Development of long life lactate sensor using thermostable mutant lactate oxidase

Stability and Stabilization of Enzyme Biosensors: The Key to Successful Application and Commercialization

Fifty-five years have passed and more than 100,000 articles have been published since the first report of an electrochemical enzyme biosensor. However, very few biosensors have reached practical application and commercialization. The bulk of the research effort has been on increasing sensitivity and selectivity. In contrast, the number of publications dealing with stability or stabilization of enzyme biosensors is very small. Here, we critically review enzyme stabilization strategies as well as the progress that has been done in the past 20 years with respect to enzyme biosensor stabilization. Glucose oxidase, lactate oxidase, alcohol oxidase, and xanthine oxidase are the focus of this review because of their potential applications in food. The inconsistency in reporting biosensor stability was identified as a critical hurdle to research progress in this area. Fundamental questions that remain unanswered are outlined.

Electrochemical lactate biosensor based upon chitosan/carbon nanotubes modified screen-printed graphite electrodes for the determination of lactate in embryonic cell cultures

l-lactate is an essential metabolite present in embryonic cell culture. Changes of this important metabolite during the growth of human embryo reflect the quality and viability of the embryo. In this study, we report a sensitive, stable, and easily manufactured electrochemical biosensor for the detection of lactate within embryonic cell cultures media. Screen-printed disposable electrodes are used as electrochemical sensing platforms for the miniaturization of the lactate biosensor. Chitosan/multi walled carbon nanotubes composite have been employed for the enzymatic immobilization of the lactate oxidase enzyme. This novel electrochemical lactate biosensor analytical efficacy is explored towards the sensing of lactate in model (buffer) solutions and is found to exhibit a linear response towards lactate over the concentration range of 30.4 and 243.9 µM in phosphate buffer solution, with a corresponding limit of detection (based on 3-sigma) of 22.6 µM and exhibits a sensitivity of 3417 ± 131 µAM(-1) according to the reproducibility study. These novel electrochemical lactate biosensors exhibit a high reproducibility, with a relative standard deviation of less than 3.8% and an enzymatic response over 82% after 5 months stored at 4 °C. Furthermore, high performance liquid chromatography technique has been utilized to independently validate the electrochemical lactate biosensor for the determination of lactate in a commercial embryonic cell culture medium providing excellent agreement between the two analytical protocols.

Printed microwells with highly stable thin-film enzyme coatings for point-of-care multiplex bioassay of blood samples

A paper-based colorimetric biosensor suitable for point-of-care bioassay of blood samples is developed using highly stable enzyme thin-film coatings confined within inkjet printed polymeric microwells.

The Ala95-to-Gly substitution in Aerococcus viridans l-lactate oxidase revisited - structural consequences at the catalytic site and effect on reactivity with O2 and other electron acceptors

Aerococcus viridansl-lactate oxidase (avLOX) is a biotechnologically important flavoenzyme that catalyzes the conversion of L-lactate and O₂ into pyruvate and H₂O₂. The enzymatic reaction underlies different biosensor applications of avLOX for blood L-lactate determination. The ability of avLOX to replace O₂ with other electron acceptors such as 2,6-dichlorophenol-indophenol (DCIP) allows the possiblity of analytical and practical applications. The A95G variant of avLOX was previously shown to exhibit lowered reactivity with O₂ compared to wild-type enzyme and therefore was employed in a detailed investigation with respect to the specificity for different electron acceptor substrates. From stopped-flow experiments performed at 20 °C (pH 6.5), we determined that the A95G variant (fully reduced by L-lactate) was approximately three-fold more reactive towards DCIP (1.0 ± 0.1 × 10(6) M(-1) ·s(-1) ) than O₂, whereas avLOX wild-type under the same conditions was 14-fold more reactive towards O₂(1.8 ± 0.1 × 10(6) m(-1) ·s(-1)) than DCIP. Substituted 1,4-benzoquinones were up to five-fold better electron acceptors for reaction with L-lactate-reduced A95G variant than wild-type. A 1.65-Å crystal structure of oxidized A95G variant bound with pyruvate was determined and revealed that the steric volume created by removal of the methyl side chain of Ala95 and a slight additional shift in the main chain at position Gly95 together enable the accomodation of a new active-site water molecule within hydrogen-bond distance to the N5 of the FMN cofactor. The increased steric volume available in the active site allows the A95G variant to exhibit a similar trend with the related glycolate oxidase in electron acceptor substrate specificities, despite the latter containing an alanine at the analogous position.

Engineering of Aerococcus viridans L-lactate oxidase for site-specific PEGylation: characterization and selective bioorthogonal modification of a S218C mutant

A defined bioconjugate of Aerococcus viridans L-lactate oxidase and poly(ethylene glycol) 5000 was prepared and characterized in its structural and functional properties in comparison to the unmodified enzyme. Because the L-lactate oxidase in the native form does not contain cysteines, we introduced a new site for chemical modification via thiol chemistry by substituting the presumably surface-exposed serine-218, a nonconserved residue in the amino acid sequence, with cysteine. The resulting S218C mutant was isolated from Escherichia coli and shown in kinetic assays to be similarly (i.e., about half as) active as the native enzyme, thus validating the structure-guided design of the mutation. Using maleimide-activated methoxypoly(ethylene glycol) 5000 in about 10-fold molar excess over protein, the S218C mutant was converted in high yield (94%) into PEGylated derivative, while the native enzyme was totally unreactive under equivalent conditions. PEGylation caused only a relatively small decrease (30%) in the specific activity of the S218C mutant, and it did not change the protein stability. PEGylation went along with enhancement of the apparent size of the homotetrameric L-lactate oxidase in gel permeation chromatography, from 170 kDa to 250 kDa. The protein hydrodynamic diameter determined by dynamic light scattering increased from 11.9 nm in unmodified S218C mutant to 16.4 nm in the PEGylated form. Site-selective PEGylation of the mutated L-lactate oxidase, using orthogonal maleimide-thiol coupling, could therefore facilitate incorporation of the enzyme into biosensors currently employed for determination of blood L-lactate levels, and it could also support different applications of the enzyme in applied biocatalysis.

A multilayer membrane amperometric glucose sensor fabricated using planar techniques for large-scale production

This paper reports on a multilayer membrane amperometric glucose sensor fabricated using planar techniques. It is characterized by good reproducibility and suitable for large-scale production. The glucose sensor has 82 electrode sets formed on a single glass substrate, each with a platinum working electrode (WE), a platinum counter electrode (CE) and an Ag/AgCl reference electrode (RE). The electrode sets are coated with a membrane consisting of five layers: gamma-aminopropyltriethoxysilane (gamma-APTES), Nafion, glucose oxidase (GOX), gamma-APTES and perfluorocarbon polymer (PFCP), in that order. Tests have shown that the sensor has acceptably low dispersion (relative standard deviation, R.S.D.=42.9%, n=82), a wide measurement range (1.11-111 mM) and measurement stability over a 27-day period. Measurements of the glucose concentration in a control human urine sample demonstrated that the sensor has very low dispersion (R.S.D.=2.49%, n=10).

Effect of mutations at Glu160 and Val198 on the thermostability of lactate oxidase

We have obtained two types of thermostable mutant lactate oxidase - one that exhibited an E-to-G point mutation at position 160 (E160G) through error-prone PCR-based random mutagenesis, and another that exhibited an E-to-G mutation at position 160 and a V-to-I mutation at position 198 (E160G/V198I) through DNA shuffling-based random mutagenesis - both of which we have previously reported. Our molecular modeling of lactate oxidase suggests that the substitution of G for E at position 160 reduces the electrostatic repulsion between the negative charges of E160 and E130 in the (beta/alpha)8 barrel structure, but a thermal-inactivation experiment on the five kinds of single-mutant lactate oxidase at position 160 (E160A, E160Q, E160H, E160R, and E160K) showed that the side-chain volume of the amino acid at position 160 mainly contributes to the thermostability of lactate oxidase. We also produced V198I single-mutant lactate oxidase through site-directed mutagenesis, and analysed the thermostability of wild-type, V198I, E160G, and E160G/V198I lactate oxidase enzymes. The half-life of E160G/V198I lactate oxidase at 70 degrees C was about three times longer than that of E160G lactate oxidase, and was about 20 times longer than that of wild-type lactate oxidase. In contrast, the thermostability of the V198I lactate oxidase was almost identical to that of wild-type lactate oxidase. This indicates that the V198I mutation alone does not affect lactate oxidase thermostability, but does affect it when combined with the E160G mutation.

Coupling the lactate oxidase to electrodes by ionotropic gelation of biopolymer

A direct ionotropic gelation of the polycationic biopolymer chitosan (CHIT) with the polyanionic enzyme lactate oxidase (LOx) was used to form thin biopolymer-enzyme films on the surface of platinum electrodes. The electrochemical assays of such films revealed a well-defined capacity of CHIT for the retention of LOx. The stoichiometry of the CHIT-LOx polyelectrolyte complexes was found to be approximately 1:40, i.e., on average, 1 CHIT chain retained 40 molecules of LOx in the CHIT-LOx films. The enzyme retention was ascribed to strong electrostatic interactions between the LOx and a fraction of the protonated amino groups on the CHIT chains. Although the LOx is inherently unstable outside its natural matrix, it displayed high surface activity of 0.26 units cm(-)(2) in the matrix of CHIT. This correlated with good stability of the biopolymer-enzyme films as demonstrated by a constant response of Pt/CHIT-LOx electrodes to lactate during continuous 24-h testing. When compared to other single-film lactate sensors, the Pt/CHIT-LOx electrodes displayed the best combination of analytical properties in terms of a low detection limit (50 nM), high sensitivity (0.23 A M(-)(1) cm(-)(2)), and fast response time (<1 s). Such a performance validated the CHIT-LOx system as an attractive sensing element for the development of new lactate biosensors.

A fiber-optic lactate sensor based on bacterial cytoplasmic membranes

A new type of fiber-optic biosensor based on bacterial cytoplasmic membranes (CPM) as the biological recognition element and an oxygen sensitive dye layer as the transducer is described for the detection of lactate. CPMs from bacteria with an induced lactate oxidase system are adsorbed onto a cellulose disk. The disk is fixed mechanically over an oxygen sensitive siloxane layer on the distal end of an optical fiber. This system detects lactate with no interference from glucose, fructose or glutamic acid.