Enzyme-Coated Micro-Crystals: An Almost Forgotten but Very Simple and Elegant Immobilization Strategy

The immobilization of enzymes using protein coated micro-crystals (PCMCs) was reported for the first time in 2001 by Kreiner and coworkers. The strategy is very simple. First, an enzyme solution must be prepared in a concentrated solution of one compound (salt, sugar, amino acid) very soluble in water and poorly soluble in a water-soluble solvent. Then, the enzyme solution is added dropwise to the water soluble solvent under rapid stirring. The components accompanying the enzyme are called the crystal growing agents, the solvent being the dehydrating agent. This strategy permits the rapid dehydration of the enzyme solution drops, resulting in a crystallization of the crystal formation agent, and the enzyme is deposited on this crystal surface. The reaction medium where these biocatalysts can be used is marked by the solubility of the PCMC components, and usually these biocatalysts may be employed in water soluble organic solvents with a maximum of 20% water. The evolution of these PCMC was to chemically crosslink them and further improve their stabilities. Moreover, the PCMC strategy has been used to coimmobilize enzymes or enzymes and cofactors. The immobilization may permit the use of buffers as crystal growth agents, enabling control of the reaction pH in the enzyme environments. Usually, the PCMC biocatalysts are very stable and more active than other biocatalysts of the same enzyme. However, this simple (at least at laboratory scale) immobilization strategy is underutilized even when the publications using it systematically presented a better performance of them in organic solvents than that of many other immobilized biocatalysts. In fact, many possibilities and studies using this technique are lacking. This review tried to outline the possibilities of this useful immobilization strategy.

Improved Enantioselectivity of Subtilisin Carlsberg towards Secondary Alcohols by Protein Engineering

Generally, the catalytic activity of subtilisin Carlsberg (SC) for transacylation reactions with secondary alcohols in organic solvent is low. Enzyme immobilization and protein engineering was performed to improve the enantioselectivity of SC towards secondary alcohols. Possible amino-acid residues for mutagenesis were found by combining available literature data with molecular modeling. SC variants were created by site-directed mutagenesis and were evaluated for a model transacylation reaction containing 1-phenylethanol in THF. Variants showing high E values (>100) were found. However, the conversions were still low. A second mutation was made, and both the E values and conversions were increased. Relative to that shown by the wild type, the most successful variant, G165L/M221F, showed increased conversion (up to 36 %), enantioselectivity (E values up to 400), substrate scope, and stability in THF.

Optical resolution of various amino acids using a supported liquid membrane encapsulating a surfactant-protease complex

We have encapsulated a surfactant-protease complex (the main protease used being alpha-chymotrypsin) in an organic phase of a supported liquid membrane (SLM) for the optical resolution of various amino acids. L-Isomers of amino acids were enantioselectively permeated through the SLM. The mechanism of the amino acid permeation through the SLM was considered to be as follows; an L-amino acid was enantioselectively esterified with ethanol by a surfactant-protease complex encapsulated in the SLM, and the resulting L-amino acid ethyl ester dissolved into the organic phase of the SLM and diffused across the SLM. Another surfactant-alpha-chymotrypsin complex in the receiving phase catalyzed ester hydrolysis to produce the initial L-amino acid and ethanol, which are water-soluble. Thus, the L-amino acid was selectively transported to the receiving phase through the SLM on the basis of the molecular recognition of the surfactant-protease complex in the SLM. It was found that the catalytic activity and enantioselectivity of the surfactant-protease complex governed the permeate flux of amino acids and the enantiomeric excess in the membrane separation.

A Supported Liquid Membrane Encapsulating a Surfactant-Lipase Complex for the Selective Separation of Organic Acids

We have developed a novel, lipase-facilitated, supported liquid membrane (SLM) for the selective separation of organic acids by encapsulating a surfactant-lipase complex in the liquid membrane phase. This system exhibited a high transport efficiency for 3-phenoxypropionic acid and enabled the selective separation of organic acids due to the different solubilities of the acids in the organic phase and the variable substrate specificity of the surfactant-lipase complex in the liquid membrane phase. We found that various parameters, such as the amount of surfactant-lipase complex in the SLM, the lipase concentration in the receiving phase, and the ethanol concentration in the feed phase, affected the transport behavior of organic acids. The optimum conditions were 5 g L(-1) of the surfactant-CRL complex in the SLM (CRL=lipase from Candida rugosa), 8 g L(-1) of PPL in the receiving phase (PPL=lipase from porcine pancreas), and an ethanol concentration of 50 vol %. Furthermore, we achieved high enantioselective transport of (S)-ibuprofen attributable to the enantioselectivity of the surfactant-CRL complex.

Variations in the enantioselectivity of salt-activated subtilisin induced by lyophilization

Including excess salt during lyophilization has been shown to increase the activity of freeze-dried subtilisin Carlsberg (SC) in anhydrous media by over 20,000-fold [Ru et al. (1999) Biotechnol Bioeng 63:233-241]. In the present study, salt-activated SC (KCl-SC) showed a 30% enhancement in enantioselectivity compared to the salt-free enzyme in a variety of organic solvents. Activity toward both enantiomers of N-acetyl-phenylalanine methyl ester (APME) increased in tandem by 2-3 orders of magnitude in all solvents, indicating that the mechanism of salt activation is inherent to the enzyme and does not strongly favor one enantiomer over the other. However, activity and enantioselectivity of salt-activated SC could be manipulated through changes in the lyophilization conditions. Variations in lyophilization time, initial KCl concentration, and initial lyophilization volume altered enantioselectivity over 2-fold. The changes in enantioselectivity reflected the activity for the L enantiomer, while the activity toward the D enantiomer was mostly unaffected. The results indicate that the lyophilization time and final water content of the KCl-SC are important determinants of enzyme activity for the L enantiomer, suggesting that the favored reaction is more sensitive to the structural integrity of the salt-activated enzyme.

Covalent modification of subtilisin Bacillus lentus cysteine mutants with enantiomerically pure chiral auxiliaries causes remarkable changes in activity

Methanethiosulfonate reagents may be used to introduce virtually unlimited structural modifications in enzymes via reaction with the thiol group of cysteine. The covalent coupling of enantiomerically pure (R) and (S) chiral auxiliary methanethiosulfonate ligands to cysteine mutants of subtilisin Bacillus lentus induces spectacular changes in catalytic activity between diastereomeric enzymes. Amidase and esterase kinetic assays using a low substrate approximation were used to establish kcat/KM values for the chemically modified mutants, and up to 3-fold differences in activity were found between diastereomeric enzymes. Changing the length of the carbon chain linking the phenyl or benzyl oxazolidinone ligand to the mutant N62C by a methylene unit reverses which diastereomeric enzyme is more active. Similarly, changing from a phenyl to benzyl oxazolidinone ligand at S166C reverses which diastereomeric enzyme is more active. Chiral modifications at S166C and L217C give CMMs having both high esterase kcat/KM's and high esterase to amidase ratios with large differences between diastereomeric enzymes.

On enzymatic activity in organic solvents as a function of enzyme history

Catalytic activities of alpha-chymotrypsin and subtilisin Carlsberg in various hydrous organic solvents were measured as a function of how the enzyme suspension had been prepared. In one method, lyophilized enzyme was directly suspended in the solvent containing 1% water. In another, the enzyme was precipitated from its aqueous solution by a 100-fold dilution with an anhydrous solvent. In most cases, the reaction rate in a given nonaqueous enzymatic system strongly (up to an order of magnitude) depended on the mode of enzyme preparation. The magnitude of this dependence was markedly affected by the nature of the solvent and enzyme. A mechanistic hypothesis proposed to explain the observed dependencies was verified in additional experiments in which the water contents and enzyme history were further varied.