Structure-function studies of [2Fe-2S] ferredoxins

A three-domain iron-sulfur flavoprotein obtained through gene fusion of ferredoxin and ferredoxin-NADP+ reductase from spinach leaves

Ferredoxin and ferredoxin-NADP+ reductase are the two last partners of the photosynthetic electron-transfer chain, responsible for the final reduction of NADP+ to NADPH. Herein, we report the engineering and characterization of a novel protein molecule in which the electron-carrier protein (ferredoxin I) and the reductase (a flavoprotein) were covalently linked in a single polypeptide chain by gene fusion. The gene was obtained by joining the cDNAs encoding the respective proteins and subsequently by deleting the intervening sequence between them by site-directed mutagenesis. No extra amino acid residues were introduced between the C-terminus of ferredoxin I and the N-terminus of the flavoenzyme. The chimera was purified to homogeneity and characterized. The M(r) of the chimera apoprotein was 45,800 as determined by mass spectrometry, in agreement with the expected value of 45,846. Both flavin and iron-sulfur cluster were in 1:1 ratio with respect to the apoprotein. The chimera was found active as a diaphorase and, more interestingly, highly efficient as a cytochrome c reductase, without need for free ferredoxin addition in the assay medium. Several lines of evidence indicate that the ferredoxin and the reductase in the chimera assume a configuration quite similar to that in the dissociable physiological complex. Thus, the fusion protein could be a useful tool for studying the mechanism of protein-protein recognition and electron transfer in the ferredoxin-ferredoxin-NADP+ reductase system.

On the role of the acidic cluster Glu 92-94 of spinach ferredoxin I

The role of the acidic cluster Glu 92-94 of spinach ferredoxin I in the interaction both with the photosystem I multisubunit complex and the ferredoxin-NADP+ reductase, either membrane-bound or purified, was studied by kinetic characterization of site-directed mutants. Three mutants of ferredoxin have been produced to evaluate the effects of elimination of one or two negative charges in the three specific positions of the acidic cluster. Kinetic characterization of the ferredoxin mutants E92A/E93A, E93A and E93A/E94A as electron carriers in the photosynthetic electron transport chain, allowed to establish that the two latter mutants were nearly indistinguishable from the wild-type protein in their ability to be photoreduced by photosystem I and as electron donor to the reductase in the NADP+ photoreduction with thylakoid membranes. The E92A/E93A ferredoxin mutant behaved very similarly to E92 mutants previously characterized. Thus, the elimination of the carboxyl groups adjacent to residue 92 did not further impaired ferredoxin I main function, i.e., as an electron carrier in NADP+ photoreduction. The two double mutants showed a reduced rate in the cross-linking of ferredoxin to the reductase promoted by a soluble carbodiimide, indicating an involvement of the acidic cluster in the formation of the active covalent complex between the two proteins.

Structure-function relationships in Anabaena ferredoxin: correlations between X-ray crystal structures, reduction potentials, and rate constants of electron transfer to ferredoxin:NADP+ reductase for site-specific ferredoxin mutants

A combination of structural, thermodynamic, and transient kinetic data on wild-type and mutant Anabaena vegetative cell ferredoxins has been used to investigate the nature of the protein-protein interactions leading to electron transfer from reduced ferredoxin to oxidized ferredoxin:NADP+ reductase (FNR). We have determined the reduction potentials of wild-type vegetative ferredoxin, heterocyst ferredoxin, and 12 site-specific mutants at seven surface residues of vegetative ferredoxin, as well as the one- and two-electron reduction potentials of FNR, both alone and in complexes with wild-type and three mutant ferredoxins. X-ray crystallographic structure determinations have been carried out for six of the ferredoxin mutants. None of the mutants showed significant structural changes in the immediate vicinity of the [2Fe-2S] cluster, despite large decreases in electron-transfer reactivity (for E94K and S47A) and sizable increases in reduction potential (80 mV for E94K and 47 mV for S47A). Furthermore, the relatively small changes in Calpha backbone atom positions which were observed in these mutants do not correlate with the kinetic and thermodynamic properties. In sharp contrast to the S47A mutant, S47T retains electron-transfer activity, and its reduction potential is 100 mV more negative than that of the S47A mutant, implicating the importance of the hydrogen bond which exists between the side chain hydroxyl group of S47 and the side chain carboxyl oxygen of E94. Other ferredoxin mutations that alter both reduction potential and electron-transfer reactivity are E94Q, F65A, and F65I, whereas D62K, D68K, Q70K, E94D, and F65Y have reduction potentials and electron-transfer reactivity that are similar to those of wild-type ferredoxin. In electrostatic complexes with recombinant FNR, three of the kinetically impaired ferredoxin mutants, as did wild-type ferredoxin, induced large (approximately 40 mV) positive shifts in the reduction potential of the flavoprotein, thereby making electron transfer thermodynamically feasible. On the basis of these observations, we conclude that nonconservative mutations of three critical residues (S47, F65, and E94) on the surface of ferredoxin have large parallel effects on both the reduction potential and the electron-transfer reactivity of the [2Fe-2S] cluster and that the reduction potential changes are not the principal factor governing electron-transfer reactivity. Rather, the kinetic properties are most likely controlled by the specific orientations of the proteins within the transient electron-transfer complex.

Probing the influence of mutations on the stability of a ferredoxin by mass spectrometry

Hydrogen/deuterium exchange, which depends on solvent accessibility, can be probed by mass spectrometry (MS) to get information on protein conformation or protein-ligand interaction. In this work, the conformational properties of the cyanobacterium Anabaena wild-type ferredoxin as well as of two single-site mutants (Phe 65 Ala and Arg 42 Ala) were studied. After incubation of the wild type and mutant proteins in deuterated water and quenching of the exchange at low pH, the proteins were rapidly digested at high enzyme-to-substrate ratio using immobilized pepsin, and the resulting peptides were characterized using ESI-MS. We have identified specific regions for which the H-bonding or solvent accessibility properties were perturbed by the mutations. These results show that this approach can provide local information on the influence of mutations, even for a highly structured protein like ferredoxin, and sometimes in regions distant from the mutation point.

Solution structure of ferredoxin from the thermophilic cyanobacterium Synechococcus elongatus and its thermostability

The three-dimensional structure of ferredoxin, purified from the thermophilic cyanobacterium Synechococcus elongatus, was determined in aqueous solution by two-dimensional proton nuclear magnetic resonance. In addition to the 946 distance constraints from nuclear Overhauser effect connectivities, we added 241 distance constraints derived from the crystal structure of Spirulina platensis ferredoxin to the 19 residues close to the [2Fe-2S] iron-sulfur center, where crosspeaks disappeared due to paramagnetic effects. The atomic root-mean-square difference of the ten converged structures from the mean structure was 0.61(+/-0.12) A for backbone atoms (N, C(alpha), C'). The main-chain structure was almost the same as the crystal structures of other mesophile ferredoxins, but comparison of the side-chain structures revealed an extension of the hydrophobic core, a unique hydrophobic patch on the surface of the large beta-sheet, and two unique charge networks in this thermostable ferredoxin structure, some of which might contribute to thermostability.

Specific aspects of electron transfer from adrenodoxin to cytochromes p450scc and p45011beta

An analysis of the electron transfer kinetics from the reduced [2Fe-2S] center of bovine adrenodoxin and its mutants to the natural electron acceptors, cytochromes P450scc and P45011beta, is the primary focus of this paper. A series of mutant proteins with distinctive structural parameters such as redox potential, microenvironment of the iron-sulfur cluster, electrostatic properties, and conformational stability was used to provide more detailed insight into the contribution of the electronic and conformational states of adrenodoxin to the driving forces of the complex formation of reduced adrenodoxin with cytochromes P450scc and P45011beta and electron transfer. The apparent rate constants of P450scc reduction were generally proportional to the adrenodoxin redox potential under conditions in which the protein-protein interactions were not affected. However, the effect of redox potential differences was shown to be masked by structural and electrostatic effects. In contrast, no correlation of the reduction rates of P45011beta with the redox potential of adrenodoxin mutants was found. Compared with the interaction with P450scc, however, the hydrophobic protein region between the iron-sulfur cluster and the acidic site on the surface of adrenodoxin seems to play an important role for precise complementarity in the tightly associated complex with P45011beta.

Reconstitution of the 2Fe-2S center and g = 1.89 electron paramagnetic resonance signal into overproduced Nostoc sp. PCC 7906 Rieske protein

The Rieske 2Fe-2S protein is a distinguishing subunit of the photosynthetic electron transport cytochrome b6f complex in chloroplast and cyanobacterial thylakoid membranes. We have constructed plasmids for overproduction in Escherichia coli of fusion, full-length, and truncated forms of the Rieske (PetC) protein from the cyanobacterium Nostoc sp. PCC 7906. A glutathione S-transferase/Rieske fusion protein was used to prepare specific chicken egg-yolk antibodies against the Rieske protein. Expression of the nonfusion petC gene in a T7 RNA polymerase promoter vector produced copious quantities of the full-length Rieske protein predominantly as inclusion bodies. The highly enriched, Rieske protein from inclusion bodies has been denatured in guanidine hydrochloride and refolded and the characteristic 2Fe-2S cluster reconstituted in vitro by incubation with iron and sulfide under reducing conditions. Purification by chromatography on Whatman DE52 cellulose and ultrafiltration through a 30000 molecular weight cutoff membrane yielded pure and predominantly monomeric Rieske protein. Reconstituted Rieske preparations showed intense and highly characteristic gx = 1.74, gy = 1.89, and gz = 2.03 "Rieske-type" electron paramagnetic resonance signals at 15 K. Two methods of reconstitution yielded Rieske preparations in which 20-60% of the protein contained 2Fe-2S clusters as determined by EPR spin quantitation. The reconstituted Rieske protein was soluble and stable at 4 degrees C in buffers containing nonionic detergents and showed a redox midpoint potential of +321 mV at pH 7.0 as determined by optical circular dichroism (CD) spectroscopy. These data demonstrate the in vitro restoration of a Cys and His liganded 2Fe-2S cluster and provide the basis for mutational and structural analysis of a PetC Rieske protein of oxygenic photosynthesis.

Cysteine ligand swapping on a deletable loop of the [2Fe-2S] ferredoxin from Clostridium pasteurianum

The [2Fe-2S] ferredoxin from Clostridium pasteurianum is unique among ferredoxins, both by its sequence and by the distribution of its cysteine residues (in positions 11, 14, 24, 56, and 60). In previous investigations, a combination of site-directed mutagenesis and of spectroscopic techniques showed that cysteines 11, 56, and 60 are ligands of the [2Fe-2S] cluster in the wild type protein and that cysteine 14 is not, but the status of cysteine 24 remained unclear. New mutated forms of this ferredoxin have been obtained and characterized. The data show that cysteine 24 is a ligand of the cluster in the wild type protein. When cysteine 24 is mutated into alanine, it is replaced as a cluster ligand by cysteine 14. The fourth ligand of the cluster can also be a cysteine residue newly introduced in position 16 when both cysteines 14 and 24 are replaced by alanine. These results suggest that the region encompassing cysteines 14 and 24 is a solvent-exposed flexible loop, in agreement with structure predictions. A number of nondeleterious deletions of variable length (3-14 residues) have been performed in the region of residues 17-32. The deletions were found to modify only marginally the spectroscopic properties of the [2Fe-2S] cluster but resulted in variations of its redox potential over a range of nearly 100 mV. This is the first instance of ligand swapping in a [2Fe-2S] protein, and the first time in any ferredoxin that a large loop has been excised from the structure without preventing the assembly of the iron-sulfur chromophore. Some of the molecular variants described here also highlight the similarities between the C. pasteurianum [2Fe-2S] ferredoxin and the 25 kDa subunit of the proton-translocating NADH: ubiquinone oxidoreductase of Paracoccus denitrificans.

Insights into protein adaptation to a saturated salt environment from the crystal structure of a halophilic 2Fe-2S ferredoxin

Haloarcula marismortui is an archaebacterium that flourishes in the world's saltiest body of water, the Dead Sea. The cytosol of this organism is a supersaturated salt solution in which proteins are soluble and active. The crystal structure of a 2Fe-2S ferredoxin from H. marismortui determined at 1.9 A is similar to those of plant-type 2Fe-2S ferredoxins of known structure, with two important distinctions. The entire surface of the protein is coated with acidic residues except for the vicinity of the iron-sulphur cluster, and there is an insertion of two amphipathic helices near the N-terminus. These form a separate hyperacidic domain whose postulated function to provide extra surface carboxylates for solvation. These data and the fact that bound surface water molecules have on the average 40% more hydrogen bonds than in a typical non-halophilic protein crystal structure support the notion that haloadaptation involves better water binding capacity.

Mutations of Glu92 in ferredoxin I from spinach leaves produce proteins fully functional in electron transfer but less efficient in supporting NADP+ photoreduction

Ferredoxin I in spinach chloroplasts fulfils the role of distributing electrons of low redox potential produced by photosystem I to several metabolic routes, NADP+ reduction being the major output. To investigate the role of Glu92, which is conserved in the chloroplast-type ferredoxins, mutations of this residue to either Gln, Ala or Lys were obtained through site-directed mutagenesis. A Glu93Ala mutant was also designed. The four mutants of ferredoxin I were overproduced in Escherichia coli, purified and characterised. The different migration in nondenaturing gel electrophoresis of wild-type and mutant proteins confirmed that the desired mutation was present in the expressed proteins. Spectral and physical properties of the mutants were similar to those of wild-type ferredoxin; electron-transfer properties were, however, quite different in the case of the mutants at position 92. Unexpectedly, these mutant ferredoxins were found to be twice as active as the wild-type protein in supporting the NADPH--cytochrome c reductase reaction catalysed by ferredoxin--NADP+ reductase. However, interactions of the mutant ferredoxins with the isolated thylakoid membranes deprived of endogenous ferredoxin showed that the mutants were less capable of supporting NADP+ photoreduction than the wild-type protein: both V and the apparent Km for reduced ferredoxin were influenced. On the other hand, the Kd values for the complex between oxidised ferredoxin and the reductase, measured at low ionic strength, were substantially changed only in the case of the Glu-->Lys mutation. With this mutant the rate of cross-linking between the two proteins induced by a carbodiimide was also decreased. It was found that the redox potentials of the iron-sulfur cluster of the mutants were more positive by 73-93 mV than that of ferredoxin I. Thus, the behavior of the ferredoxin mutants can be rationalised in terms of the effect of the side-chain replacement on the electrochemical properties of the [2Fe-2S] cluster and of an impairment in the interaction with the reductase under physiological conditions.

The role of threonine 54 in adrenodoxin for the properties of its iron-sulfur cluster and its electron transfer function

The amino acid in position 54 of adrenodoxin is strongly conserved among ferredoxins, consisting of a threonine or serine. Its role was studied by analyzing mutants T54S and T54A of bovine adrenodoxin. Absorption, circular dichroism, fluorescence, and electron paramagnetic resonance spectra of mutant T54S show that this substitution has no influence on the formation and stability of the ferredoxin. The redox potential of this mutant, however, was lowered by 55 mV as compared with native adrenodoxin, indicating a role for this residue in redox potential modulation. Incorporation of the iron-sulfur cluster was not impaired in the T54A mutant, although structural features of the oxidized protein were considerably changed. The decreased stability of the T54A mutant as compared with the wild type and mutant T54S indicates that a hydrogen bond donor at this position stabilizes the protein. Both mutants have been shown to be functionally active. Replacement of threonine 54 by serine or alanine, however, leads to rearrangements at the recognition sites for its redox partners. This is reflected by decreased Km and Kd values of both mutants for the cytochromes P450, whereas only T54A displayed a decreased Km value in cytochrome c reduction. Substrate conversion was accelerated (2.2- and 2.4-fold for mutants T54A and T54S, respectively) in the CYP11B1-, but not in the CYP11A1-dependent reaction.

Modified ligands to FA and FB in photosystem I. I. Structural constraints for the formation of iron-sulfur clusters in free and rebound PsaC

Cysteines 14, 21, 34, 51, or 58 in PsaC of photosystem I (PS I) were replaced with aspartic acid (C21D and C58D), serine (C14S, C34S, and C51S), and alanine (C14A, C34A, and C51A). When free in solution, the C34S and C34A holoproteins contained two S = 1/2 ground state [4Fe-4S] clusters; all other mutant proteins contained [3Fe-4S] clusters and [4Fe-4S] clusters; in addition, there was evidence in C14S, C51S, C14A, and C51A for high spin (S = 3/2) [4Fe-4S] clusters, presumably in the modified site. These findings are consistent with the assignment of C14, C21, C51, and C58, but not C34, as ligands to FA and FB. The [4Fe-4S] clusters in the unmodified sites in C14S, C51S, C14A, and C51A remained highly electronegative, with Em values ranging from -495 to -575 mV. The [3Fe-4S] clusters in the modified sites were driven 400 to 450 mV more oxidizing than the native [4Fe-4S] clusters, with Em values ranging from -98 mV to -171 mV. A C14D/C51D double mutant contains [3Fe-4S] and S = 1/2 [4Fe-4S] clusters, showing that the 3Cys.1Asp motif is also able to accommodate a low spin cubane. When C34S, C34A, C14S, C51S, C14A, and C51A were rebound to P700-FX cores, electron transfer to FA/FB was regained, but functional reconstitution has not yet been achieved for C21D, C58D, or C14D/C51D. These data imply that PsaC requires two iron-sulfur clusters to refold, one of which must be a cubane. Since two [4Fe-4S] clusters are found in all reconstituted PS I complexes, the presence of two cubanes in free PsaC may be a necessary precondition for binding to P700-FX cores.

Amino acid sequences of heterotrophic and photosynthetic ferredoxins from the tomato plant (Lycopersicon esculentum Mill.)

Several forms (isoproteins) of ferredoxin in roots, leaves, and green and red pericarps in tomato plants (Lycopersicon esculentum Mill.) were earlier identified on the basis of N-terminal amino acid sequence and chromatographic behavior (Green et al. 1991). In the present study, a large scale preparation made possible determination of the full length amino acid sequence of the two ferredoxins from leaves. The ferredoxins characteristic of fruit and root were sequenced from the amino terminus to the 30th residue or beyond. The leaf ferredoxins were confirmed to be expressed in pericarp of both green and red fruit. The ferredoxins characteristic of fruit and root appeared to be restricted to those tissue. The results extend earlier findings in demonstrating that ferredoxin occurs in the major organs of the tomato plant where it appears to function irrespective of photosynthetic competence.

A mixed-ligand iron-sulfur cluster (C556SPaB or C565SPsaB) in the Fx-binding site leads to a decreased quantum efficiency of electron transfer in photosystem I

The proposed structure of Photosystem I depicts two cysteines on the PsaA polypeptide and two cysteines on the PsaB polypeptide in a symmetrical environment, each providing ligands for the interpolypeptide Fx cluster. We studied the role of Fx in electron transfer by substituting serine for cysteine (C565SPsaB and C556SPsaB), thereby introducing the first example of a genetically engineered, mixed-ligand [4Fe-4S] cluster into a protein. Optical kinetic spectroscopy shows that after a single-turnover flash at 298 K, the contribution of A1- (lifetime of 10 microseconds, 40% of total and lifetime of 100 microseconds, 20% of total) and Fx- (lifetime of 500-800 microseconds, 10-15% of total) to the overall P700+ back reaction have increased in C565SPsaB and C556SPsaB at the expense of the back reaction from [FA/FB]-. The electron paramagnetic resonance spectrum of Fx shows g-values of 2.04, 1.94, and 1.81 in both mutants and a similarly decreased amount of FA and FB reduced at 15 K after a single-turnover flash. These results indicate that the mixed-ligand (3 cysteines, 1 serine) Fx cluster is an inefficient electron carrier, but that a small leak through Fx still permits FA and FB to be reduced quantitatively when the samples are frozen during continuous illumination. The data confirm that Fx is a necessary intermediate in the electron transfer pathway from A1 to FA and FB in Photosystem I.

Effect of cysteine to serine mutations on the properties of the [4Fe-4S] center in Escherichia coli fumarate reductase

Site-directed mutants of Escherichia coli fumarate reductase in which FrdB Cys148, Cys151, Cys154, and Cys158 are replaced individually by Ser have been constructed and overexpressed in a strain of E. coli lacking a wild-type copy of fumarate reductase and succinate dehydrogenase. The consequences of these mutations on bacterial growth, enzymatic activity, and the EPR properties of the constituent iron-sulfur clusters have been investigated. The Cys154Ser and Cys158Ser FrdB mutations result in enzymes with negligible activity that have largely dissociated from the cytoplasmic membrane and consequently are incapable of supporting cell growth under conditions requiring a functional fumarate reductase. EPR studies indicate that these effects are associated with loss of both the [3Fe-4S] and [4Fe-4S] clusters. In contrast the Cys148Ser and Cys151Ser FrdB mutations result in functional membrane bound enzymes that are able to support growth under anaerobic and aerobic conditions. EPR studies of these mutants indicate that all three of the constituent Fe-S clusters are assembled, and the redox and spectroscopic properties of the [2Fe-2S] and [3Fe-4S] clusters are unchanged compared to the wild-type enzyme. In both mutants the [4Fe-4S] cluster is assembled with one non-cysteinyl ligand, and the available data suggest serinate coordination. The physicochemical consequences are perturbation of the intercluster spin interaction between the S = 1/2 [4Fe-4S]+ and S = 2 [3Fe-FS]0 clusters and a 60-mV decrease in redox potential for the [4Fe-FS]2+,+ cluster in the FrdB Cys148Ser mutant, and a S = 1/2 to S = 3/2 spin state conversion for the [4Fe-4S]+ cluster and a 72-mV decrease in redox potential for the [4Fe-4S]2+,+ cluster in the FrdB Cys151Ser mutant. Taken together with the previous FrdB Cys to Ser mutagenesis results [Werth, M. T., Cecchini, G., Manodori, A., Ackrell, B. A. C., Schröder, I., Gunsalus, R. P., & Johnson, M. K. (1990) Proc. Natl. Acad. Sci. U.S.A. 87, 8965-8969; Manodori, A., Cecchini, G., Schröder, I., Gunsalus, R. P., Werth, M. T., & Johnson, M. K. (1992) Biochemistry 31, 2703-2712], the results provide strong support for the proposal that all three clusters are located in the FrdB subunit with Cys57, Cys62, Cys65, and Cys77 ligating the [2Fe-2S] cluster, Cys148, Cys151, Cys154, and Cys214 ligating the [4Fe-4S] cluster, and Cys158, Cys204, and Cys210 ligating the [3Fe-4S] cluster. The role of the low potential [4Fe-4S] cluster in mediating electron transfer from menaquinol to the FAD active site is discussed in light of these mutagenesis results.

Direct electrochemistry and EPR spectroscopy of spinach ferredoxin mutants with modified electron transfer properties

Mutations of the conserved residue Glu-92 to lysine, glutamine, and alanine have been performed in the recombinant ferredoxin I of spinach leaves. The purified ferredoxin mutants were found twice as active with respect to wild-type protein in the NADPH-cytochrome c reductase reaction catalyzed by ferredoxin-NADP+ reductase in the presence of ferredoxin. Cyclic voltammetry and EPR measurements showed that the mutations cause a change in the [2Fe-2S] cluster geometry, whose redox potential becomes approximately 80 mV less negative. These data point to a role of the Glu-92 side-chain in determining the low redox potential typical of the [2Fe-2S] cluster of chloroplast and cyanobacterial ferredoxins. Also a ferredoxin/ferredoxin-NADP+ reductase chimeric protein obtained by gene fusion was overproduced in Escherichia coli and purified. Fusion of the ferredoxin with its reductase causes only minor effects to the iron-sulfur cluster, as judged by cyclic voltammetry and EPR measurements.

Structure-function relationships in the ferredoxin/ferredoxin: NADP+ reductase system from Anabaena

We have used a combination of laser flash photolysis time-resolved spectrophotometry and site-specific mutagenesis of surface amino acid residues to investigate the structural factors which influence electron transfer from Anabaena ferredoxin to its physiological partner ferredoxin-NADP+ reductase. Two ferredoxin residues (E94 and F65) are found to be highly critical interaction sites, whereas other nearby residues are found to be either inconsequential or to have only moderate effects. Basic residues near the N-terminus of the reductase are also found to exert a significant influence on interprotein electron transfer. The mechanistic implications of these results are discussed.

Mutations of surface residues in Anabaena vegetative and heterocyst ferredoxin that affect thermodynamic stability as determined by guanidine hydrochloride denaturation

The stability properties of oxidized wild-type (wt) and site-directed mutants in surface residues of vegetative (Vfd) and heterocyst (Hfd) ferredoxins from Anabaena 7120 have been characterized by guanidine hydrochloride (Gdn-HCl) denaturation. For Vfd it was found that mutants E95K, E94Q, F65Y, F65W, and T48A are quite similar to wt in stability. E94K is somewhat less stable, whereas E94D, F65A, F65I, R42A, and R42H are substantially less stable than wt. R42H is a substitution found in all Hfds, and NMR comparison of the Anabaena 7120 Vfd and Hfd showed the latter to be much less stable on the basis of hydrogen exchange rates (Chae YK, Abildgaard F, Mooberry ES, Markley JL, 1994, Biochemistry 33:3287-3295); we also find this to be true with respect to Gdn-HCl denaturation. Strikingly, the Hfd mutant H42R is more stable than the wt Hfd by precisely the amount of stability lost in Vfd upon mutating R42 to H (2.0 kcal/mol). On the basis of comparison of the X-ray crystal structures of wt Anabaena Vfd and Hfd, the decreased stabilities of F65A and F65I can be ascribed to increased solvent exposure of interior hydrophobic groups. In the case of Vfd mutants E94K and E94D, the decreased stabilities may result from disruption of a hydrogen bond between the E94 and S47 side chains. The instability of the R42 mutants is also most probably due to decreased hydrogen bonding capabilities.(ABSTRACT TRUNCATED AT 250 WORDS)