Pyrroloquinoline quinone (PQQ) is reduced to pyrroloquinoline quinol (PQQH2) by vitamin C, and PQQH2 produced is recycled to PQQ by air oxidation in buffer solution at pH 7.4

Development of an electrochemical impedance spectroscopy immunosensor for insulin monitoring employing pyrroloquinoline quinone as an ingestible redox probe

Contemporary electrochemical impedance spectroscopy (EIS)-based biosensors face limitations in their applicability for in vivo measurements, primarily due to the necessity of using a redox probe capable of undergoing oxidation and reduction reactions in solution. Although previous investigations have demonstrated the effectiveness of EIS-based biosensors in detecting various target analytes using potassium ferricyanide as a redox probe, its unsuitability for blood or serum measurements, attributed to its inherent toxicity, poses a significant challenge. In response to this challenge, our study adopted a unique approach, focusing on the use of ingestible materials, by exploring naturally occurring substances within the body, with a specific emphasis on pyrroloquinoline quinone (PQQ). Following an assessment of PQQ's electrochemical attributes, we conducted a comprehensive series of EIS measurements. This involved the thorough characterization of the sensor's evolution, starting from the bare electrode and progressing to the immobilization of antibodies. The sensor's performance was then evaluated through the quantification of insulin concentrations ranging from 1 pM to 100 nM. A single frequency was identified for insulin measurements, offering a pathway for potential in vivo applications by combining PQQ as a redox probe with EIS measurements. This innovative approach holds promise for advancing the field of in vivo biosensing based on the EIS method.

Antiviral Effects of Pyrroloquinoline Quinone through Redox Catalysis To Prevent Coronavirus Infection

The global spread of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) coronavirus disease (COVID-19) is ongoing. Therefore, effective prevention of virus infection is required. Pyrroloquinoline quinone (PQQ), a natural compound found in various foods and human breast milk, plays a role in various physiological processes and is associated with health benefits. In this study, we aimed to determine the effects of PQQ on preventing coronavirus infections using a proxy Feline Infectious Peritonitis Virus (FIPV; belongs to the coronavirus family). In plaque reduction assays, we showed that pre- and post-PQQ-treated viruses were less infectious. IC50 was 87.9 and 5.1 μM for pre- and post-PQQ-treated viral infections, respectively. These results suggest that PQQ decreased the virion stability and viral replication. RT-qPCR confirmed these results. TEM findings showed that PQQ damaged viral capsids and aggregated viral particles, leading to inhibited virus attachment and entry into the host cells. PQQ was optimized by the addition of ascorbic acid and glutamic acid, which increased the number of redox cycles of PQQ and increased reactive oxygen species production by 14 times. In vitro, PQQ inhibited 3 CLpro/Mpro enzymes (an enzyme critical for viral replication) activity of SARS-CoV-2. Our results demonstrate the antiviral effect of PQQ on coronavirus, mainly by disrupting virion stability and loss of infectivity (occurring outside the host cell), due to increased redox activity. Furthermore, PQQ may hinder viral replication (inside the host cell) by 3 CLpro/Mpro enzyme inhibition. In summary, this study demonstrates the antiviral effect of PQQ and its potential application in coronavirus diseases.

Revisiting the role of electron donors in lytic polysaccharide monooxygenase biochemistry

The plant cell wall is rich in carbohydrates and many fungi and bacteria have evolved to take advantage of this carbon source. These carbohydrates are largely locked away in polysaccharides and so these organisms deploy a range of enzymes that can liberate individual sugars from these challenging substrates. Glycoside hydrolases (GHs) are the enzymes that are largely responsible for bringing about this sugar release; however, 12 years ago, a family of enzymes known as lytic polysaccharide monooxygenases (LPMOs) were also shown to be of key importance in this process. LPMOs are copper-dependent oxidative enzymes that can introduce chain breaks within polysaccharide chains. Initial work demonstrated that they could activate O2 to attack the substrate through a reaction that most likely required multiple electrons to be delivered to the enzyme. More recently, it has emerged that LPMO kinetics are significantly improved if H2O2 is supplied to the enzyme as a cosubstrate instead of O2. Only a single electron is required to activate an LPMO and H2O2 cosubstrate and the enzyme has been shown to catalyse multiple turnovers following the initial one-electron reduction of the copper, which is not possible if O2 is used. This has led to further studies of the roles of the electron donor in LPMO biochemistry, and this review aims to highlight recent findings in this area and consider how ongoing research could impact our understanding of the interplay between redox processes in nature.

Antioxidants other than vitamin C may be detected by glucose meters: Immediate relevance for patients with disorders targeted by antioxidant therapies

Owing to their ease of use, glucose meters are frequently used in research and medicine. However, little is known of whether other non-glucose molecules, besides vitamin C, interfere with glucometry. Therefore, we sought to determine whether other antioxidants might behave like vitamin C in causing falsely elevated blood glucose levels, potentially exposing patients to glycemic mismanagement by being administered harmful doses of glucose-lowering drugs. To determine whether various antioxidants can be detected by seven commercial glucose meters, human blood samples were spiked with various antioxidants ex vivo and their effect on the glucose results were assessed by Parkes error grid analysis. Several of the glucose meters demonstrated a positive bias in the glucose measurement of blood samples spiked with vitamin C, N-acetylcysteine, and glutathione. With the most interference-sensitive glucose meter, non-blood solutions of 1 mmol/L N-acetylcysteine, glutathione, cysteine, vitamin C, dihydrolipoate, and dithiothreitol mimicked the results seen on that glucose meter for 0.7, 1.0, 1.2, 2.6, 3.7 and 5.5 mmol/L glucose solutions, respectively. Glucose meter users should be alerted that some of these devices might produce spurious glucose results not only in patients on vitamin C therapy but also in those being administered other antioxidants. As discussed herein, the clinical relevance of the data is immediate in view of the current use of antioxidant therapies for disorders such as the metabolic syndrome, diabetes, cardiovascular diseases, and coronavirus disease 2019.

Mechanism of Pyrroloquinoline Quinone-Dependent Hydride Transfer Chemistry from Spectroscopic and High-Resolution X-ray Structural Studies of the Methanol Dehydrogenase from Methylococcus capsulatus (Bath)

The active site of methanol dehydrogenase (MDH) contains a rare disulfide bridge between adjacent cysteine residues. As a vicinal disulfide, the structure is highly strained, suggesting it might work together with the pyrroloquinoline quinone (PQQ) prosthetic group and the Ca²⁺ ion in the catalytic turnover during methanol (CH₃OH) oxidation. We purify MDH from Methylococcus capsulatus (Bath) with the disulfide bridge broken into two thiols. Spectroscopic and high-resolution X-ray crystallographic studies of this form of MDH indicate that the disulfide bridge is redox active. We observe an internal redox process within the holo-MDH that produces a disulfide radical anion concomitant with a companion PQQ radical, as evidenced by an optical absorption at 408 nm and a magnetically dipolar-coupled biradical in the EPR spectrum. These observations are corroborated by electron-density changes between the two cysteine sulfurs of the disulfide bridge as well as between the bound Ca²⁺ ion and the O5-C5 bond of the PQQ in the high-resolution X-ray structure. On the basis of these findings, we propose a mechanism for the controlled redistribution of the two electrons during hydride transfer from the CH₃OH in the alcohol oxidation without formation of the reduced PQQ ethenediol, a biradical mechanism that allows for possible recovery of the hydride for transfer to an external NAD⁺ oxidant in the regeneration of the PQQ cofactor for multiple catalytic turnovers. In support of this mechanism, a steady-state level of the disulfide radical anion is observed during turnover of the MDH in the presence of CH₃OH and NAD⁺.

Pyrroloquinoline quinone inhibits the production of inflammatory cytokines via the SIRT1/NF-κB signal pathway in weaned piglet jejunum

The small intestine is an important digestive organ and plays a vital role in the life of a pig. In this study, we explored the regulatory role and molecular mechanism of pyrroloquinoline quinone (PQQ) on intestinal health and to discussed the interaction between PQQ and vitamin C (VC). A total of 160 healthy piglets weaned at 21 d were randomly divided into four treatment groups according to 2 × 2 factoring. The results showed that dietary PQQ could significantly decrease the levels of plasma globulin, albumin/globulin (A/G), indirect bilirubin (IBIL), blood urea nitrogen (BUN), creatinine (CREA) (P < 0.05 for each), total bilirubin, (TBIL) (P < 0.01), diamine oxidase (DAO) (P < 0.01) and immunoglobulin G (IgG) (P < 0.0001) and increase the levels of immunoglobulin A (IgA) and immunoglobulin M (IgM) (P < 0.0001) in the plasma of weaned piglets. Similarly, dietary VC could significantly decrease the levels of plasma globulin, A/G, DAO (P < 0.05 for each) and IgG (P < 0.0001) and increase the levels of IgA and IgM (P < 0.0001) in the plasma of weaned piglets. In addition, dietary PQQ increased (P < 0.05) the mRNA levels of antioxidant genes (NQO1, UGT1A1, and EPHX1), thereby enhancing (oxidized) nicotinamide adenine dinucleotide (NAD+) concentration and sirtuin 1 (SIRT1) activity in tissues. However, the addition of 200 mg kg-1 VC to the diet containing PQQ reduced most of the effects of PQQ. We further show that PQQ reduced (P < 0.05) the expression of inflammation-related genes (IL-2, IL-6, TNF-α, and COX-2) via the SIRT1/NF-κB deacetylation signaling. In conclusion, our data reveals that PQQ exerts a certain protective effect on the intestines of piglets, but higher concentrations of VC react with PQQ, which inhibits the regulatory mechanism of PQQ.

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 protease activity of human ATG4B is regulated by reversible oxidative modification

Macroautophagy/autophagy plays a pivotal role in cytoplasmic material recycling and metabolic turnover, in which ATG4B functions as a "scissor" for processing pro-LC3 and lipidated LC3 to drive the autophagy progress. Mounting evidence has demonstrated the tight connection between ROS and autophagy during various pathological situations. Coincidentally, several studies have shown that ATG4B is potentially regulated by redox modification, but the underlying molecular mechanism and its relationship with autophagy is ambiguous. In this study, we verified that ATG4B activity was definitely regulated in a reversible redox manner. We also determined that Cys292 and Cys361 are essential sites of ATG4B to form reversible intramolecular disulfide bonds that respond to oxidative stress. Interestingly, we unraveled a new phenomenon that ATG4B concurrently formed disulfide-linked oligomers at Cys292 and Cys361, and that both sites underwent redox modifications thereby modulating ATG4B activity. Finally, increased autophagic flux and decreased oxidation sensitivity were observed in Cys292 and Cys361 double site-mutated cells under normal growth conditions. In conclusion, our research reveals a novel molecular mechanism that oxidative modification at Cys292 and Cys361 sites regulates ATG4B function, which modulates autophagy.Abbreviations: Air-ox: air-oxidation; ATG4B: autophagy related 4B cysteine peptidase; BCNU: 1,3-bis(2-chloroethyl)-1-nitrosourea; CBB: Coomassie Brilliant Blue; CM: complete medium; CQ: chloroquine; DTT: dithiothreitol; GSH: reduced glutathione; GSNO: S-nitrosoglutathione; GSSG: oxidized glutathione; HMW: high molecular weight; H₂O₂: hydrogen peroxide; NAC: N-acetyl-L-cysteine; NEM: N-ethylmaleimide; PE: phosphatidylethanolamine; PTM: post-translational modification; ROS, reactive oxygen species; WT: wild type.

Synthesis and crystal structure of pyrroloquinoline quinol (PQQH2) and pyrroloquinoline quinone (PQQ)

Pyrroloquinoline quinone (PQQ) is a water-soluble quinone compound first identified as a cofactor of alcohol- and glucose-dehydrogenases (ADH and GDH) in bacteria. For example, in the process of ADH reaction, alcohol is oxidized to the corresponding aldehyde, and inversely PQQ is reduced to pyrroloquinoline quinol (PQQH2). PQQ and PQQH2molecules play an important role as a cofactor in ADH and GDH reactions. However, crystal structure analysis has not been performed for PQQ and PQQH2. In the present study, the synthesis of PQQH2powder crystals was performed under air, by utilizing vitamin C as a reducing agent. By reacting a trihydrate of disodium salt of PQQ (PQQNa2·3H2O) with excess vitamin C in H2O at 293 and 343 K, yellowish brown and black powder crystals of PQQH2having different properties were obtained in high yield, respectively. The former was PQQH2trihydrate (PQQH2·3H2O) and the latter was PQQH2anhydrate (PQQH2). Furthermore, sodium-free red PQQ powder crystal (a monohydrate of PQQ, PQQ·H2O) was prepared by the reaction of PQQNa2·3H2O with HCl in H2O. Single crystals of PQQH2and PQQ were prepared from Me2SO/CH3CN mixed solvent, and we have succeeded in the crystal structure analyses of PQQH2and PQQ for the first time.