mutants (IFABP-V60C/L38K/E93E and -V60C/E51K/E93E) proved to be unstable, a second approach was explored. By N-methylation of the pyridoxamine, a cationic cofactor was created and tethered to Cys60 of IFABP-V60C/L38K and -V60C/E51K; this latter strategy had the effect of permanently installing a positive charge on the cofactor. These chemogenetic assemblies catalyze the transamination between alpha-ketoglutarate and various amino acids with enantioselectivities of up to 96% ee. The pH profile of the initial rates is bell shaped and similar to native aminotransferases. The k(cat) values and the turnover numbers for these new constructs are the highest achieved to date in our system. This success was only made possible by the unique flexibility of the underlying enzyme design concept employed, which permits full control of both the protein scaffold and the catalytically active group.

Enzyme design by chemical modification of protein scaffolds

Covalent modification methods allow an almost unlimited range of functionality to be introduced into proteins. In concert with genetic techniques, chemical strategies have had significant impact in the field of enzyme design. Major recent developments include introducing catalytic activity into inactive proteins, modifying the selectivity and/or reactivity of existing enzymes and designing novel enzyme-based biosensors. New chemical methods promise to further increase the range of functionality that can be incorporated into proteins. These results suggest that semi-synthetic methods will play a key role in the development of future biocatalysts.