Thermostable variants constructed via the structure-guided consensus method also show increased stability in salts solutions and homogeneous aqueous-organic media

Enzyme instability is a major factor preventing widespread adoption of enzymes for catalysis. Stability at high temperatures and in the presence of high salt concentrations and organic solvents would allow enzymes to be employed for transformations of compounds not readily soluble in low temperature or in purely aqueous systems. Furthermore, many redox enzymes require costly cofactors for function and consequently a robust cofactor regeneration system. In this work, we demonstrate how thermostable variants developed via an amino acid sequence-based consensus method also showed improved stability in solutions with high concentrations of kosmotropic and chaotropic salts and water-miscible organic solvents. This is invaluable to protein engineers since deactivation in salt solutions and organic solvents is not well understood, rendering a priori design of enzyme stability in these media difficult. Variants of glucose 1-dehydrogenase (GDH) were studied in solutions of different salts along the Hofmeister series and in the presence of varying amounts of miscible organic solvent. Only the most stable variants showed little deactivation dependence on salt-type and salt concentration. Kinetic stability, expressed by the deactivation rate constant k(d,obs), did not always correlate with thermodynamic stability of variants, as measured by melting temperature T(m). However, a strong correlation (R(2) > 0.95) between temperature stability and organic solvent stability was found when plotting T(50)(60) versus C(50)(60) values. All GDH variants retained stability in homogeneous aqueous-organic solvents with >80% v/v of organic solvent.

Operational stability of enzymes. Acylase-catalyzed resolution of N-acetyl amino acids to enantiomerically pure L-amino acids

The method of measuring enzyme deactivation by monitoring necessary addition of fresh enzyme to keep a constant degree of conversion in a CSTR at constant [E] x tau, the product of concentration of active enzyme [E] and residence time tau, was successfully applied to acylase I from porcine kidney and Aspergillus oryzae fungus. Fungal enzyme was found to be more stable than kidney enzyme. Activation by both Co2+ and Zn2+ ions also yielded increased operational enzyme stability: Co2+ and Zn2+ are better stabilizers than activators. Mg2+ and Ca2+ are found to be neither activators nor stabilizers. Fungal acylase partially deactivated by exposition to a metal-free medium in the CSTR was reactivated by addition of Zn2+, demonstrating that loss of Zn2+ from the enzyme molecule is mainly responsible for deactivation in a continuous reactor.

Synthesis of dipeptides by suspension-to-suspension conversion via thermolysin catalysis: from analytical to preparative scale

When using proteases in direct reversal of their normal hydrolytic function, the equilibrium position is very important in limiting the attainable yield in equilibrium-controlled enzymic peptide synthesis. Analysis of the equilibrium position reveals a favourable shift towards the peptide product if starting materials are largely undissolved in the reaction medium and the product precipitates. This approach enabled us to obtain high peptide yields in thermolysin-catalysed reactions in high-density aqueous media with an equimolar supply of substrates. The easy scale-up (up to mol-scale) of this approach is demonstrated by two examples. Z-His-Phe-NH2 and Z-Asp-Phe-OMe, precursors for cyclo-[-His-Phe-] and the low-calorie sweetener Aspartame, respectively, were synthesized in preparative yields of 84-88%.

An enzymatic route to L-ornithine from arginine--activation, selectivity and stabilization of L-arginase

The non-proteinogenic amino acid L-ornithine (L-Orn) can be conveniently obtained by enzymatic hydrolysis of arginine (Arg) with arginase (EC 3.5.3.1). Arginase from calf liver (Vmax = 459 mumol/(min.mg), Km = 25.5 mM) is inhibited competitively by L-Orn (Ki = 480 mM). The enzyme was found to be completely enantioselective (E-value > 100) so that D,L-Arg can be split into D-Arg and L-Orn. Operational stability at 25 degrees C (deactivation rate constant kdeact = 3.8 x 10(-3) h-1; tau 1/2 = 182 h) is sufficient for use in a continuous process but is significantly smaller than temperature stability (kdeact = 4.1 x 10(-4) h-1; tau 1/2 = 1682 h); mechanical stress through stirring and unsteady Mn2+ supply owing to oxidation in the continuous process are believed to cause the difference. Addition of ascorbic acid stabilizes calf liver arginase at temperatures higher than 25 degrees C (at 60 degrees C, delta delta G not equal to = 2.9 kJ/mol).

Cation and strong co-solute effects on protein kinetic stability

Degradation of proteins is important for the operating life of biocatalysts and the shelf life of protein pharmaceuticals. We have previously found that the deactivating effects of salts on proteins can be correlated to an indicator of ion hydration, the B-viscosity coefficient of the anion in solution. Here, we test the influence of cations on protein kinetic stability by observing deactivation of mRFP (monomeric red fluorescent protein) in ammonium, caesium and chloride salt solutions, and we find that mRFP deactivation does not depend on cation hydration. We also measure mRFP deactivation in solutions containing denaturants (guanidinium chloride or urea) or stabilizing co-solutes (glycerol or sucrose) frequently encountered in many protein formulations to test whether hydration of these co-solutes can be used to indicate their relative effects on protein kinetic stability. We find that mRFP deactivation in solutions containing kosmotropic salts or stabilizers reaches a limiting rate and that hydration of denaturants is not an indicator of their denaturing strength.