Practical insights on enzyme stabilization

Enzymes are efficient catalysts designed by nature to work in physiological environments of living systems. The best operational conditions to access and convert substrates at the industrial level are different from nature and normally extreme. Strategies to isolate enzymes from extremophiles can redefine new operational conditions, however not always solving all industrial requirements. The stability of enzymes is therefore a key issue on the implementation of the catalysts in industrial processes which require the use of extreme environments that can undergo enzyme instability. Strategies for enzyme stabilization have been exhaustively reviewed, however they lack a practical approach. This review intends to compile and describe the most used approaches for enzyme stabilization highlighting case studies in a practical point of view.

(R,S)-2-chlorophenoxyl pyrazolides as novel substrates for improving lipase-catalyzed hydrolytic resolution

The best reaction condition of Candida antartica lipase B as biocatalyst, 3-(2-pyridyl)pyrazole as leaving azole, and water-saturated methyl t-butyl ether as reaction medium at 45°C were first selected for performing the hydrolytic resolution of (R,S)-2-(4-chlorophenoxyl) azolides (1-4). In comparison with the kinetic resolution of (R,S)-2-phenylpropionyl 3-(2-pyridyl)pyrazolide or (R,S)-α-methoxyphenylacetyl 3-(2-pyridyl)pyrazolide at the same reaction condition, excellent enantioselectivity with more than two order-of-magnitudes higher activity for each enantiomer was obtained. The resolution was then extended to other (R,S)-3-(2-pyridyl)pyrazolides (5-7) containing 2-chloro, 3-chloro, or 2,4-dichloro substituent, giving good (E > 48) to excellent (E > 100) enantioselectivity. The thermodynamic analysis for 1, 2, and 4-7 demonstrates profound effects of the acyl or leaving moiety on varying enthalpic and entropic contributions to the difference of Gibbs free energies. A thorough kinetic analysis further indicates that on the basis of 6, the excellent enantiomeric ratio for 4 and 7 is due to the higher reactivity of (S)-4 and lower reactivity of (R)-7, respectively.

Modification of Porcine Pancreatic Lipase with Z‐Proline

Porcine pancreatic lipase was modified with Z-proline via the constitution of amide bonds between the free amino groups of lipase and the carboxyl groups of Z-proline, which were activated by 1-ethyl-3-(3-dimethylaminopropyl)-carbodiimide (EDC). Different amounts of Z-proline were bound to lipase. Modification degree was determined by 2,4,6-trinitrobenzene sulphonic acid (TNBS), by means of the decrease in free amino groups on lipase. The reason for choosing Z-proline was its unique structural characteristics, protected amino groups, and its effect on protein conformation by reducing the flexibility of the lipase molecule, thus achieving stabilization against changes in pH and temperature. After the modification, porcine pancreatic lipase was found to have new physicochemical characteristics, such as optimum alkaline pH stability and thermal stability at elevated temperatures.

The role of conformational flexibility of enzymes in the discrimination between amino acid and ester substrates for the subtilisin-catalyzed reaction in organic solvents

To investigate how the conformational flexibility of subtilisin affects its ability to discriminate between enantiomeric amino acid and ester substrates for the subtilisin-catalyzed reaction in an organic solvent, the flexibility around the active site and the surface of subtilisin was estimated from the mobility of a spin label bound to subtilisin by ESR spectroscopy. Many studies on enzyme flexibility focus on the active site. Both the surface and active site flexibility play an important role in the enantioselectivity enhancement of the enzyme-catalyzed reaction. It was found, however, that the different behavior observed for the enantioselectivity between the amino acid and ester substrates could be correlated with the flexibility around the surface rather than the flexibility at the active site of subtilisin. In other words, for the ester substrates, the greater flexibility around the surface of subtilisin induced by a conformational change resulting from the presence of an additive such as DMSO is essential for the enantioselectivity enhancement. This model is also supported by the Michaelis-Menten kinetic parameters for each enantiomeric substrate. Our findings provide insight into the enantioselectivity enhancement for the resolution of enantiomers for enzyme-catalyzed reactions in organic solvents.

Chemical modification of biocatalysts

Although several powerful methods exist for the redesign of enzyme structure and function these are typically limited to the 20 most abundant proteinogenic amino acids. The use of chemical modification overcomes this limitation to allow virtually unlimited alteration of amino acid sidechain structures. If heterogeneous mixtures of enzyme products are to be avoided, however, the required chemistry should be efficient, selective and compatible with aqueous conditions. Recent advances have been made in the modification of proteinases, aminotransferases and redox enzymes.

Methods to increase enantioselectivity of lipases and esterases

Lipases and esterases are frequently used in the synthesis of optically pure compounds; however, natural enzymes do not always show sufficiently high enantioselectivity. Variation of the structure of the substrates, modification of the reaction system or protein engineering (e.g. the expression of pure enzymes, rational design or directed evolution) are strategies that can be employed to improve the distinction between two enantiomers or enantiotopic groups.