Biocatalyst behaviour in low-water systems

Homotropic cooperative binding of organic solvent vapors by solid trypsin

Homotropic cooperative binding was observed at vapor sorption of organic solvents (acetonitrile, propionitrile, ethanol, 1-propanol, 2-propanol, nitroethane) by dried solid trypsin from porcine pancreas (0.05 g H2O/g protein). The vapor sorption isotherms were obtained by the static method of gas chromatographic headspace analysis at 298 K for 'vapor solvent+solid trypsin' systems in the absence of the liquid phase. All isotherms have a sigmoidal shape with significant sorbate uptake only above the threshold of sorbate thermodynamic activity. On the sorption isotherms of non-hydroxylic sorbates the saturation of trypsin by organic solvent was observed above the sorbate threshold activity. The formation of inclusion compounds with phase transition between solvent-free and solvent-saturated trypsin is supposed. Approximation of obtained isotherms by the Hill equation gives the inclusion stoichiometry S, inclusion free energy, and the Hill constant N of clathrates. The inclusion stoichiometry S depends significantly on the size and shape of sorbate molecules and changes from S=31 mol of sorbate per mol of trypsin for ethanol to S=6 for nitroethane. The inclusion free energies determined for the standard states of pure liquid sorbate and infinitely dilute solution in toluene are in the range from -0.5 to -1.2 kJ/mol and from -3.1 to -8.1 kJ/mol, respectively, per 1 mol of sorbate. The Hill constants are relatively high: from N=5.6 for 1-propanol to N approximately equal to 10(3) for nitroethane. The implication of the obtained results for the interpretation of solvent effects on the enzyme activity and stability in low-water medium is discussed.

Novel microbial lipases: catalytic activity in reactions in organic media

2,000 microbial strains were isolated from soil samples and tested to determine their lipolytic activity by employing screening techniques on solid and in liquid media. Culture broths were initially tested with 1,2-O-dilauryl-rac-glycero-3-glutaric acid-resorufinyl ester, a chromogenic substrate specific for lipases. Fourteen lipase-producing microorganisms were selected and their taxonomic identification was carried out. Hydrolysis of tributyrin or olive oil and the esterification of oleic acid with heptanol were selected to preliminary evaluate the catalytic activity of these lipases. All the selected lipases catalysed this esterification reaction with good yields. Resolution of (R,S)-2-(4-isobutylphenyl) propionic acid, (R,S)-1-phenylethanol, (R,S) 1-phenylethylamine and of (R) or (S) glycidol were performed to evaluate the stereoselectivity of these novel enzymes as biocatalysts in reactions in organic media. Lipases from the fungi Fusarium oxysporum and Ovadendron sulphureo-ochraceum gave the best yields and enantioselectivities in the resolution of racemic ibuprofen and 1-phenylethanol. Several lipases displayed a high stereoselectivity in the resolution of chiral amines by an alcoxycarbonylation reaction.

Improving enzymes by using them in organic solvents

The technological utility of enzymes can be enhanced greatly by using them in organic solvents rather than their natural aqueous reaction media. Studies over the past 15 years have revealed not only that this change in solvent is feasible, but also that in such seemingly hostile environments enzymes can catalyse reactions impossible in water, become more stable, and exhibit new behaviour such as 'molecular memory'. Of particular importance has been the discovery that enzymatic selectivity, including substrate, stereo-, regio- and chemoselectivity, can be markedly affected, and sometimes even inverted, by the solvent. Enzyme-catalysed reactions in organic solvents, and even in supercritical fluids and the gas phase, have found numerous potential applications, some of which are already commercialized.

Effect of beta-galactosidase hydration on alcoholysis reaction in organic one-phase liquid systems

Alcoholysis reactions were performed in organic one-phase liquid systems with E. coli beta-galactosidase to produce heptyl-beta-galactoside from lactose and 1-heptanol. The reaction rate was highly dependent on the amount of water solubilized in the alcohol. A larger amount of water led to a system of two liquid phases in which the alcoholysis rate was 73% faster than in the one-phase system. No hydrolysis reaction of either lactose or product was observed in one-phase liquid systems up to 20 h, independent of the water content. Solubility of lactose in the organic phase increased with the water content in the system and the reaction followed the Michaelis-Menten model. Water activity was calculated for heptanol containing different amounts of water and the obtained values were used to estimate the hydration of beta-galactosidase from known models. Enzyme activity correlated with sorbed water, similar to the behavior reported for lysozyme in low water environments. It is concluded that water contribution to enzyme hydration dominates the rate of reaction compared to its effect on lactose solubilization.

Alcoholysis and reverse hydrolysis reactions in organic one-phase system with a hyperthermophilic beta-glycosidase

Alcoholysis and reverse hydrolysis reactions were performed enzymatically in one-phase water-saturated 1-heptanol systems. Lactose or glucose was used as substrate to produce heptyl-beta-galactoside and/or heptyl-beta-glucoside, respectively. When alcoholysis of lactose was performed at 37 degrees C with beta-galactosidase from Escherichia coli, the initial rate was 14 nmol/mL min, and the limiting factors were the poor solubility of the substrate in 1-heptanol and low thermal stability of the enzyme. When a hyperthermophilic beta-glycosidase was used at 90 degrees C, the rate was 3.14-fold higher; in this case a higher concentration of soluble lactose in the water-saturated heptanol was available to the enzyme due to the higher temperature. The hyperthermophilic beta-glycosidase was also able to use glucose and galactose as substrates to achieve the reverse hydrolysis reaction. As a consequence, when lactose was used as substrate, heptyl-beta-galactoside was formed by alcoholysis, while the released glucose moiety was used in a secondary reverse hydrolysis reaction to produce heptyl-beta-glucoside. Both reactions followed Michaelis-Menten kinetics behavior. Neither lactose nor heptyl glycosides were hydrolyzed by this enzyme in water-saturated heptanol. However, the conversion was limited by a strong product inhibition and the formation of oligosaccharides, especially at high substrate concentrations, reducing the final glycoside yield.

Cold-adapted enzymes: from fundamentals to biotechnology

Psychrophilic enzymes produced by cold-adapted microorganisms display a high catalytic efficiency and are most often, if not always, associated with high thermosensitivity. Using X-ray crystallography, these properties are beginning to become understood, and the rules governing their adaptation to cold appear to be relatively diverse. The application of these enzymes offers considerable potential to the biotechnology industry, for example, in the detergent and food industries, for the production of fine chemicals and in bioremediation processes.

Supramolecular interactions of solid human serum albumin with binary mixtures of solvent vapors

Sorption isotherms of organic compounds on solid human serum albumin (HSA) from binary vapor mixtures were determined by gas chromatographic headspace analysis. The shape of sorption isotherms depends on molecular structure of studied sorbates. The 'active' compounds capable to sorb effectively on dry HSA increase the sorption of 'passive' compounds unable to be sorbed by dry HSA in absence of the third component. The critical hydration of HSA is required for sorption activation of 'passive' sorbates if water is taken as 'active' component. Ethanol and acetonitrile exhibit such activation effect without threshold. 'Passive' sorbates are able to produce cooperative activation effect on the sorption of 'active' component. Hydration history effect is observed for sorption on prehydrated HSA and HSA hydrated in situ. Obtained results were interpreted in terms of clathrate formation by 'passive' sorbate (substrate) and 'active' component inside the HSA (receptor) binding centers.

Optimization of Pseudomonas cepacia lipase preparations for catalysis in organic solvents

The activity of different lipase (from Pseudomonas cepacia) forms, such as crude powder (crude PC), purified and lyophilized with PEG (PEG + PC), covalently linked to PEG (PEG-PC), cross-linked enzyme crystals (CLEC-PC), and immobilized in Sol-Gel-AK (Sol-Gel-AK-PC) was determined, at various water activities (aw), in carbon tetrachloride, benzene and 1,4-dioxane. The reaction of vinyl butyrate with 1-octanol was employed as a model and both transesterification (formation of 1-octyl butyrate) and hydrolysis (formation of butyric acid from vinyl butyrate) rates were determined. Both rates depended on the lipase form, solvent employed, and aw value. Hydrolysis rates always increased as a function of aw, while the optimum of aw for transesterification depended on the enzyme form and nature of the solvent. At proper aw, some lipase forms such as PEG + PC, PEG-PC, and Sol-Gel-AK-PC had a total activity in organic solvents (transesterification plus hydrolysis) which was close to (39 and 48%) or even higher than (130%) that displayed by the same amount of lipase protein in the hydrolysis of tributyrin-one of the substrates most commonly used as standard for the assay of lipase activity-in aqueous buffer. Instead, CLEC-PC and crude PC were much less active in organic solvents (2 and 12%) than in buffer. The results suggest that enzyme dispersion and/or proper enzyme conformation (favored by interaction with PEG or the hydrophobic Sol-Gel-AK matrix) are essential for the expression of high lipase activity in organic media.

Studies on kinetic parameters and stability of aminoacylase in non-conventional media

Catalytic properties and conformational stability of aminoacylase (N-acylamino acid amidohydrolase, EC 3.5.1.14) were studied in water-N,N-dimethylformamide (DMF) and water-dioxane solvent mixtures. Beside the prompt inhibition the solvents caused further inactivation during incubations. In the presence of 5% DMF content the inactivation proceeds with a well-measurable rate (t1/2 39 min), while in the case of 20% DMF the enzyme practically lost its starting activity during 50 min incubation (t1/2 13 min). The K(m) value of the enzyme increased about three times with increasing DMF concentrations up to about 2.6 M DMF, while the Vmax value decreased practically to zero in the same concentration range.

1H-NMR on line monitoring of water activity during lipase catalysed esterification

1H-NMR spectroscopy is used to determine simultaneously the water activity (aw) and the time course of an esterification reaction catalysed by a lipase. Chemical shifts signals of hydroxylic hydrogens in fast exchange (i.e the average hydroxylic signal of acid, alcohol and water) varies with water activity and ester content. Calibration curves have been established from model media composed of the substrates and various ester contents, at different water activities, in order to mimic a reaction medium. One relationship is established between water activity, hydroxylic hydrogen signal chemical shift and ester content. In order to estimate the water activity evolution as a function of time, this last relationship is applied to the hydroxylic hydrogen chemical shift measured in a reaction medium where the Rhizomucor miehei lipase in a powder form is suspended in the liquid substrates. This alternative way of determining the water activity based on hydroxylic hydrogen chemical shift presents some advantages over more classical means, i.e. time saved and inaccuracies avoided by monitoring without handling the sample.

Activity and mobility of subtilisin in low water organic media: hydration is more important than solvent dielectric

The relationship between hydration, catalytic activity and protein dynamics was investigated for subtilisin Carlsberg in organic solvents with low water content. The organic media were cyclohexane, dichloromethane or acetonitrile, with controlled thermodynamic water activity (aw). Catalytic rate profiles showed the same dependence on aw for the three different solvents. The structural mobility of the enzyme in air and organic media was probed by proton solid-state NMR relaxation measurements. Both spin-lattice relaxation time (T1 ) and line width at half height (apparent spin-spin relaxation time (T2)) were determined for protein which was exchanged and hydrated with D2O. We found NMR relaxation was much more dependent on aw than medium identity (despite very different dielectrics) showing that enzyme hydration is the primary determinant of mobility. Results suggest that initial hydration up to aw 0.22 causes rigidification of part of the protein structure. As aw is increased further, enzyme mobility is found to increase. Above aw 0.44, a large increase in the proportion of more mobile protons coincides with a steep rise in catalytic activity for the enzyme in each of the solvents studied.

Effect of chain length on interactions of aliphatic alcohols with suspended human serum albumin

Enthalpy changes on the immersion of human serum albumin (HSA) into n-butanol, n-propanol, ethanol and methanol containing different amounts of water have been measured calorimetrically at 25 degrees C. Water sorption isotherms on HSA were also determined in water-n-butanol and water-ethanol mixtures. From comparison of the calorimetric and sorption data, it was concluded that there is a significant enthalpy change on the HSA immersion into methanol and ethanol even under conditions where there is no change in the quantity of adsorbed water. No similar contribution was found in the n-butanol based suspensions. Water monolayer capacity evaluated from the Langmuir model decreases also significantly when going from ethanol to n-butanol. Considering this non water sorption contribution, values of the monolayer capacity and the shape of the experimental dependences, it was inferred that a relatively small change of the solvent molecule structure (from n-propanol to ethanol) affects strongly the interactions of the protein with the solvent.

Activity of L-phenylalanine ammonia-lyase in organic solvents

L-Phenylalanine ammonia-lyase (EC 4.3.1.5), (PAL) was shown to be active in a monophasic non-aqueous medium for the first time. Ultraviolet absorbance spectra of trans-cinnamic acid were shown to be similar in both water and n-octanol. High catalytic rates were observed only when the enzyme was placed in solvents containing high concentrations of water. PAL forward reaction was observed only when the water concentration in n-octanol exceeded 2.0% (v/v), which corresponds to a value of 0.8 in thermodynamic water activity (aw) terms. In n-octanol containing either 2.0 or 3.5% (v/v) H2O (and 2 mM L-phenylalanine), lyophilized and aw = 0.113 pre-equilibrated PAL powder exhibited catalytic rates 0.02 and 1.75% of the value observed in aqueous solution respectively. A freshly lyophilized (non-equilibrated) PAL preparation incubated in water-saturated n-octanol (measured [H2O] = 3.6% (v/v), L-phenylalanine concentration approximately 6.8 mM) gave catalytic activity values 17% of those observed in aqueous solution. This is the first demonstration of catalytic activity of an amino acid ammonia-lyase in monophasic organic solvent.

Why are enzymes less active in organic solvents than in water?

In order to exploit fully the biotechnological opportunities afforded by nonaqueous enzymology, the issue of often drastically diminished enzymatic activity in organic solvents compared with that in water must be addressed and resolved. Recent studies have made great strides towards elucidating causes of this phenomenon of activity loss. None of these causes is insurmountable; by designing strategies that systematically target them, enzymatic activity in organic solvents can be readily enhanced by multiple orders of magnitude and ultimately brought to the aqueous-like level.