Sensitive catecholamine measurement using a monoenzymatic recycling system

Quantitative determination of dopamine in the presence of interfering substances supported by machine learning tools

In the field of neuroscience as well as in the clinical setting, the neurotransmitter dopamine (DA) is an analyte which is important for research as well as medical purposes. There are plenty of methods available to measure dopamine quantitatively, with voltammetric ones such as differential pulse voltammetry (DPV) being among the most convenient and simple ones. However, dopamine often occurs, either naturally or because of the requirements of involved enzymatic systems, alongside substances that can influence the signal it produces upon electrochemical conversion. An example for such substances is the magnesium ion, which itself is not electrochemically active in the potential range needed for DA oxidation, but influences the dopamine signal. We have characterized the properties of DPV signals subject to the interaction between DA and Mg2+ and show that, although these properties are changing in a nonlinear fashion when both concentrations are varying, relatively simple linear mathematical models can be used to determine dopamine concentrations quantitatively in the presence of magnesium ions. The focus of this study is thus, the mathematical treatment of experimental data in order to overcome an analytical problem and not the investigation of the chemical background of DA-Mg2+ interaction.

PQQ-GDH - Structure, function and application in bioelectrochemistry

This review summarizes the basic features of the PQQ-GDH enzyme as one of the sugar converting biocatalysts. Focus is on the membrane -bound and the soluble form. Furthermore, the main principles of enzymatic catalysis as well as studies on the physiological importance are reviewed. A short overview is given on developments in protein engineering. The major part, however, deals with the different fields of application in bioelectrochemistry. This includes approaches for enzyme-electrode communication such as direct electron transfer, mediator-based systems, redox polymers or conducting polymers and holoenzyme reconstitution, and covers applied areas such as biosensing, biofuel cells, recycling schemes, enzyme competition, light-directed sensing, switchable detection schemes, logical operations by enzyme electrodes and immune sensing.

The cooperativity effect in the reaction of soluble quinoprotein (PQQ-containing) glucose dehydrogenase is not due to subunit interaction but to substrate-assisted catalysis

Soluble quinoprotein (PQQ-containing) glucose dehydrogenase (sGDH, EC 1.1.99.35) catalyzes the oxidation of β-d-glucose to d-glucono-δ-lactone. Although sGDH has many analytical applications, the relationship between activity and substrate concentration is not well established. Previous steady-state kinetic studies revealed a negative cooperativity effect which has recently been ascribed to subunit interaction. To investigate this conclusion, stopped-flow kinetic experiments were carried out on the reaction in which oxidized enzyme (E_ox ) was reduced with substrates to E_red . The appearance of E_red is observed to be preceded by formation of an intermediate enzyme form, Int, which is mono-exponentially formed from E_ox . However, the rate of conversion of Int into E_red depends hyperbolically on the concentration of substrate (leading to a 35-fold stimulation in the case of glucose). Evidence is provided that substrate not only binds to E_ox but also to Int and E_red as well, and that the binding to Int causes the significant stimulation of Int decay. It is proposed that a proton shuffling step is involved in the decay, which is facilitated by binding of substrate to Int. Substituting the PQQ-activating Ca by a Ba ion lowered all reaction rates but did not change the stimulation factor. In summary, the previous proposal that the cooperativity effect of sGDH is due to interaction between its substrate-loaded subunits is incorrect; it is due to substrate-assisted catalysis of the enzyme.

ENZYMES

EC 1.1.99.35 - soluble quinoprotein glucose dehydrogenase.

Effect of substrate inhibition and cooperativity on the electrochemical responses of glucose dehydrogenase. Kinetic characterization of wild and mutant types

Thanks to its insensitivity to dioxygen and to its good catalytic reactivity, and in spite of its poor substrate selectivity, quinoprotein glucose dehydrogenase (PQQ-GDH) plays a prominent role among the redox enzymes that can be used for analytical purposes, such as glucose detection, enzyme-based bioaffinity assays, and the design of biofuel cells. A detailed kinetic analysis of the electrochemical catalytic responses, leading to an unambiguous characterization of each individual steps, seems a priori intractable in view of the interference, on top of the usual ping-pong mechanism, of substrate inhibition and of cooperativity effects between the two identical subunits of the enzyme. Based on simplifications suggested by extended knowledge previously acquired by standard homogeneous kinetics, it is shown that analysis of the catalytic responses obtained by means of electrochemical nondestructive techniques, such as cyclic voltammetry, with ferrocene methanol as a mediator, does allow a full characterization of all individual steps of the catalytic reaction, including substrate inhibition and cooperativity and, thus, allows to decipher the reason that makes the enzyme more efficient when the neighboring subunit is filled with a glucose molecule. As a first practical illustration of this electrochemical approach, comparison of the native enzyme responses with those of a mutant (in which the asparagine amino acid in position 428 has been replaced by a cysteine residue) allowed identification of the elementary steps that makes the mutant type more efficient than the wild type when cooperativity between the two subunits takes place, which is observed at large mediator and substrate concentrations. A route is thus opened to structure-reactivity relationships and therefore to mutagenesis strategies aiming at better performances in terms of catalytic responses and/or substrate selectivity.

Application of cellulose-based self-assembled tri-enzyme system in a pseudo-reagent-less biosensor for biogenic catecholamine detection

Amorphous cellulose was used as a specific carrier for the deposition of self-assembled multienzyme complexes capable of catalyzing coupled reactions. Naturally glycosylated fungal cellobiohydrolases (CBHs) of glycosyl hydrolase families 6 and 7 were specifically deposited onto the cellulose surface through their family I cellulose-binding modules (CBM). Naturally glycosylated fungal laccase was then deposited onto the preformed glycoprotein layer pretreated by ConA, through the interaction of mannosyl moieties of fungal glycoproteins with the multivalent lectin. The formation of a cellulase-ConA-laccase composite was proven by direct and indirect determination of activity of immobilized laccase. In the absence of cellulases and ConA, no laccase deposition onto the cellulose surface was observed. Finally, basidiomycetous cellobiose dehydrogenase (CDH) was deposited onto the cellulose surface through the specific interaction of its FAD domain with cellulose. The obtained paste was applied onto the surface of a Clark-type oxygen electrode and covered with a dialysis membrane. In the presence of traces of catechol or dopamine as mediators, the obtained immobilized multienzyme composite was capable of the coupled oxidation of cellulose by dissolved oxygen, thus providing the basis for a sensitive assay of the mediator. Swollen amorphous cellulose plays three different roles in the obtained biosensor as: (i) a gelforming matrix that captures the analyte and its oxidized intermediate, (ii) a specific carrier for protein self-assembly, and (iii) a source of excess substrate for a pseudo-reagent-less assay with signal amplification. The detection limit of such a tri-enzyme biosensor is 50-100 nM dopamine.

Biosensor based on cellobiose dehydrogenase for detection of catecholamines

A cellobiose dehydrogenase (CDH)-modified graphite electrode was designed for amperometric detection of catecholamines in the flow injection mode, by their recycling between the graphite electrode (+300 mV vs Ag|AgCl) and the reduced FAD cofactor of adsorbed CDH, resulting in an amplified response signal. The high efficiency of the enzyme-catecholamine reaction leads to a detection limit below 1 nM and a sensitivity of 15.8 A.M(-1) x cm(-2) (approximately 1150 nA/microM) for noradrenaline, with a coverage of less than 2.5 microg of CDH adsorbed on the electrode surface (0.073 cm(2)). Working parameters such as pH, cellobiose concentration, carrier buffer, and applied potential were optimized, using hydroquinone as a model analyte. The sensitivity, linear range, and amplification factor can be modulated by the steady-state concentration of cellobiose in the flow buffer. The response of the sensor decreases only 2% when run continuously for 4 h in the flow injection mode. The response peak maximum is obtained within 6 s at a flow rate of 0.5 mL/min, representing the time of the entire sample segment to pass the electrode. CDH enzymes from Phanerochaete chrysosporium and Sclerotium rolfsii were investigated, providing different characteristics of the sensor, with sensors made with CDH from P. chrysosporium being the better ones.

Amplification of amperometric biosensor responses by electrochemical substrate recycling. 3. Theoretical and experimental study of the phenol-polyphenol oxidase system immobilized in Laponite hydrogels and layer-by-layer self-assembled structures

The amperometric response toward phenol of PPO-based rotating disk bioelectrodes is analyzed on the basis of a kinetic model taking into account internal and external mass transport effects and a CEC' electroenzymatic mechanism. Monophenolase activity of PPO catalyses the oxidation of phenol to o-quinone (step C). o-Quinone can then enter an amplification recycling process involving electrochemical reduction (step E) and enzymatic reoxidation (step C': catecholase activity). The rate-limiting steps such as monophenolase activity, catecholase recycling, permeability of the membrane, and activity and accessibility of the catalytic enzyme sites are theoretically considered and experimentally demonstrated for different electrode configurations including PPO immobilized in Laponite hydrogels and layer-by-layer self-assembled multilayers of PPO and poly(diallyldimethylammonium).