Analysis of the interaction of a hybrid system consisting of bovine adrenodoxin reductase and flavodoxin from the cyanobacterium Anabaena PCC 7119

Evolution of the acceptor side of photosystem I: ferredoxin, flavodoxin, and ferredoxin-NADP+ oxidoreductase

The development of oxygenic photosynthesis by primordial cyanobacteria ~2.7 billion years ago led to major changes in the components and organization of photosynthetic electron transport to cope with the challenges of an oxygen-enriched atmosphere. We review herein, following the seminal contributions as reported by Jaganathan et al. (Functional genomics and evolution of photosynthetic systems, vol 33, advances in photosynthesis and respiration, Springer, Dordrecht, 2012), how these changes affected carriers and enzymes at the acceptor side of photosystem I (PSI): the electron shuttle ferredoxin (Fd), its isofunctional counterpart flavodoxin (Fld), their redox partner ferredoxin-NADP⁺ reductase (FNR), and the primary PSI acceptors F _x and F _A/F _B. Protection of the [4Fe-4S] centers of these proteins from oxidative damage was achieved by strengthening binding between the F _A/F _B polypeptide and the reaction center core containing F _x, therefore impairing O₂ access to the clusters. Immobilization of F _A/F _B in the PSI complex led in turn to the recruitment of new soluble electron shuttles. This function was fulfilled by oxygen-insensitive [2Fe-2S] Fd, in which the reactive sulfide atoms of the cluster are shielded from solvent by the polypeptide backbone, and in some algae and cyanobacteria by Fld, which employs a flavin as prosthetic group and is tolerant to oxidants and iron limitation. Tight membrane binding of FNR allowed solid-state electron transfer from PSI bridged by Fd/Fld. Fine tuning of FNR catalytic mechanism led to formidable increases in turnover rates compared with FNRs acting in heterotrophic pathways, favoring Fd/Fld reduction instead of oxygen reduction.

The long goodbye: the rise and fall of flavodoxin during plant evolution

Ferredoxins are electron shuttles harbouring iron-sulfur clusters that connect multiple oxido-reductive pathways in organisms displaying different lifestyles. Some prokaryotes and algae express an isofunctional electron carrier, flavodoxin, which contains flavin mononucleotide as cofactor. Both proteins evolved in the anaerobic environment preceding the appearance of oxygenic photosynthesis. The advent of an oxygen-rich atmosphere proved detrimental to ferredoxin owing to iron limitation and oxidative damage to the iron-sulfur cluster, and many microorganisms induced flavodoxin expression to replace ferredoxin under stress conditions. Paradoxically, ferredoxin was maintained throughout the tree of life, whereas flavodoxin is absent from plants and animals. Of note is that flavodoxin expression in transgenic plants results in increased tolerance to multiple stresses and iron deficit, through mechanisms similar to those operating in microorganisms. Then, the question remains open as to why a trait that still confers plants such obvious adaptive benefits was not retained. We compare herein the properties of ferredoxin and flavodoxin, and their contrasting modes of expression in response to different environmental stimuli. Phylogenetic analyses suggest that the flavodoxin gene was already absent in the algal lineages immediately preceding land plants. Geographical distribution of phototrophs shows a bias against flavodoxin-containing organisms in iron-rich coastal/freshwater habitats. Based on these observations, we propose that plants evolved from freshwater macroalgae that already lacked flavodoxin because they thrived in an iron-rich habitat with no need to back up ferredoxin functions and therefore no selective pressure to keep the flavodoxin gene. Conversely, ferredoxin retention in the plant lineage is probably related to its higher efficiency as an electron carrier, compared with flavodoxin. Several lines of evidence supporting these contentions are presented and discussed.

Cytochromes P450 as promising catalysts for biotechnological application: chances and limitations

Cytochromes P450 (CYPs) belong to the superfamily of heme b containing monooxygenases with currently more than 21,000 members. These enzymes accept a vast range of organic molecules and catalyze diverse reactions. These extraordinary capabilities of CYP systems that are unmet by other enzymes make them attractive for biotechnology. However, the complexity of these systems due to the need of electron transfer from nicotinamide adenine dinucleotide (phosphate) (NAD(P)H) via redox partner proteins for the initial hydroxylation step limits a broader technical implementation of CYP enzymes. There have been several reviews during the past years tackling the potential CYPs for synthetic application. The aim of this review is to give a critical overview about possibilities and chances for application of these interesting catalysts as well as to discuss drawbacks and problems related to their use. Solutions to overcome these limitations will be demonstrated, and several selected examples of successful CYP applications under industrial conditions will be reviewed.

The importance of flavodoxin for environmental stress tolerance in photosynthetic microorganisms and transgenic plants. Mechanism, evolution and biotechnological potential

Ferredoxins are electron shuttles harboring iron-sulfur clusters which participate in oxido-reductive pathways in organisms displaying very different lifestyles. Ferredoxin levels decline in plants and cyanobacteria exposed to environmental stress and iron starvation. Flavodoxin is an isofunctional flavoprotein present in cyanobacteria and algae (not plants) which is induced and replaces ferredoxin under stress. Expression of a chloroplast-targeted flavodoxin in plants confers tolerance to multiple stresses and iron deficit. We discuss herein the bases for functional equivalence between the two proteins, the reasons for ferredoxin conservation despite its susceptibility to aerobic stress and for the loss of flavodoxin as an adaptive trait in higher eukaryotes. We also propose a mechanism to explain the tolerance conferred by flavodoxin when expressed in plants.

Heterologous ferredoxin reductase and flavodoxin protect Cos-7 cells from oxidative stress

BACKGROUND

Ferredoxin-NADP(H) reductase (FNR) from Pisum sativum and Flavodoxin (Fld) from Anabaena PCC 7119 have been reported to protect a variety of cells and organisms from oxidative insults. In this work, these two proteins were expressed in mitochondria of Cos-7 cells and tested for their efficacy to protect these cells from oxidative stress in vitro.

PRINCIPAL FINDINGS

Cos-7/pFNR and Cos-7/pFld cell lines expressing FNR and Fld, respectively, showed a significantly higher resistance to 24 h exposure to 300-600 µM hydrogen peroxide measured by LDH retention, MTT reduction, malondialdehyde (MDA) levels and lipid peroxide (LPO; FOX assay) levels. However, FNR and Fld did not exhibit any protection at shorter incubation times (2 h and 4 h) to 4 mM hydrogen peroxide or to a 48 h exposure to 300 µM methyl viologen. We found enhanced methyl viologen damage exerted by FNR that may be due to depletion of NADPH pools through NADPH-MV diaphorase activity as previously observed for other overexpressed enzymes.

SIGNIFICANCE

The results presented are a first report of antioxidant function of these heterologous enzymes of vegetal and cyanobacterial origin in mammalian cells.

Structural and mechanistic aspects of flavoproteins: photosynthetic electron transfer from photosystem I to NADP+

This minireview covers the research carried out in recent years into different aspects of the function of the flavoproteins involved in cyanobacterial photosynthetic electron transfer from photosystem I to NADP(+), flavodoxin and ferredoxin-NADP(+) reductase. Interactions that stabilize protein-flavin complexes and tailor the midpoint potentials in these proteins, as well as many details of the binding and electron transfer to protein and ligand partners, have been revealed. In addition to their role in photosynthesis, flavodoxin and ferredoxin-NADP(+) reductase are ubiquitous flavoenzymes that deliver NAD(P)H or low midpoint potential one-electron donors to redox-based metabolisms in plastids, mitochondria and bacteria. They are also the basic prototypes for a large family of diflavin electron transferases with common functional and structural properties. Understanding their mechanisms should enable greater comprehension of the many physiological roles played by flavodoxin and ferredoxin-NADP(+) reductase, either free or as modules in multidomain proteins. Many aspects of their biochemistry have been extensively characterized using a combination of site-directed mutagenesis, steady-state and transient kinetics, spectroscopy and X-ray crystallography. Despite these considerable advances, various key features of the structural-function relationship are yet to be explained in molecular terms. Better knowledge of these systems and their particular properties may allow us to envisage several interesting applications of these proteins beyond their physiological functions.

Exploring the biochemical properties and remediation applications of the unusual explosive-degrading P450 system XplA/B

Widespread contamination of land and groundwater has resulted from the use, manufacture, and storage of the military explosive hexa-hydro-1,3,5-trinitro-1,3,5-triazine (RDX). This contamination has led to a requirement for a sustainable, low-cost method to remediate this problem. Here, we present the characterization of an unusual microbial P450 system able to degrade RDX, consisting of flavodoxin reductase XplB and fused flavodoxin-cytochrome P450 XplA. The affinity of XplA for the xenobiotic compound RDX is high (K(d) = 58 muM) and comparable with the K(m) of other P450s toward their natural substrates (ranging from 1 to 500 muM). The maximum turnover (k(cat)) is 4.44 per s, only 10-fold less than the fastest self-sufficient P450 reported, BM3. Interestingly, the presence of oxygen determines the final products of RDX degradation, demonstrating that the degradation chemistry is flexible, but both pathways result in ring cleavage and release of nitrite. Carbon monoxide inhibition is weak and yet the nitroaromatic explosive 2,4,6-trinitrotoluene (TNT) is a potent inhibitor. To test the efficacy of this system for the remediation of groundwater, transgenic Arabidopsis plants expressing both xplA and xplB were generated. They are able to remove saturating levels of RDX from liquid culture and soil leachate at rates significantly faster than those of untransformed plants and xplA-only transgenic lines, demonstrating the applicability of this system for the phytoremediation of RDX-contaminated sites.

Survey of the year 2004 commercial optical biosensor literature

The year 2004 represents a milestone for the biosensor research community: in this year, over 1000 articles were published describing experiments performed using commercially available systems. The 1038 papers we found represent an approximately 10% increase over the past year and demonstrate that the implementation of biosensors continues to expand at a healthy pace. We evaluated the data presented in each paper and compiled a 'top 10' list. These 10 articles, which we recommend every biosensor user reads, describe well-performed kinetic, equilibrium and qualitative/screening studies, provide comparisons between binding parameters obtained from different biosensor users, as well as from biosensor- and solution-based interaction analyses, and summarize the cutting-edge applications of the technology. We also re-iterate some of the experimental pitfalls that lead to sub-optimal data and over-interpreted results. We are hopeful that the biosensor community, by applying the hints we outline, will obtain data on a par with that presented in the 10 spotlighted articles. This will ensure that the scientific community at large can be confident in the data we report from optical biosensors.