HighlightsPhenyl-/aminoalkylsilane-grafted silica gels were used as supports for CaLB.Adsorption onto mixed-function-grafted silica supports resulted in active CaLB.CaLB adsorption and glutardialdehyde cross-linking resulted in enhanced durability.The novel CaLB biocatalysts were used in kinetic resolutions of a secondary alcohol and an amine.The novel CaLBs were robust biocatalysts in continuous-flow biotransformations. AbstractAdsorption onto solid supports has proven to be an easy and effective way to improve the mechanical and catalytic properties of lipases. Covalent binding of lipases onto the support surface enhances the active lifetime of the immobilized biocatalysts. Our study indicates that mesoporous silica gels grafted with various functions are ideal supports for both adsorptive and covalent binding for lipase B from Candida antarctica (CaLB). Adsorption of CaLB on phenyl-functionalized silica gels improved in particular its specific activity, whereas adsorption on aminoalkyl-modified silica gels enabling covalent binding with the proper reagents resulted in only moderate specific activity. In addition, adsorption on silica gels modified by mixtures of phenyl-and aminoalkyl silanes significantly increased the productivity of CaLB. Furthermore, CaLB adsorbed onto a phenyl/aminoalkyl-modified surface and then treated with glutardialdehyde (GDA) as cross-linking agent provided a biocatalyst of enhanced durability. Adsorbed and cross-linked CaLB was resistant to detergent washing that would otherwise physically deactivate adsorbed CaLB preparations. The catalytic properties of our best immobilized CaLB variants, including temperaturedependent behavior were compared between 0 and 70 °C with those of two commercial CaLB biocatalysts in the continuous-flow kinetic resolutions of racemic 1-phenylethanol rac-1a and 1-phenylethanamine rac-1b.

Besides the post‐translationally cyclizing catalytic Ala‐Ser‐Gly triad, Tyr110 and its equivalents are of the most conserved residues in the active site of phenylalanine ammonia‐lyase (PAL, EC 4.3.1.5), histidine ammonia‐lyase (HAL, EC 4.3.1.3) and other related enzymes. The Tyr110Phe mutation results in the most pronounced inactivation of PAL indicating the importance of this residue. The recently published X‐ray structures of PAL revealed that the Tyr110‐loop was either missing (for Rhodospridium toruloides) or far from the active site (for Petroselinum crispum). In bacterial HAL (∼500 amino acids) and plant and fungal PALs (∼710 amino acids), a core PAL/HAL domain (∼480 amino acids) with ≥ 30% sequence identity along the different species is common. In plant and fungal PAL a ∼100‐residue long C‐terminal multi‐helix domain is present. The ancestor bacterial HAL is thermostable and, in all of its known X‐ray structures, a Tyr83‐loop‐in arrangement has been found. Based on the HAL structures, a Tyr110‐loop‐in conformation of the P. crispum PAL structure was constructed by partial homology modeling, and the static and dynamic behavior of the loop‐in/loop‐out structures were compared. To study the role of the C‐terminal multi‐helix domain, Tyr‐loop‐in/loop‐out model structures of two bacterial PALs (Streptomyces maritimus, 523 amino acids and Photorhabdus luminescens, 532 amino acids) lacking this C‐terminal domain were also built. Molecular dynamics studies indicated that the Tyr‐loop‐in conformation was more rigid without the C‐terminal multi‐helix domain. On this basis it is hypothesized that a role of this C‐terminal extension is to decrease the lifetime of eukaryotic PAL by destabilization, which might be important for the rapid responses in the regulation of phenylpropanoid biosynthesis.

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Abstract:The bioimprinting effect in sol-gel immobilization of lipases was studied to develop efficient novel immobilized biocatalysts with significantly improved properties for biotransformations in continuous-flow systems. The bioimprinting candidates were selected systematically among the substrate mimics already found in the active site of experimental lipase structures. Four lipases (Lipase AK, Lipase PS, CaLB and CrL) were immobilized by a sol-gel process with nine bioimprinting candidates using various combinations of tetraethoxysilane (TEOS), phenyltriethoxysilane (PhTEOS), octyltriethoxysilane (OcTEOS) and dimethyldiethylsilane (DMDEOS) as silica precursors. The biocatalytic properties of the immobilized lipases were characterized by enantiomer selective acylation of various racemic secondary alcohols in two different multisubstrate systems (mixture A: a series of alkan-2-ols rac-1a-e and mixture B: heptan-2-ol rac-1f and 1-phenylethanol rac1g). Except with Lipase AK, the most significant activity enhancement was found with the imprinting molecules already found as substrate mimics in Xray structures of various lipases. The synthetic usefulness of the best biocatalysts was demonstrated by the kinetic resolution of racemic 1-(thiophen-2-yl)ethanol (rac-1h) in batch and continuous-flow systems.