Bioelectrocatalysis by redox enzymes at modified electrodes

Self-assembled monolayers of thiolated compounds are used as promoters for protein-electrode reactions. They provide an anchor group based on thiol chemisorptions and also a functional group for effective interaction with the protein. These interactions are often governed by electrostatic attraction. For example, for positively charged proteins, such as cytochrome c and the selenoprotein glutathione peroxidase, mercaptoalkanoic acids have been used. Clay modification of the electrode surface has been found to facilitate the heterogeneous electron transfer process for heme proteins, e.g. cytochrome c, cytochrome P450 and myoglobin. Interestingly, nucleic acids at carbon electrodes and thiol-modified double stranded oligonucleotides act as promoters of the redox communication to proteins, whereas the mechanism is still subject to controversy interpretations. By interacting the protein immobilised at the electrode with species in solution, signal chains have been constructed. The interaction can result in a simple co-ordination or redox reaction, depending on the nature of the reaction partners. For analytical purposes, e.g. biosensors, the electrochemical redox conversion of the immobilised protein is evaluated.     

Structures of redox enzymes

There is an ever increasing flood of structural information and over 1,000 protein structures have been deposited in the Protein Data Base between January 1999 and January 2000. Major advances in the past year in the field of redox enzymes have included the structures of nitric oxide synthases in ligand-free and ligand-bound complexes, and the determination of the multi-subunit mitochondrial bc1 complex. The first,structures of flavocytochrome have also appeared providing insight into novel electron and proton pathways.     

The mechanism of reduction of single-site redox proteins by ascorbic acid.

The reduction of single-site haem and copper redox proteins by ascorbic acid was studied as a function of pH. Evidence is presented that indicates that the double-deprotonated ascorbate anion, ascorbate2-, is the reducing agent, and the pH-independent second-order rate constants for reduction by this species are given. Investigation of the temperature dependences of these rate constants have yielded the values of the activation parameters (delta H++ and delta S++) for reduction. These values, together with ligand-replacement studies, suggest that ascorbate2- acts as an outer-sphere reductant for these proteins. Reasons to account for the apparent inability of ascorbic acid to reduce the alkaline conformer of mammalian ferricytochrome c are suggested.     

Pulmonary reduction of an intravascular redox polymer

Pulmonary endothelial cells in culture reduce external electron acceptors via transplasma membrane electron transport (TPMET). In studying endothelial TPMET in intact lungs, it is difficult to exclude intracellular reduction and reducing agents released by the lung. Therefore, we evaluated the role of endothelial TPMET in the reduction of a cell-impermeant redox polymer, toluidine blue O polyacrylamide (TBOP(+)), in intact rat lungs. When added to the perfusate recirculating through the lungs, the venous effluent TBOP(+) concentration decreased to an equilibrium level reflecting TBOP(+) reduction and autooxidation of its reduced (TBOPH) form. Adding superoxide dismutase (SOD) to the perfusate increased the equilibrium TBOP(+) concentration. Kinetic analysis indicated that the SOD effect could be attributed to elimination of the superoxide product of TBOPH autooxidation rather than of superoxide released by the lungs, and experiments with lung-conditioned perfusate excluded release of other TBOP(+) reductants in sufficient quantities to cause significant TBOP(+) reduction. Thus the results indicate that TBOP(+) reduction is via TPMET and support the utility of TBOP(+) and the kinetic model for investigating TPMET mechanisms and their adaptations to physiological and pathophysiological stresses in the intact lung.     

Use and improvement of microbial redox enzymes for environmental purposes

Industrial development may result in the increase of environmental risks. The enzymatic transformation of polluting compounds to less toxic or even innocuous products is an alternative to their complete removal. In this regard, a number of different redox enzymes are able to transform a wide variety of toxic pollutants, such as polynuclear aromatic hydrocarbons, phenols, azo dyes, heavy metals, etc. Here, novel information on chromate reductases, enzymes that carry out the reduction of highly toxic Cr(VI) to the less toxic insoluble Cr(III), is discussed. In addition, the properties and application of bacterial and eukaryotic proteins (lignin-modifying enzymes, peroxidases and cytochromes) useful in environmental enzymology is also discussed.     

A note on some old and some possible new redox indicators

Summary Starting with the frequently used dye 2,6-dichloroindophenol some of the chemical properties of the quinonimide class of dyes are described. Consideration of the effects of completing a hetero-six-membered ring, as in the azine, thiazine and oxazine classes, on the properties of the compounds is suggested as a lead towards the development of some redox indicators perhaps more desirable than the indophenols. This approach developed from a study of the Liebermann nitroso-reaction for phenols. Some properties of the redox indicators which resulted from that work are described in relation to energy transduction in the chloroplast.