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Enhanced stability in organic solvents is a desirable feature for enzymes implemented under industrial conditions. Lipases potential as biocatalysts is mainly limited by denaturation in polar alcohols. In this study we focused on selected solvent tunnels in lipase from Geobacillus stearothermophilus T6 to improve its stability in methanol during biodiesel synthesis. Using rational mutagenesis, bulky aromatic residues were incorporated in order to occupy solvent channels and induce aromatic interactions leading to a better inner core packing. Each solvent tunnel was systematically analyzed with respect to its chemical and structural characteristics. Selected residues were replaced with Phe, Tyr or Trp. Overall, 16 mutants were generated and screened in 60% methanol, from which 3 variants showed elevated stability up to 81-fold compared with wild-type. All stabilizing mutations were found in the longest tunnel detected in the "closed-lid" x-ray structure. Combining the Phe substitutions created double mutant A187F/L360F with an increase in Tm of +7°C in methanol and a 3-fold increase in biodiesel synthesis yield from waste chicken oil. Kinetic analysis with p-nitrophenyl laurate revealed that all mutants displayed lower hydrolysis rates (kcat), though mostly their stability properties determined the transesterification capability. Seven crystal structures of different variants were solved disclosing new p-p or CH/p intramolecular interactions emphasizing the significance of aromatic interactions to improved solvent stability. This rational approach could be implemented in other enzymes for stabilization in organic solvents. [ABSTRACT FROM AUTHOR]

Tyrosinase belongs to the type 3 copper enzyme family, containing a dinuclear copper center, CuA and CuB. It is mainly responsible for melanin production in a wide range of organisms. Although copper ions are essential for the activity of tyrosinase, the mechanism of copper uptake is still unclear. We have recently determined the crystal structure of tyrosinase from Bacillus megaterium (TyrBm) and revealed that this enzyme has tighter binding of CuA in comparison with CuB. Investigating copper accumulation in TyrBm, we found that the presence of copper has a more significant effect on the diphenolase activity. By decreasing the concentration of copper, we increased the diphenolase to monophenolase activity ratio twofold. Using a rational design approach, we identified five variants having an impact on copper uptake. We have found that a major role of the highly conserved Asn205 residue is to stabilize the orientation of the His204 imidazole ring in the binding site, thereby promoting the correct coordination of CuB. Further investigation of these variants revealed that Phe197, Met61, and Met184, which are located at the entrance to the binding site, not only play a role in copper uptake, but are also important for enhancing the diphenolase activity. We propose a mechanism of copper accumulation by the enzyme as well as an approach to changing the selectivity of TyrBm towards l-dopa production. Graphical abstract: [Figure not available: see fulltext.] [ABSTRACT FROM AUTHOR]

Tyrosinases are type 3 copper enzymes that are involved in the production of melanin and have two copper ions in the active site. Here, the crystallization and primary analysis of a tyrosinase from Bacillus megaterium is reported. The purified protein was crystallized in the absence or presence of zinc ions and the crystals diffracted to a resolution of 2.0 Å. Crystals obtained in the presence of zinc belonged to space group P212121, while crystals grown in the absence of zinc belonged to space group P21. In both space groups the asymmetric unit contained a dimer, with minor differences in the crystal density and in packing interactions. [ABSTRACT FROM AUTHOR]