Enzyme regulation in C4 photosynthesis: purification, properties, and activities of thioredoxins from C4 and C3 plants

NTRC links built-in thioredoxin to light and sucrose in regulating starch synthesis in chloroplasts and amyloplasts

Plants have an unusual plastid-localized NADP-thioredoxin reductase C (NTRC) containing both an NADP-thioredoxin reductase (NTR) and a thioredoxin (Trx) domain in a single polypeptide. Although NTRC is known to supply reductant for detoxifying hydrogen peroxide in the dark, its other functions are unknown. We now report that NTRC plays a previously unrecognized role in the redox regulation of ADP-glucose pyrophosphorylase (AGPase), a central enzyme of starch synthesis. When supplied NADPH, NTRC activated AGPase in vitro in a redox reaction that required the active site cysteines of both domains of the enzyme. In leaves, AGPase was activated in planta either by light or external feeding of sucrose in the dark. Leaves of an Arabidopsis NTRC KO mutant showed a decrease both in the extent of redox activation of AGPase and in the enhancement of starch synthesis either in the light (by 40-60%) or in the dark after treatment with external sucrose (by almost 100%). The light-dependent activation of AGPase in isolated chloroplasts, by contrast, was unaffected. In nonphotosynthetic tissue (roots), KO of NTRC decreased redox activation of AGPase and starch synthesis in response to light or external sucrose by almost 90%. The results provide biochemical and genetic evidence for a role of NTRC in regulating starch synthesis in response to either light or sucrose. The data also suggest that the Trx domain of NTRC and, to a lesser extent, free Trxs linked to ferredoxin enable amyloplasts of distant sink tissues to sense light used in photosynthesis by leaf chloroplasts and adjust heterotrophic starch synthesis accordingly.

The ferredoxin/thioredoxin system of oxygenic photosynthesis

Forty years ago, ferredoxin (Fdx) was shown to activate fructose 1,6-bisphosphatase in illuminated chloroplast preparations, thereby laying the foundation for the field now known as "redox biology." Enzyme activation was later shown to require the ubiquitous protein thioredoxin (Trx), reduced photosynthetically by Fdx via an enzyme then unknown-ferredoxin:thioredoxin reductase (FTR). These proteins, Fdx, FTR, and Trx, constitute a regulatory ensemble, the "Fdx/Trx system." The redox biology field has since grown beyond all expectations and now embraces a spectrum of processes throughout biology. Progress has been notable with plants that possess not only the plastid Fdx/Trx system, but also the earlier known NADP/Trx system in the cytosol, endoplasmic reticulum, and mitochondria. Plants contain at least 19 types of Trx (nine in chloroplasts). In this review, we focus on the structure and mechanism of action of members of the photosynthetic Fdx/Trx system and on biochemical processes linked to Trx. We also summarize recent evidence that extends the Fdx/Trx system to amyloplasts-heterotrophic plastids functional in the biosynthesis of starch and other cell components. The review highlights the plant as a model system to uncover principles of redox biology that apply to other organisms.

Redox regulation: a broadening horizon

Initially discovered in the context of photosynthesis, regulation by change in the redox state of thiol groups (S-S <--> 2SH) is now known to occur throughout biology. Several systems, each linking a hydrogen donor to an intermediary disulfide protein, act to effect changes that alter the activity of target proteins: the ferredoxin/thioredoxin system, comprised of reduced ferredoxin, a thioredoxin, and the enzyme, ferredoxin-thioredoxin reductase; the NADP/thioredoxin system, including NADPH, a thioredoxin, and NADP-thioredoxin reductase; and the glutathione/glutaredoxin system, composed of reduced glutathione and a glutaredoxin. A related disulfide protein, protein disulfide isomerase (PDI) acts in protein assembly. Regulation linked to plastoquinone and signaling induced by reactive oxygen species (ROS) and other agents are also being actively investigated. Progress made on these systems has linked redox to the regulation of an increasing number of processes not only in plants, but in other types of organisms as well. Research in areas currently under exploration promises to provide a fuller understanding of the role redox plays in cellular processes, and to further the application of this knowledge to technology and medicine.

The oxygen evolving enhancer protein 1 (OEE) of photosystem II in green algae exhibits thioredoxin activity

A thioredoxin-like chloroplast protein of the fructosebisphosphatase-stimulating f-type, but with an unusually high molecular mass of 28 kDa has previously been identified and purified to homogeneity in a fractionation scheme for resolution of the acid- and heat-stable, regular-size (12kDa) thioredoxins of the unicellular green algae, Scenedesmus obliquus. An apparently analogous protein of 26 kDa was described in a cyanobacterium, Anabaena sp., but no such large thioredoxin species f exists in the thioredoxin profiles of higher plants. The structure of the 28 kDa protein, which had been envisaged to represent a precursor, or fusion product of the two more specialized, common chloroplast thioredoxins f and m has now been determined by amino acid sequencing. Although it exhibits virtually all the properties and enzyme-modulating activities of a thioredoxin proper this algal protein, surprisingly, does not belong to the thioredoxin family of small redox proteins but is identical with OEE (oxygen evolving enhancer) protein 1, an auxiliary component of the photosystem II manganese cluster. Extracts of Chlorella vulgaris and Chlamydomonas reinhardtii also contain heat-stable protein fractions of 23-26 kDa capable of specifically stimulating chloroplast fructosebisphosphatase in vitro. In contrast, OEE protein 1 from spinach is not able to modulate FbPase or NADP malate dehydrogenase from spinach chloroplasts. A dual function of the OEE protein in algal photosynthesis is envisaged.

Hybrids from pea chloroplast thioredoxins f and m: physicochemical and kinetic characteristics

Two hybrid thioredoxins (Trx) have been constructed from cDNA clones coding for pea chloroplast Trxs m and f. The splitting point was the Avall site situated between the two cysteines of the regulatory cluster. One hybrid, Trx m/f, was purified from Escherichia coli-expressed cell lysates as a high yielding 12 kDa protein. Western blot analysis showed a positive reaction with antibodies against pea Trxs m and f and, like the parenteral pea Trx m, displayed an acidic pI (5.0) and a high thermal stability. In contrast, the opposite hybrid Trx f/m appeared in E. coli lysates as inclusion bodies, where it was detected by Western blot against pea Trx f antibodies as a 40 kDa protein. Trx f/m was very unstable, sensitive to heat denaturation, and could not be purified. Trx m/f showed a higher affinity for pea chloroplast fructose-1,6-bisphosphatase (FBPase) and a smaller Trx/FBPase saturation ratio than both parenterals; however, the FBPase catalytic rate was lower than that with Trxs m and f. Surprisingly, the hybrid Trx m/f appeared to be incompetent in the activation of pea NADP-malate dehydrogenase. Computer-assisted models of pea Trxs m and f, and of the chimeric Trx m/f, showed a change in the orientation of the alpha 4-helix in the hybrid, which could explain the kinetic modifications with respect to Trxs m and f. We conclude that the stability of Trxs lies on the N-side of the regulatory cluster, and is associated with the acidic character of this fragment and, as a consequence, with the acidic pl of the whole molecule. In contrast, the ability of FBPase binding and enzyme catalysis depends on the structure on the C-side of the regulatory cysteines.

Identification of residues of spinach thioredoxin f that influence interactions with target enzymes

The necessity for two types of thioredoxins (Trx f and m) within chloroplasts of higher plants that mediate the same redox chemistry with various target enzymes is not well understood. To approach this complex issue, we have applied site-directed mutagenesis to the identification of residues of Trx f that affect its binding to and selectivity for target enzymes. Based upon amino acid sequence alignments and the three-dimensional structure of Escherichia coli thioredoxin, putative key residues of Trx f were replaced with residues found at corresponding positions of Trx m to generate the mutants K58E, Q75D, N74D, and deletion mutants DeltaAsn-74 and DeltaAsn-77. Kinetics of activation of oxidized recombinant sorghum leaf NADP-dependent malate dehydrogenase and oxidized spinach chloroplastic fructose-1,6-bisphosphatase by wild-type Trx f, wild-type Trx m, and Trx f mutants were compared. All of the mutants are less efficient than wild-type Trx f in the activation of fructose-1,6-bisphosphatase and are altered in both S0.5 and Vmax. In contrast to literature reports, the activation of NADP-dependent malate dehydrogenase does not display rate saturation kinetics with respect to the concentration of Trx f, thereby signifying very weak interactions between the two proteins. The mutants of Trx f likewise interact only weakly with NADP-dependent malate dehydrogenase, but the apparent second-order rate constants for activation are increased compared to that with wild-type Trx f. Thus, Lys-58, Asn-74, Gln-75, and Asn-77 of Trx f contribute to its interaction with target enzymes and influence target protein selectivity.

Kinetic evidence for protein complexes between thioredoxin and NADP-malate dehydrogenase and presence of a thioredoxin binding site at the N-terminus of the enzyme

The kinetics of activation of NADP-malate dehydrogenase (MDH; EC 1.1.1.82) from soybean and spinach leaves by the chloroplast thioredoxins isolated from the same plants, by the corresponding storage forms of the soybean chloroplast thioredoxins from soybean seeds, and by the bacterial Escherichia coli thioredoxin have been studied. The Hill equation has been applied to evaluate the saturation kinetics. The observed variable thioredoxin saturation characteristics (Vmax 0.37-14.5 mumol NADPH min-1 mg enzyme-1; K0.5 0.15-1.33 microM; Hill coefficient h 0.90-3.04) indicate that the activation of NADP-MDH depends strongly on the individual thioredoxin used. Thus, thioredoxin action is not solely due to simple reductive activation of the NADP-MDH. Specific thioredoxin complex formation between thioredoxin and NADP-MDH must be included into the mechanism of the activation process. To study the regulatory consequences of the specific thioredoxin/NADP-MDH complexes we investigated the saturation kinetics of the substrates NADPH and oxaloacetate in presence of different concentrations of each individual thioredoxin species. The kinetic characteristics of the substrates (S0.5, Vmax, and Hill coefficients h) varied individually in response to the different thioredoxin species substantiating our model of thioredoxin/ NADP-MDH complex formation. Aminopeptidase-K-truncated pea NADP-MDH has been used to demonstrate that the N-terminal 37 amino residues are involved in providing a specific thioredoxin binding site. The fact that the versatile light-dependent regulation of numerous enzyme activities by only two thioredoxin species in chloroplasts cannot be accomplished without the formation of thioredoxin/target enzyme complexes is discussed in detail.

Completion of the thioredoxin reaction mechanism: kinetic evidence for protein complexes between thioredoxin and fructose 1,6-bisphosphatase

The activation of chloroplast fructose 1,6-bisphosphatase from spinach and soybean leaves by the two chloroplast thioredoxins isolated from the same plants has been studied. The thioredoxin saturation characteristics (Vmax: 0.15-103.2 mumol Pi/min per mg enzyme; K0.5: 0.0048-0.84 microM; Hill coefficient n: 1.02-3.80) indicate that in addition to the reductive activation by thioredoxin specific complex formation between thioredoxin and fructose 1,6-bisphosphatase is responsible for fine regulation of the enzyme activity. This complex formation has been inserted into the thioredoxin mechanism and the physiological consequences discussed. Obviously, physiologically relevant investigations of the thioredoxin-dependent regulation of fructose 1,6-bisphosphatase activity can only be performed in homologous enzyme-thioredoxin combinations. Dithiothreitol and E. coli thioredoxin are no complete substitutes in regulatory studies.

Redox regulation of maize NADP-malic enzyme by thiol-disulfide interchange: effect of reduced thioredoxin on activity

Incubation of C4 NADP-malic enzyme from maize leaves with the oxidant o-iodosobenzoate leads to the reversible and complete inactivation of the enzyme. The time-course of inactivation is biphasic with the rate depending on the o-iodosobenzoate concentration. The inactivation is partially prevented by L-malate, NADP and Mg2+ alone, while NADP plus Mg2+ afford total protection. The complete reversal of the inactivation by the reductive agents dithiothreitol and 2-mercaptoethanol suggests that the modification of the enzyme by o-iodosobenzoate occurs concomitant with the oxidation of one or more pairs of sulfhydryl groups to the disulfide state, producing a conformationally altered form of the protein or directly modifying the active site. Titration of free thiol groups before and after inactivation of maize malic enzyme by o-iodosobenzoate shows a decrease in the accessible groups from 7 to 5, suggesting inactivation is accompanied by oxidation of two vicinal thiols. The oxidized form of the enzyme is rapidly reactivated by incubation with chemical and photochemically reduced thioredoxin in vitro, while the 'dark' activity of the enzyme is enhanced to the level of the 'light' activity by dithiothreitol. This evidence suggests that a reversible reduction and oxidation of disulfide bonds may take place during the regulation of the enzyme, indicating that the redox state of the disulfide bonds of C4 NADP-malic enzyme from maize leaves is important for the expression of maximal catalytic activity.

Isolation and characterization of different forms of thioredoxins from the green alga Acetabularia mediterranea: identification of an NADP/thioredoxin system in the extrachloroplastic fraction

A procedure has been developed for the simultaneous purification to apparent homogeneity of chloroplast thioredoxins f and m, and nonchloroplast thioredoxin h, from the green alga Acetabularia mediterranea. In the chloroplast fraction, three thioredoxins were isolated: one f type thioredoxin (Mr 13.4 kDa) and two m type thioredoxin forms (Mr of 12.9 and 13.8 kDa). A Western blot analysis of crude and purified chloroplast thioredoxin preparations revealed that Acetabularia thioredoxin m was immunologically related to its higher-plant counterparts whereas thioredoxin f was not. In the nonchloroplast fraction, a single form of thioredoxin h (Mr 13.4 kDa) and its associated enzyme NADP-thioredoxin reductase (NTR) were evidenced. Acetabularia NTR was partially purified and shown to be an holoenzyme composed of two 33.0-kDa subunits as is the case for other plant and bacterial NTRs. Similarity was confirmed by immunological tests: the algal enzyme was recognized by antibodies to spinach and Escherichia coli NTRs. Acetabularia thioredoxin h seemed to be more distant from higher-plant type h thioredoxins as recognition by antibodies to thioredoxin h from spinach and wheat was weak. The algal thioredoxin h was also slightly active with spinach and E. coli NTRs. These results suggest that in green algae as in the green tissues of higher plants the NADP and chloroplast thioredoxin systems are present simultaneously, and might play an important regulatory role in their respective cellular compartments.

Mutation of a negatively charged amino acid in thioredoxin modifies its reactivity with chloroplastic enzymes

A new over-expression system has been set up for Escherichia coli thioredoxin, yielding 55 mg purified protein/10 g fresh cells. This system has been used to produce thioredoxin modified by site-directed mutagenesis. Taking advantage of the structural and enzymatic similarity between E. coli and spinach m-type thioredoxin, Asp61 of E. coli thioredoxin has been changed into Asn in order to investigate the impact of the suppression of a charged residue on the interaction of thioredoxin with target enzymes. The modification did not significantly alter the structure of the protein. Neither the rate of reduction of insulin and 5,5'-dithio-bis(2-nitrobenzoic acid) by the reduced thioredoxin, nor the reduction by NADPH-dependent thioredoxin reductase, have been modified. The major effect of the mutation was observed for chloroplast enzyme activation with thioredoxin reduced by dithiothreitol and with thioredoxin reduced by ferredoxin-dependent thioredoxin reductase in a light-activation reconstituted chloroplast system. The substitution of the negatively charged Asp61 by the neutral Asn led to an increase in the efficiency of spinach fructose-1,6-bisphosphatase activation by the dithiothreitol-reduced thioredoxin, and to an increase in both spinach fructose-1,6-bisphosphatase and corn NADP-dependent malate dehydrogenase activities in the light-activation system. This suggests that the suppression of the negative charge improves the reactivity of thioredoxin with chloroplast enzymes such as fructose-1,6-bisphosphatase and ferredoxin-dependent thioredoxin reductase.

Human thioredoxin reactivity-structure/function relationship

The reactivity of human thioredoxin (HTR) was tested in several reactions. HTR was as efficient as E. coli or plant and algal thioredoxins when assayed with E. coli ribonucleotide reductase or for the reduction of insulin. On the other hand, HTR was poorly reduced by NADPH and the E. coli flavoenzyme NADPH thioredoxin reductase as monitored in the DTNB reduction test. When reduced with dithiothreitol (DTT), HTR was much less efficient than thioredoxin m and thioredoxin f, the respective specific thioredoxins for the chloroplast enzymes NADP-malate dehydrogenase (NADP-MDH) and fructose 1,6 bisphosphatase (FBPase). Finally, HTR could be used in the photoactivation of NADP-MDH although less efficiently than thioredoxin m, proving nevertheless that it can be reduced by the iron sulfur enzyme ferredoxin thioredoxin reductase in the presence of photoreduced ferredoxin. Based on sequence comparisons, it was expected that HTR would display a reactivity similar to chloroplast thioredoxin f rather than to thioredoxin m. However the observed behavior of FTR did not exactly fit this prediction. The results are discussed in relation to the structural data available for the proteins.

Novel forms of ferredoxin and ferredoxin-NADP reductase from spinach roots

Ferredoxin and the enzyme catalyzing its reduction by NADPH, ferredoxin-NADP reductase (ferredoxin-NADP+ oxidoreductase or FNR), were found to be present in roots of spinach (Spinacia oleracea). Localization experiments with endosperm of germinating castor beans (Ricinus communis), a classical nonphotosynthetic tissue for cell fractionation studies, confirmed that ferredoxin and FNR are localized in the plastid fraction. Both proteins were purified from spinach roots and found to resemble their leaf counterparts in activity, spectral properties, and complex formation, but to differ in amino acid composition and amino terminal sequence. The results indicate that the primary structures of the FNR and ferredoxin of spinach roots differ from that of the corresponding leaf proteins. Together with earlier findings, the present results provide evidence that nonphotosynthetic plastids, including those of roots, are capable of reducing ferredoxin with heterotrophically generated NADPH.

Modulation of spinach chloroplast NADP-glyceraldehyde-3-phosphate dehydrogenase by chaotropic anions

Neutral salts enhanced the specific activity of chloroplast NADP-glyceraldehyde-3-phosphate dehydrogenase (D-glyceraldehyde-3-phosphate:NADP+ oxidoreductase (phosphorylating), EC 1.2.1.13) from spinach leaves. The ordering of the respective anions, according to the concentration for maximal stimulation, yielded the lyotropic (Hofmeister) series [SCN- (0.05 M), ClO-4 (0.08 M), Cl3CCO-2 (0.24 M), I- (0.35 M), Br- (0.6 M), Cl- (1.0 M)]; the more chaotropic the anion the less its concentration for maximal activation. Neither the NAD-linked activity of the chloroplast enzyme nor glyceraldehyde-3-phosphate dehydrogenases originating from cyanobacteria and rabbit muscle were stimulated by neutral salts. Chaotropic anions also enhanced the catalytic capacity of the chloroplast enzyme at concentrations lower than those required for the activation process. In the presence of 0.12 M NaBr the rate of catalysis was maximum whereas the highest conversion from the inactive to an active form was observed at 0.6 M NaBr. On the other hand, nonstimulatory concentrations of chaotropic anions lowered the concentration of ATP, Pi, and NADPH required for maximum stimulation of the specific activity (concerted hysteresis). On the basis that the enhancement of NADP-glyceraldehyde-3-phosphate dehydrogenase (and other chloroplast enzymes) by chaotropic anions paralleled the effect of organic solvents and reduced thioredoxin, it appeared that the modification of hydrophobic (intramolecular) interactions participates in the mechanism of light-mediated regulation.

Concerted action of cosolvents, chaotropic anions and thioredoxin on chloroplast fructose-1,6-bisphosphatase. Reactivity to iodoacetamide

The incubation of chloroplast fructose-1,6-bisphosphatase with both dithiothreitol and protein denaturants made sulfhydryl groups available for reaction with [1-14C]iodoacetamide (10-12 mol iodoacetamide incorporated/mol enzyme). Digestion of S-carboxyamidomethylated enzyme with trypsin and polyacrylamide gel electrophoresis, in the presence of sodium dodecylsulfate, yielded two 14C-labeled fragments whose apparent molecular mass were 10 kDa and 16 kDa. In the absence of either dithiothreitol or protein denaturants the incorporation of iodoacetamide to the enzyme was lower than 4 mol. When chloroplast fructose-1,6-bisphosphatase was initially incubated with dithiothreitol (2.5 mM) and (a) high concentrations of both fructose 1,6-bisphosphate (4 mM) and Ca2+ (0.3 mM) or (b) low concentrations of both fructose 1,6-bisphosphate (0.8 mM) and Ca2+ (0.05 mM) in the presence of either 2-propanol (15%, by vol.), trichloroacetate (0.15 M) or chloroplast thioredoxin-f (0.5 microM) and subsequently subjected to proteolysis and electrophoresis, S-carboxyamidomethylated tryptic fragments had similar molecular masses. Thus, conditions that stimulated the specific activity of chloroplast fructose-1,6-bisphosphatase caused conformational changes which favoured both the reduction of disulfide bridges and the exposure of sulfhydryl groups. In this aspect, thioredoxin exerted structural and kinetic effects similar to compounds not involved in redox reactions (organic solvents, chaotropic anions). These results indicated that the modification of hydrophobic (intramolecular) interactions in chloroplast fructose-1,6-bisphosphatase constituted the underlying mechanism in light-activation by the ferredoxin-thioredoxin system.

Reduction of protein mixed disulfides (dethiolation) by Escherichia coli thioredoxin: a study with glycogen phosphorylase b and creatine kinase

The role of thioredoxin in the reduction of protein mixed disulfides (dethiolation) was studied by electrofocusing methodology with glycogen phosphorylase b and creatine kinase as substrates for the reaction. Glycogen phosphorylase b was effectively dethiolated by Escherichia coli thioredoxin with dithiothreitol as the reductant, while creatine kinase could not be dethiolated by this mechanism. The rate of dethiolation of phosphorylase b was dependent on the concentration of thioredoxin up to a maximum at 20 microM when the concentration of phosphorylase b was 4 microM in monomer. Rat heart contained a thioredoxin reductase activity that could use added E. coli thioredoxin to dethiolate phosphorylase b and the same concentration of thioredoxin as above was required. This activity was not expressed with creatine kinase as the substrate. Cardiac tissue was shown to have a similar endogenous dethiolating activity. These results suggest that thioredoxin may play an important role in dethiolating specific proteins that might become S-thiolated during oxidative stress of cardiac tissue.

Evidence for function of the ferredoxin/thioredoxin system in the reductive activation of target enzymes of isolated intact chloroplasts

Results obtained with isolated intact chloroplasts maintained aerobically under light and dark conditions confirm earlier findings with reconstituted enzyme assays and indicate that the ferredoxin/thioredoxin system functions as a light-mediated regulatory thiol chain. The results were obtained by application of a newly devised procedure in which a membrane-permeable thiol labeling reagent, monobromobimane (mBBr), reacts with sulfhydryl groups and renders the derivatized protein fluorescent. The mBBr-labeled protein in question is isolated individually from chloroplasts by immunoprecipitation and its thiol redox status is determined quantitatively by combining sodium dodecyl sulfate-polyacrylamide gel electrophoresis and fluorescence measurements. The findings indicate that each member of the ferredoxin/thioredoxin system containing a catalytically active thiol group is reduced in isolated intact chloroplasts after a 2-min illumination. The extents of reduction were FTR, 38%; thioredoxin m, 75% (11-kDa form) and 87% (13-kDa form); thioredoxin f, 95%. Reduction of each of these components was negligible both in the dark and when chloroplasts were transferred from light to dark conditions. The target enzyme, NADP-malate dehydrogenase, also underwent net reduction in illuminated intact chloroplasts. Fructose-1,6-bisphosphatase showed increased mBBr labeling under these conditions, but due to interfering gamma globulin proteins it was not possible to determine whether this was a result of net reduction as is known to take place in reconstituted assays. Related experiments demonstrated that mBBr, as well as N-ethylmaleimide, stabilized photoactivated NADP-malate dehydrogenase and fructose-1,6-bisphosphatase so that they remained active in the dark. By contrast, phosphoribulokinase, another thioredoxin-linked enzyme, was immediately deactivated following mBBr addition. These latter results provide new information on the relation between the regulatory and active sites of these enzymes.

An NADP/thioredoxin system in leaves: purification and characterization of NADP-thioredoxin reductase and thioredoxin h from spinach

An NADP/thioredoxin system, consisting of NADPH, NADP-thioredoxin reductase (NTR), and its thioredoxin, thioredoxin h, has been previously described for heterotrophic plant tissues, i.e., wheat seeds and cultured carrot cells. Until now there was no evidence for this system in green leaves. Here, we report the identification of protein components of the NADP/thioredoxin system in leaves of several species. Thioredoxin h and NTR, which were both recovered in the extrachloroplastic fraction, were purified to apparent homogeneity from spinach leaves. This represents the first time that NTR has been characterized from a plant source. Similar to that from bacterial and mammalian sources, spinach leaf NTR was a flavoprotein (Mr 68,000) composed of two subunits of identical molecular mass (Mr 33,000) that resembled Escherichia coli NTR immunologically. Spinach thioredoxin h existed in two forms (Mr of 13,500 and 12,000) and was highly specific for plant NTR. Thioredoxin h and NTR partially purified from spinach roots showed properties similar to their counterparts from leaves. Spinach cytosolic thioredoxin h differed from chloroplast thioredoxin m or f from the same source but was similar to thioredoxin h from wheat seed in immunological properties.

Evidence for the presence of a [2Fe-2S] ferredoxin in bean sprouts

An iron-sulfur protein with properties similar to those of ferredoxins found in the leaves of higher plants has been isolated from bean sprouts--a non-photosynthetic plant tissue. The bean sprout protein has a molecular mass of 12.5 kDa and appears to contain a single [2Fe-2S] cluster. The absorbance and circular dichroism spectra of the bean sprout protein resemble those of spinach leaf ferredoxin and the bean sprout protein can replace spinach ferredoxin as an electron donor for NADP+ reduction, nitrite reduction and thioredoxin reduction by spinach leaf enzymes. Although the reduced bean sprout protein (Em = -440 mV) is a slightly stronger reductant than spinach ferredoxin and appears to be less acidic than spinach ferredoxin, the two proteins are similar enough so that the bean sprout protein is recognized by an antibody raised against spinach ferredoxin.

Ferredoxin-thioredoxin reductase: a catalytically active dithiol group links photoreduced ferredoxin to thioredoxin functional in photosynthetic enzyme regulation

The mechanism by which the ferredoxin-thioredoxin system activates the target enzyme, NADP-malate dehydrogenase, was investigated by analyzing the sulfhydryl status of individual protein components with [14C]iodoacetate and monobromobimane. The data indicate that ferredoxin-thioredoxin reductase (FTR)--an iron-sulfur enzyme present in oxygenic photosynthetic organisms--is the first member of a thiol chain that links light to enzyme regulation. FTR possesses a catalytically active dithiol group localized on the 13 kDa (similar) subunit, that occurs in all species investigated and accepts reducing equivalents from photoreduced ferredoxin and transfers them stoichiometrically to the disulfide form of thioredoxin m. The reduced thioredoxin m, in turn, reduces NADP-malate dehydrogenase, thereby converting it from an inactive (S-S) to an active (SH) form. The means by which FTR is able to combine electrons (from photoreduced ferredoxin) with protons (from the medium) to reduce its active disulfide group remains to be determined.

Ferredoxin-thioredoxin reductase, an iron-sulfur enzyme linking light to enzyme regulation in oxygenic photosynthesis: purification and properties of the enzyme from C3, C4, and cyanobacterial species

Ferredoxin-thioredoxin reductase (FTR), an enzyme involved in the light regulation of chloroplast enzymes, was purified to homogeneity from leaves of spinach (a C3 plant) and corn (a C4 plant) and from cells of a cyanobacterium (Nostoc muscorum). The enzyme is a yellowish brown iron-sulfur protein, containing four nonheme iron and labile sulfide groups, that catalyzes the activation of NADP-malate dehydrogenase and fructose 1,6-bisphosphatase in the presence of ferredoxin and of thioredoxin m and f, respectively. FTR is synonymous with the protein earlier called ferralterin. FTR showed an Mr of about 30,000 (determined by sedimentation equilibrium ultracentrifugation, amino acid composition, gel filtration, and gradient gel electrophoresis) and was composed of two dissimilar subunits (as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis). One of the FTR subunits from each source was similar both in Mr (about 13,000) and immunological properties, while the other subunit (of variable molecular weight) was characteristic of a particular organism. The similar subunit contained a disulfide group that was rapidly reduced by a dithiol (dithiothreitol) but not by monothiols (2-mercaptoethanol or reduced glutathione). Homogeneous FTR formed a tight noncovalent complex with ferredoxin on affinity columns. The basis for the structural variation in the different FTR enzymes remains to be determined.

Thioredoxin and NADP-thioredoxin reductase from cultured carrot cells

Dark-grown carrot (Daucus carota L.) tissue cultures were found to contain both protein components of the NADP/thioredoxin system--NADP-thioredoxin reductase and the thioredoxin characteristic of heterotrophic systems, thioredoxin h. Thioredoxin h was purified to apparent homogeneity and, like typical bacterial counterparts, was a 12-kdalton (kDa) acidic protein capable of activating chloroplast NADP-malate dehydrogenase (EC 1.1.1.82) more effectively than fructose-1,6-bisphosphatase (EC 3.1.3.11). NADP-thioredoxin reductase (EC 1.6.4.5) was partially purified and found to be an arsenite-sensitive enzyme composed of two 34-kDa subunits. Carrot NADP-thioredoxin reductase resembled more closely its counterpart from bacteria rather than animal cells in acceptor (thioredoxin) specificity. Upon greening of the cells, the content of NADP-thioredoxin-reductase activity, and, to a lesser extent, thioredoxin h decreased. The results confirm the presence of a heterotrophic-type thioredoxin system in plant cells and raise the question of its physiological function.