Quinones such as ubiquinone are the lipid soluble electron and proton carriers in the membranes of mitochondria, chloroplasts and oxygenic bacteria. Quinones undergo controlled redox reactions bound to specific sites in integral membrane proteins such as the cytochrome bc1 oxidoreductase. The quinone reactions in bacterial photosynthesis are amongst the best characterized, presenting a model to understand how proteins modulate cofactor chemistry. The free energy of ubiquinone redox reactions in aqueous solution and in the QA and QB sites of the bacterial photosynthetic reaction centers (RCs) are compared. In the primary QA site ubiquinone is reduced only to the anionic semiquinone (Q•−) while in the secondary QB site the product is the doubly reduced, doubly protonated quinol (QH2). The ways in which the protein modifies the relative energy of each reduced and protonated intermediate are described. For example, the protein stabilizes Q•− while destabilizing Q= relative to aqueous solution through electrostatic interactions. In addition, kinetic and thermodynamic mechanisms for stabilizing the intermediate semiquinones are compared. Evidence for the protein sequestering anionic compounds by slowing both on and off rates as well as by binding the anion more tightly is reviewed.

Bacterial reaction centers (RCs) catalyze a series of electron-transfer reactions reducing a neutral quinone to a bound, anionic semiquinone. The dissociation constants and association rates of 13 tailless neutral and anionic benzo- and naphthoquinones for the Q(A) site were measured and compared. The K(d) values for these quinones range from 0.08 to 90 microM. For the eight neutral quinones, including duroquinone (DQ) and 2,3-dimethoxy-5-methyl-1,4-benzoquinone (UQ(0)), the quinone concentration and solvent viscosity dependence of the association rate indicate a second-order rate-determining step. The association rate constants (k(on)) range from 10(5) to 10(7) M(-)(1) s(-)(1). Association and dissociation rate constants were determined at pH values above the hydroxyl pK(a) for five hydroxyl naphthoquinones. These negatively charged compounds are competitive inhibitors for the Q(A) site. While the neutral quinones reach equilibrium in milliseconds, anionic hydroxyl quinones with similar K(d) values take minutes to bind or dissociate. These slow rates are independent of ionic strength, solvent viscosity, and quinone concentration, indicating a first-order rate-limiting step. The anionic semiquinone, formed by forward electron transfer at the Q(A) site, also dissociates slowly. It is not possible to measure the association rate of the unstable semiquinone. However, as the protein creates kinetic barriers for binding and releasing anionic hydroxyl quinones without greatly increasing the affinity relative to neutral quinones, it is suggested that the Q(A) site may do the same for anionic semiquinone. Thus, the slow semiquinone dissociation may not indicate significant thermodynamic stabilization of the reduced species in the Q(A) site.

Chronic rhinosinusitis (CRS) is a highly prevalent disease in the adult and pediatric population. It causes significant burden and the management is considered one of the most costly public health conditions. Comorbidities include asthma, aspirin sensitivity, and nasal polyposis. Staphylococcus aureus biofilms and exotoxins that act as superantigens have been implicated to play an important pathological role in the incidence, maintenance, and ongoing burden of CRS. A better understanding of the interplay between bacterial factors, host factors, and the environment will facilitate better management of this disease. This literature review focuses on these factors and highlights current research in this field.

Quinones are electron and proton carriers that play a primary role in the aerobic metabolism of virtually every cell in nature. Most physiological quinones are benzoquinones. They undergo highly regulated redox reactions in the mitochondria, Golgi apparatus, plasma membrane and endoplasmic reticulum. Important consequences of these electron transfer reactions are the production of and protection against reactive oxygen species (ROS). Quinones have been extensively studied for both their cytotoxic as well as cellular protective properties and they have been particularly useful in rational drug design. The role of quinones in medicine is explored in this literature review with a particular focus on renal diseases. Due to their high basal metabolism and detoxification role, the kidneys are particularly sensitive to oxidative stress. Regardless of the underlying etiology, ROS plays an important role in both acute kidney injury (AKI) and chronic kidney diseases (CKD). Depending on the oxidative state of the kidney, quinones can be nephrotoxoic or nephro-protective. Many factors play a role in the interaction between quinones and the kidney and the consequences of this are just beginning to be explored.Electronic supplementary materialThe online version of this article (doi:10.1186/2193-1801-2-139) contains supplementary material, which is available to authorized users.

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