Modification of galactitol dehydrogenase from Rhodobacter sphaeroides D for immobilization on polycrystalline gold surfaces

Characterization of the sorbitol dehydrogenase SmoS from Sinorhizobium meliloti 1021

Sinorhizobium meliloti 1021 is a Gram-negative alphaproteobacterium with a robust capacity for carbohydrate metabolism. The enzymes that facilitate these reactions assist in the survival of the bacterium across a range of environmental niches, and they may also be suitable for use in industrial processes. SmoS is a dehydrogenase that catalyzes the oxidation of the commonly occurring sugar alcohols sorbitol and galactitol to fructose and tagatose, respectively, using NAD⁺ as a cofactor. The main objective of this study was to evaluate SmoS using biochemical techniques. The nucleotide sequence was codon-optimized for heterologous expression in Escherichia coli BL21 (DE3) Gold cells and the protein was subsequently overexpressed and purified. Size-exclusion chromatography and X-ray diffraction experiments suggest that SmoS is a tetramer. SmoS was crystallized, and crystals obtained in the absence of substrate diffracted to 2.1 Å resolution and those of a complex with sorbitol diffracted to 2.0 Å resolution. SmoS was characterized kinetically and shown to have a preference for sorbitol despite having a higher affinity for galactitol. Computational ligand-docking experiments suggest that tagatose binds the protein in a more energetically favourable complex than fructose, which is retained in the active site over a longer time frame following oxidation and reduces the rate of the reaction. These results supplement the inventory of biomolecules with potential for industrial applications and enhance the understanding of metabolism in the model organism S. meliloti.

Tandem anaerobic-aerobic degradation of ranitidine, diclofenac, and simvastatin in domestic sewage

There is a consensus among scientists that domestic sewage treatment plants are the main sources of drugs entry into the aquatic environment. Therefore, this work studies the biodegradation of the drugs ranitidine (RNT), diclofenac (DCF), and simvastatin (SVT) (50 μg L^-1, each), in real domestic sewage, using a continuous anaerobic-aerobic reactor with immobilized biomass and an anaerobic batch reactor. The continuous anaerobic-aerobic reactor was operated for 6 months with hydraulic retention time (HRT) of 8 h. The initial degradation rates and the maximum oxidation capacities (MOC) of the system were estimated, achieving 90, 72, and 62% removals and 100, 93, and 72% of MOC for RNT, DCF and SVT, respectively, as well as 71% removal of soluble chemical oxygen demand (COD). RNT was degraded throughout the reactor, while DCF was degraded mainly in the two anaerobic chambers and SVT in the first anaerobic chamber. Anaerobic batches were used for the identification of biodegradation by-products (2,6-dichloro-N-(2-methylphenyl) aniline and simvastatin acid), the evaluation of the specific methanogenic activity (SMA) inhibition, and the estimation of acute and chronic ecotoxicities using the ECOSAR 1.11 software. The present study showed that, even at environmental concentrations, RNT, DCF, and SVT were capable of inhibiting the SMA. Lipophilicities dictated the behavior of those three drugs. The greater their lipophilicities, the greater the SMA inhibition and their ecotoxicity.

Immobilization of Cysteine-Tagged Proteins on Electrode Surfaces by Thiol-Ene Click Chemistry

Thiol-ene click chemistry can be exploited for the immobilization of cysteine-tagged dehydrogenases in an active form onto carbon electrodes (glassy carbon and carbon felt). The electrode surfaces have been first modified with vinylphenyl groups by electrochemical reduction of the corresponding diazonium salts generated in situ from 4-vinylaniline. The grafting process has been optimized in order to not hinder the electrochemical regeneration of NAD(+)/NADH cofactor and soluble mediators such as ferrocenedimethanol and [Cp*Rh(bpy)Cl](+). Having demonstrated the feasibility of thiol-ene click chemistry for attaching ferrocene moieties onto those carbon surfaces, the same approach was then applied to the immobilization of d-sorbitol dehydrogenases with cysteine tag. These proteins can be effectively immobilized (as pointed out by XPS), and the cysteine tag (either 1 or 2 cysteine moieties at the N terminus of the polypeptide chain) was proven to maintain the enzymatic activity of the dehydrogenase upon grafting. The bioelectrode was applied to electroenzymatic enantioselective reduction of d-fructose to d-sorbitol, as a case study.

Sol-gel Approaches for Elaboration of Polyol Dehydrogenase-Based Bioelectrodes

This review describes the input of sol-gel chemistry to the immobilization of polyol dehydrogenases on electrodes, for applications in bioelectrocatalysis. The polyol dehydrogenases are described and their application for biosensing, biofuel cell and electrosynthesis are briefly discussed. The immobilization of proteins via sol-gel approaches is described, including a discussion on the difficulty to maintain the activity of proteins in a silica matrix and the strategies developed to offer a proper environment to the proteins by developing optimal organic-inorganic hybrid materials. Finally, the co-immobilization of the NAD + co-factor and of mediators for the elaboration of reagentless devices is presented, based on published and original data. All-in-all, sol-gel approaches appear to be a very promising for development of original electrochemical applications involving dehydrogenases in near future.

Structural insight into substrate differentiation of the sugar-metabolizing enzyme galactitol dehydrogenase from Rhodobacter sphaeroides D

Galactitol 2-dehydrogenase (GatDH) belongs to the protein superfamily of short-chain dehydrogenases. As an enzyme capable of the stereo- and regioselective modification of carbohydrates, it exhibits a high potential for application in biotechnology as a biocatalyst. We have determined the crystal structure of the binary form of GatDH in complex with its cofactor NAD(H) and of the ternary form in complex with NAD(H) and three different substrates. The active form of GatDH constitutes a homo-tetramer with two magnesium-ion binding sites each formed by two opposing C termini. The catalytic tetrad is formed by Asn(116), Ser(144), Tyr(159), and Lys(163). GatDH structurally aligns well with related members of the short-chain dehydrogenase family. The substrate binding pocket can be divided into two parts of different size and polarity. In the smaller part, the side chains of amino acids Ser(144), Ser(146), and Asn(151) are important determinants for the binding specificity and the orientation of (pro-) chiral compounds. The larger part of the pocket is elongated and flanked by polar and non-polar residues, enabling a rather broad substrate spectrum. The presented structures provide valuable information for a rational design of this enzyme to improve its stability against pH, temperature, or solvent concentration and to optimize product yield in bioreactors.