Enzyme engineering of choline oxidase for improving stability

Functioning as a flavoprotein, choline oxidase facilitates the transformation of choline into glycine betaine. Notably, choline oxidase and its resultant product, glycine betaine, find extensive applications across various industries and fields of study. However, its high sensitivity and tendency to lose functional activity at high temperatures reduces its industrial usage. MD simulation and mutation studies have revealed the role of certain residues responsible for the enzyme's thermal instability. This study focuses on inducing thermal stability to choline oxidase of A. globiformis through computational approaches at a maximum temperature of 60 °C. MD simulation analysis showed that Trp 331, Val 464 and Ser 101 contribute to structural instability, leading to the instability at 60 °C. Mutation of these residues with phenylalanine residues and simulation of the mutated enzyme at 60 °C exhibited thermostability and insignificant residual fluctuation. The re-docking and MM/GBSA analyses further validated the mutated enzyme's binding affinity and catalytic activity.Communicated by Ramaswamy H. Sarma.

Phase Behavior of Internal Mixtures of Hydrocarbon-like Primary Organic Aerosol and Secondary Aerosol Based on Their Differences in Oxygen-to-Carbon Ratios

The phase behavior, the number and type of phases, in atmospheric particles containing mixtures of hydrocarbon-like organic aerosol (HOA) and secondary organic aerosol (SOA) is important for predicting their impacts on air pollution, human health, and climate. Using a solvatochromic dye and fluorescence microscopy, we determined the phase behavior of 11 HOA proxies (O/C = 0-0.29) each mixed with 7 different SOA materials generated in environmental chambers (O/C 0.4-1.08), where O/C represents the average oxygen-to-carbon atomic ratio. Out of the 77 different HOA + SOA mixtures studied, we observed two phases in 88% of the cases. The phase behavior was independent of relative humidity over the range between 90% and <5%. A clear trend was observed between the number of phases and the difference between the average O/C ratios of the HOA and SOA components (ΔO/C). Using a threshold ΔO/C of 0.265, we were able to predict the phase behavior of 92% of the HOA + SOA mixtures studied here, with one-phase particles predicted for ΔO/C < 0.265 and two-phase particles predicted for ΔO/C ≥ 0.265. The threshold ΔO/C value provides a relatively simple and computationally inexpensive framework for predicting the number of phases in internal SOA and HOA mixtures in atmospheric models.

Mechanistic insights into primary biotransformation of diethyl phthalate in earthworm and significant SOD inhibitory effect of esterolytic products

Phthalic acid esters (PAEs) are used as plasticizer or modifier in artificially-manufactured products. Though the rapid biotransformation of phthalates in microbes and plants have been well documented, it is less studied yet in terrestrial animals, e.g. earthworm. In this study, the major biotransformation of diethyl phthalate (DEP) in Eisenia fetida was illustrated using in vitro incubation of earthworm crude enzymes. DEP could be substantially biotransformed into phthalate monoester (MEP) and a small amount of phthalic acid (PA) through esterolysis, which was verified to be driven by endogenous carboxylesterase. Despite the inferior contribution, the oxidation of DEP might also occur under the initiated electron transfer by NADPH coenzyme. The dominant metabolite MEP showed a higher inhibition of superoxide dismutase (SOD) activity than DEP with EC₅₀ of 0.0082 ± 0.0016 mmol/L, so the higher ecological risks of MEP would be marked. The inhibition effect of PA was validated to be even stronger than MEP though it was slightly generated. The direct binding interaction with SOD was proved to be an important molecular event for regulation of SOD activity. Besides the static quenching mechanism, the caused conformational changes including despiralization of α-helix and spatial reorientation of tryptophan were spectrally believed to affect binding and underlie inhibition efficiency of SOD activity.

Effect of poly and mono-unsaturated fatty acids on stability and structure of recombinant S100A8/A9

Recombinant pET 15b vectors containing the coding sequences S100A8 and S100A9 are expressed in Escherichia coli BL21 (DE3) and purified using Ni-NTA affinity chromatography. The structural changes of S100A8/A9 complex are analyzed upon interaction with poly/mono-unsaturated fatty acids (UFAs). The thermodynamic values, Gibbs free energy and the protein melting point, are obtained through thermal denaturation of protein both with and without UFAs by thermal scanning of protein emission using the fluorescence spectroscopy technique. The far-ultraviolet circular dichroism spectra show that all studied unsaturated fatty acids, including arachidonic, linoleic, alpha-linolenic and oleic acids, induce changes in the secondary structure of S100A8/A9 by reducing the α-helix and β-sheet structures. The tertiary structure of S100A8/A9 has fluctuations in the fluorescence emission spectra after the incubation of protein with UFAs. The blue-shift of emission maximum wavelength and the increase in fluorescence intensity of anilino naphthalene-8-sulfonic acid confirm that the partial unfolding is caused by the conformational changes in the tertiary structure in the presence of UFAs. The structural changes in S100A8/A9 and its lower stability in the presence of UFAs may be necessary for S100A8/A9 to play a biological role in the inflammatory milieu.