Circular dichroism analysis of penicillin G acylase covalently immobilized on silica nanoparticles

Gold Nanoparticles Are an Immobilization Platform for Active and Stable Acetylcholinesterase: Demonstration of a General Surface Protein Functionalization Strategy

Immobilizing enzymes onto abiological surfaces is a key step for developing protein-based technologies that can be useful for applications such as biosensors and biofuel cells. A central impediment for the advancement of this effort is a lack of generalizable strategies for functionalizing surfaces with proteins in ways that prevent unfolding, aggregation, and uncontrolled binding, requiring surface chemistries to be developed for each surface-enzyme pair of interest. In this work, we demonstrate a significant advancement toward addressing this problem using a gold nanoparticle (AuNP) as an initial scaffold for the chemical bonding of the enzyme acetylcholinesterase (AChE), forming the conjugate AuNP-AChE. This can then be placed onto chemically and structurally distinct surfaces (e.g., metals, semiconductors, plastics, etc.), thereby bypassing the need to develop surface functionalization strategies for every substrate or condition of interest. Carbodiimide crosslinker chemistry was used to bind surface lysine residues in AChE to AuNPs functionalized with ligands containing carboxylic acid tails. Using amino acid analysis, we found that on average, 3.3 ± 0.1 AChE proteins were bound per 5.22 ± 1.25 nm AuNP. We used circular dichroism spectroscopy to measure the structure of the bound protein and determined that it remained essentially unchanged after binding. Finally, we performed Michaelis-Menten kinetics to determine that the enzyme retained 18.2 ± 2.0% of its activity and maintained that activity over a period of at least three weeks after conjugation to AuNPs. We hypothesize that structural changes to the peripheral active site of AChE are responsible for the differences in activity of bound AChE and unbound AChE. This work is a proof-of-concept demonstration of a generalizable method for placing proteins onto chemically and structurally diverse substrates and materials without the need for surface functionalization strategies.

Conformational changes of enzymes upon immobilisation

Protein conformation plays a crucial role in determining both the catalytic efficiency and the chemo-, regio- and enantioselectivity of enzymes, thus eventually influencing their exploitability in biotechnological applications. Inevitably, immobilisation processes alter the natural molecular environment of enzymes, and quite often affect their catalytic activity through different mechanisms such as reduced accessibility of the substrate to the catalytic active centre, loss of the enzyme dynamic properties and alteration of the conformational integrity of the enzyme. This tutorial review outlines first the most common spectroscopic techniques used for investigating the conformation of immobilized proteins, and then examines how protein loading and polar and hydrophobic/hydrophilic interactions with the carrier affect the structural and dynamic features of enzymes. The nanoscale-level studies in which protein conformational changes, determined either by experimental approaches or by homology modelling, are correlated with the size and shape of the support are also discussed. Altogether, these results should provide useful information on how supports and/or enzymes have to be tailored to improve biocatalyst performance.

Synthesis of silica particles and their application as supports for alcohol dehydrogenases and cofactor immobilizations: conformational changes that lead to switch in enzyme stereoselectivity

FTIR, circular dichroism (CD) and fluorescence spectroscopies were used to characterize conformational changes in horse liver alcohol dehydrogenase (HLADH) and ketoreductase (KRED 117) upon physical and covalent immobilizations on silica particles (functionalized with amino, epoxy and thiol groups) of different sizes. Conformational changes for immobilized enzymes were associated with high and low frequency shifts of the amide I and II bands. CD spectra of native HLADH and KRED 117 characterized with a negative peak at 222nm indicating a α-helical structure. The disappearance of the negative peak in the CD spectra of immobilized enzymes and appearance of a positive peak at 222nm supported these observations. These findings demonstrated unfolding of folded enzymes and exposure of the amino acid residues during denaturation with a red shift in tryptophan fluorescence. The decrease in specific activities (by 60-70% in all cases) for both immobilized enzymes was correlated to those of conformational changes. Silica-attached enzyme-NADH systems were evaluated for enantioselective reduction of 1-(p-methoxyphenyl)-propan-2-one. Conformational changes enhanced the enantioselectivity of immobilized HLADH with a switch in its stereoselectivity. In the case of immobilized KRED 117, kinetic values (V(max) and K(m)) were lower than that of the free enzyme, without enhancing enzyme enantio- and stereoselectivity.

Experimental aspects of solid state circular dichroism

The interest of circular dichroism in the solid state is stimulated by several needs, such as the desire to get solvent free spectra, the insolubility of the sample or the intrinsic process in which the sample itself is prepared or manipulated. We approach the argument on the basis of the sampling technique, since each different case calls for specific care in getting proper results.

Liquid-chromatographic and mass-spectrometric identification of lens proteins using microwave-assisted digestion with trypsin-immobilized magnetic nanoparticles

We used a newly developed method combining trypsin-immobilized magnetic nanoparticles (TIMNs) and microwave-assisted protein digestion to study the proteins of human lens tissue. The digested proteins were identified by liquid chromatography and mass spectrometry. The lens proteins were digested under optimized conditions (digestion time 1 min, microwave power 400 W, trypsin-to-protein ratio 1:5) determined using bovine serum albumin as the standard protein, before liquid-chromatographic and mass-spectrometric analysis. Twenty-six proteins were identified with the new digestion method compared with 11 proteins identified with traditional in-solution digestion (12h). gamma-Crystallin, beta-crystallin, and superoxide dismutase 1 proteins, identified with the microwave-assisted method but not the traditional method, are related to cataract development according to some studies. The TIMNs were easily separated from the digestion products. This new digestion method could prove extremely useful for large-scale proteomic analyses.

Redirecting the inactivation pathway of penicillin amidase and increasing amoxicillin production via a thermophilic molecular chaperone

We have previously shown that a single-subunit thermosome from Methanocaldococcus jannaschii (rTHS) can stabilize enzymes in semi-aqueous media (Bergeron et al., 2008b). In the present study, rTHS was used to stabilize penicillin amidase (PGA) in methanol-water mixtures. Including methanol in the reaction medium for amoxicillin synthesis can suppress unwanted hydrolysis reactions but inactivate PGA. Inactivation and reactivation pathways proposed for PGA illustrate the predictability of enzyme stabilization by rTHS in co-solvents. Calcium was necessary for reversible dissociation of the two PGA subunits in methanol-water and the presence of calcium resulted in an enhancement of chaperone-assisted stabilization. rTHS also acted as a stabilizer in the enzymatic synthesis of the beta-lactam antibiotic amoxicillin. rTHS stabilized PGA, increasing its half-life in 35% methanol by fivefold at 37 degrees C. Stabilization by rTHS was enhanced but did not require the presence of ATP. Including rTHS in fed-batch reactions performed in methanol-water resulted in nearly 4 times more amoxicillin than when the reaction was run without rTHS, and over threefold higher selectivity towards amoxicillin synthesis compared to aqueous conditions without rTHS. The thermosome and other thermophilic chaperones may thus be generally useful for stabilizing enzymes in their soluble form and expanding the range of conditions suitable for biocatalysis.