Purification of a rotenone-insensitive NAD(P)H dehydrogenase from the inner surface of the inner membrane of red beetroot mitochondria

Ca2+-binding and Ca2+-independent respiratory NADH and NADPH dehydrogenases of Arabidopsis thaliana

Type II NAD(P)H:quinone oxidoreductases are single polypeptide proteins widespread in the living world. They bypass the first site of respiratory energy conservation, constituted by the type I NADH dehydrogenases. To investigate substrate specificities and Ca(2+) binding properties of seven predicted type II NAD(P)H dehydrogenases of Arabidopsis thaliana we have produced them as T7-tagged fusion proteins in Escherichia coli. The NDB1 and NDB2 enzymes were found to bind Ca(2+), and a single amino acid substitution in the EF hand motif of NDB1 abolished the Ca(2+) binding. NDB2 and NDB4 functionally complemented an E. coli mutant deficient in endogenous type I and type II NADH dehydrogenases. This demonstrates that these two plant enzymes can substitute for the NADH dehydrogenases in the bacterial respiratory chain. Three NDB-type enzymes displayed distinct catalytic profiles with substrate specificities and Ca(2+) stimulation being considerably affected by changes in pH and substrate concentrations. Under physiologically relevant conditions, the NDB1 fusion protein acted as a Ca(2+)-dependent NADPH dehydrogenase. NDB2 and NDB4 fusion proteins were NADH-specific, and NDB2 was stimulated by Ca(2+). The observed activity profiles of the NDB-type enzymes provide a fundament for understanding the mitochondrial system for direct oxidation of cytosolic NAD(P)H in plants. Our findings also suggest different modes of regulation and metabolic roles for the analyzed A. thaliana enzymes.

New insights into type II NAD(P)H:quinone oxidoreductases

Type II NAD(P)H:quinone oxidoreductases (NDH-2) catalyze the two-electron transfer from NAD(P)H to quinones, without any energy-transducing site. NDH-2 accomplish the turnover of NAD(P)H, regenerating the NAD(P)(+) pool, and may contribute to the generation of a membrane potential through complexes III and IV. These enzymes are usually constituted by a nontransmembrane polypeptide chain of approximately 50 kDa, containing a flavin moiety. There are a few compounds that can prevent their activity, but so far no general specific inhibitor has been assigned to these enzymes. However, they have the common feature of being resistant to the complex I classical inhibitors rotenone, capsaicin, and piericidin A. NDH-2 have particular relevance in yeasts like Saccharomyces cerevisiae and in several prokaryotes, whose respiratory chains are devoid of complex I, in which NDH-2 keep the balance and are the main entry point of electrons into the respiratory chains. Our knowledge of these proteins has expanded in the past decade, as a result of contributions at the biochemical level and the sequencing of the genomes from several organisms. The latter showed that most organisms contain genes that potentially encode NDH-2. An overview of this development is presented, with special emphasis on microbial enzymes and on the identification of three subfamilies of NDH-2.

Alternative NAD(P)H dehydrogenases of plant mitochondria

Plant mitochondria have a highly branched electron transport chain that provides great flexibility for oxidation of cytosolic and matrix NAD(P)H. In addition to the universal electron transport chain found in many organisms, plants have alternative NAD(P)H dehydrogenases in the first part of the chain and a second oxidase, the alternative oxidase, in the latter part. The alternative activities are nonproton pumping and allow for NAD(P)H oxidation with varying levels of energy conservation. This provides a mechanism for plants to, for example, remove excess reducing power and balance the redox poise of the cell. This review presents our current understanding of the alternative NAD(P)H dehydrogenases present in plant mitochondria.

Identification of AtNDI1, an internal non-phosphorylating NAD(P)H dehydrogenase in Arabidopsis mitochondria

Plant mitochondria contain non-phosphorylating NAD(P)H dehydrogenases (DHs) that are not found in animal mitochondria. The physiological function, substrate specificity, and location of enzymes within this family have yet to be conclusively determined. We have linked genome sequence information to protein and biochemical data to identify that At1g07180 (SwissProt Q8GWA1) from the Arabidopsis Genome Initiative database encodes AtNDI1, an internal NAD(P)H DH in Arabidopsis mitochondria. Three lines of evidence are presented: (a). The predicted protein sequence of AtNDI1 has high homology with other designated NAD(P)H DHs from microorganisms, (b). the capacity for matrix NAD(P)H oxidation via the rotenone-insensitive pathway is significantly reduced in the Atndi1 mutant plant line, and (c). the in vitro translation product of AtNDI1 is imported into isolated mitochondria and located on the inside of the inner membrane.

Kinetic characterization of the rotenone-insensitive internal NADH: ubiquinone oxidoreductase of mitochondria from Saccharomyces cerevisiae

Saccharomyces cerevisiae mitochondria contain an NADH:Q6 oxidoreductase (internal NADH dehydrogenase) encoded by NDI1 gene in chromosome XIII. This enzyme catalyzes the transfer of electrons from NADH to ubiquinone without the translocation of protons across the membrane. From a structural point of view, the mature enzyme has a single subunit of 53 kDa with FAD as the only prosthetic group. Due to the fact that S. cerevisiae cells lack complex I, the expression of this protein is essential for cell growth under respiratory conditions. The results reported in this work show that the internal NADH dehydrogenase follows a ping-pong mechanism, with a Km for NADH of 9.4 microM and a Km for oxidized 2,6-dichorophenolindophenol (DCPIP) of 6.2 microM. NAD+, one of the products of the reaction, did not inhibit the enzyme while the other product, reduced DCPIP, inhibited the enzyme with a Ki of 11.5 microM. Two dead-end inhibitors, AMP and flavone, were used to further characterize the kinetic mechanism of the enzyme. AMP was a linear competitive inhibitor of NADH (Ki = 5.5 mM) and a linear uncompetitive inhibitor of oxidized DCPIP (Ki = 11.5 mM), in agreement with the ping-pong mechanism. On the other hand, flavone was a partial inhibitor displaying a hyperbolic uncompetitive inhibition regarding NADH, and a hyperbolic noncompetitive inhibition with respect to oxidized DCPIP. The apparent intercept inhibition constant (Kii = 5.4 microM) and the slope inhibition constant (Kis = 7.1 microM) were obtained by non linear regression analysis. The results indicate that the ternary complex F-DCPIPox-flavone catalyzes the reduction of DCPIP, although with lower efficiency. The effect of pH on Vmax was studied. The Vmax profile shows two groups with pKa values of 5.3 and 7.2 involved in the catalytic process.

Citrate and isocitrate in plant metabolism

The relevance of citrate and isocitrate metabolism in plants is discussed in connection with the different pathways for their conversions. The routes for citrate and isocitrate conversions are incorporated into the system of cross-linked metabolic processes and may provide carbon skeletons for nitrogen assimilation and reducing equivalents for biosynthetic reactions, support the functioning of the glyoxylate cycle and play an important role in the TCA and energy metabolism as a whole. The possibility of the coupling of citrate and isocitrate metabolism with various electron transport systems is discussed from the point of view of the efficiency of the balancing cellular NAD(P)H/NAD(P)+ and ATP/ADP ratios. The role of citrate and isocitrate and their derivations as potent effectors of some enzymes is considered. Special attention is paid to the enzymes associated with citrate and isocitrate metabolism and to the mechanisms which regulate their activity. The possibilities of the coordination of the main processes of energy and biosynthetic metabolism at the level of citrate and isocitrate distribution are discussed.

Purification and characterization of a 43-kDa rotenone-insensitive NADH dehydrogenase from plant mitochondria

A 43-kDa NAD(P)H dehydrogenase was purified from red beetroot mitochondria. An antibody against this dehydrogenase was used in conjunction with the membrane-impermeable protein cross-linker 3,3'-dithiobis(sulfosuccinimidylpropionate) to localize the dehydrogenase on the matrix side of the inner membrane. Immunoblotting showed that the dehydrogenase was found in mitochondria isolated from several plant species but not from rat livers. Antibodies against the purified dehydrogenase partially inhibited rotenoneinsensitive internal NADH oxidation by inside-out submitochondrial particles. The level of rotenone-insensitive respiration with NAD-linked substrates correlated with the amount of 43-kDa NAD(P)H dehydrogenase present in mitochondria isolated from different soybean tissues. Based on these results, we conclude that the 43-kDa NAD(P)H dehydrogenase is responsible for rotenone-insensitive internal NADH oxidation in plant mitochondria.

Purification and properties of NAD(P)H: (quinone-acceptor) oxidoreductase of sugarbeet cells

NAD(P)H:(quinone-acceptor) oxidoreductase [NAD(P)H-QR], a plant cytosolic protein, was purified from cultured sugarbeet cells by a combination of ammonium sulfate fractionation, FPLC Superdex 200 gel filtration, Q-Sepharose anion-exchange chromatography, and a final Blue Sepharose CL-6B affinity chromatography with an NADPH gradient. The subunit molecular mass is 24 kDa and the active protein (94 kDa) is a tetramer. The isoelectric point is 4.9. The enzyme was characterized by ping-pong kinetics and extremely elevated catalytic capacity. It prefers NADPH over NADH as electron donor (kcat/Km ratios of 1.7 x 10(8) M-1 S-1 and 8.3 x 10(7) M-1 S-1 for NADPH and NADH, respectively, with benzoquinone as electron acceptor). The acridone derivative 7-iodo-acridone-4-carboxylic acid is an efficient inhibitor (I0.5 = 5 x 10(-5) M), dicumarol is weakly inhibitory. The best acceptor substances are hydrophilic, short-chain quinones such as ubiquinone-0 (Q-0), benzoquinone and menadione, followed by duroquinone and ferricyanide, whereas hydrophobic quinones, cytochrome c and oxygen are reduced at negligible rates at best. Quinone acceptors are reduced by a two-electron reaction with no apparent release of free semiquinonic intermediates. This and the above properties suggest some relationship of NAD(P)H-QR to DT-diaphorase, an animal flavoprotein which, however, has distinct structural properties and is strongly inhibited by dicumarol. It is proposed that NAD(P)H-QR by scavenging unreduced quinones and making them prone to conjugation may act in plant tissues as a functional equivalent of DT-diaphorase.

Functional molecular aspects of the NADH dehydrogenases of plant mitochondria

There are multiple routes of NAD(P)H oxidation associated with the inner membrane of plant mitochondria. These are the phosphorylating NADH dehydrogenase, otherwise known as Complex I, and at least four other nonphosphorylating NAD(P)H dehydrogenases. Complex I has been isolated from beetroot, broad bean, and potato mitochondria. It has at least 32 polypeptides associated with it, contains FMN as its prosthetic group, and the purified enzyme is sensitive to inhibition by rotenone. In terms of subunit complexity it appears similar to the mammalian and fungal enzymes. Some polypeptides display antigenic similarity to subunits from Neurospora crassa but little cross-reactivity to antisera raised against some beef heart complex I subunits. Plant complex I contains eight mitochondrial encoded subunits with the remainder being nuclear-encoded. Two of these mitochondrial-encoded subunits, nad7 and nad9, show homology to corresponding nuclear-encoded subunits in Neurospora crassa (49 and 30 kDa, respectively) and beef heart CI (49 and 31 kDa, respectively), suggesting a marked difference between the assembly of CI from plants and the fungal and mammalian enzymes. As well as complex I, plant mitochondria contain several type-II NAD(P)H dehydrogenases which mediate rotenone-insensitive oxidation of cytosolic and matrix NADH. We have isolated three of these dehydrogenases from beetroot mitochondria which are similar to enzymes isolated from potato mitochondria. Two of these enzymes are single polypeptides (32 and 55 kDa) and appear similar to those found in maize mitochondria, which have been localized to the outside of the inner membrane. The third enzyme appears to be a dimer comprised of two identical 43-kDa subunits. It is this enzyme that we believe contributes to rotenone-insensitive oxidation of matrix NADH. In addition to this type-II dehydrogenases, several observations suggest the presence of a smaller form of CI present in plant mitochondria which is insensitive to rotenone inhibition. We propose that this represents the peripheral arm of CI in plant mitochondria and may participate in nonphosphorylating matrix NADH oxidation.

The contribution of mitochondria to energetic metabolism in photosynthetic cells

Mitochondria fulfill important functions in photosynthetic cells not only in darkness but also in light. Mitochondrial oxidative phosphorylation is probably the main mechanism to supply ATP for extrachloroplastic functions in both conditions. Furthermore, during photosynthesis mitochondrial electron transport is important for regulation of the redox balance in the cell. This makes mitochondrial function an integral part of a flexible metabolic system in the photosynthetic cell. This flexibility is probably very important in order to allow the metabolism to override disturbances caused by the changing environment which plants are adapted to.

The outer membrane of plant mitochondria contains a calcium-dependent protein kinase and multiple phosphoproteins

Highly purified mitochondria from potato (Solanum tuberosum L. cv. Bintje) tubers were subfractionated into a matrix fraction, an inner membrane fraction and an outer membrane fraction with minimal cross-contamination. When the matrix and inner membrane fractions were incubated with [gamma-32P]ATP only one and three prominent phosphoproteins were detected after SDS-PAGE and autoradiography, respectively. In contrast, more than 20 phosphoproteins could be labelled in the outer membrane fraction, the main ones at 12, 18, 26, 43, 58, 60, 65, 74 and 110 kDa. Only one band, at 18 kDa, was detectable when the labelling was done in the presence of EGTA. We conclude that the outer membrane of plant mitochondria contains at least one Ca(2+)-dependent protein kinase and more than 20 endogenous substrates.

NAD(P)H-ubiquinone oxidoreductases in plant mitochondria

Plant (and fungal) mitochondria contain multiple NAD(P)H dehydrogenases in the inner membrane all of which are connected to the respiratory chain via ubiquinone. On the outer surface, facing the intermembrane space and the cytoplasm, NADH and NADPH are oxidized by what is probably a single low-molecular-weight, nonproton-pumping, unspecific rotenone-insensitive NAD(P)H dehydrogenase. Exogenous NADH oxidation is completely dependent on the presence of free Ca2+ with a K0.5 of about 1 microM. On the inner surface facing the matrix there are two dehydrogenases: (1) the proton-pumping rotenone-sensitive multisubunit Complex I with properties similar to those of Complex I in mammalian and fungal mitochondria. (2) a rotenone-insensitive NAD(P)H dehydrogenase with equal activity with NADH and NADPH and no proton-pumping activity. The NADPH-oxidizing activity of this enzyme is completely dependent on Ca2+ with a K0.5 of 3 microM. The enzyme consists of a single subunit of 26 kDa and has a native size of 76 kDa, which means that it may form a trimer.