Effects of charged amino-acid mutation on the solution structure of cytochrome b5 and binding between cytochrome b5 and cytochrome c

The solution structure of oxidized bovine microsomal cytochrome b(5) mutant (E48, E56/A, D60/A) has been determined through 1524 meaningful nuclear Overhauser effect constraints together with 190 pseudocontact shift constraints. The final family of 35 conformers has rmsd values with respect to the mean structure of 0.045+/-0.009 nm and 0.088+/-0.011 nm for backbone and heavy atoms, respectively. A characteristic of this mutant is that of having no significant changes in the whole folding and secondary structure compared with the X-ray and solution structures of wild-type cytochrome b(5). The binding of different surface mutants of cytochrome b(5) with cytochrome c shows that electrostatic interactions play an important role in maintaining the stability and specificity of the protein complex formed. The differences in association constants demonstrate the electrostatic contributions of cytochrome b(5) surface negatively charged residues, which were suggested to be involved in complex formation in the Northrup and Salemme models, have cumulative effect on the stability of cyt c-cyt b(5) complex, and the contribution of Glu48 is a little higher than that of Glu44. Moreover, our result suggests that the docking geometry proposed by Northrup, which is involved in the participation of Glu48, Glu56, Asp60, and heme propionate of cytochrome b(5), do occur in the association between cytochrome b(5) and cytochrome c.     

Polymer-induced phase separation in aqueous micellar solutions of octyl-beta-D-thioglucoside for extraction of membrane proteins

A water-soluble polymer such as polyethylene glycol (PEG), Dextran T-500 (Dx), or diethylaminoethyl-Dextran (DEAE-Dx) induced aqueous micellar solutions of octyl-beta-D-thioglucoside (OTG) to phase separation at 0 degrees C. One of the two phases thus formed is a surfactant-depleted aqueous solution (aqueous phase) of a water-soluble polymer and the other a concentrated OTG solution (surfactant-rich phase). In a combination of OTG with PEG or Dx, cytochrome P450 (P450) and cytochrome b(5) (b(5)) were well extracted into the surfactant-rich phase. The extraction yield of P450 was slightly greater than that of b(5). In contrast to PEG and Dx, DEAE-Dx markedly reduced the extraction of b(5), while that of P450 remained almost unchanged. DEAE-Dx served the dual functions of inducing the phase separation and preventing the extraction of b(5) into the surfactant-rich phase. This depressed extraction of b(5) was reversed by the addition of potassium phosphate. DEAE-Dx and potassium phosphate proved effective in controlling the extractability of b(5). The polymer-induced phase separation provides a new basis for highly efficient extraction of membrane proteins under mild conditions that should be acceptable for thermolabile membrane proteins under physiological conditions. (c) 1997 John Wiley & Sons, Inc. Biotechnol Bioeng 56: 311-318, 1997.     

Surface interactions in the complex between cytochrome f and the E43Q/D44N and E59K/E60Q plastocyanin double mutants as determined by 1H-NMR chemical shift analysis

A combination of site-directed mutagenesis and NMR chemical shift perturbation analysis of backbone and side-chain protons has been used to characterize the transient complex of the photosynthetic redox proteins plastocyanin and cytochrome f. To elucidate the importance of charged residues on complex formation, the complex of cytochrome f and E43Q/D44N or E59K/E60Q spinach plastocyanin double mutants was studied by full analysis of the (1)H chemical shifts by use of two-dimensional homonuclear NMR spectra. Both mutants show a significant overall decrease in chemical shift perturbations compared with wild-type plastocyanin, in agreement with a large decrease in binding affinity. Qualitatively, the E43Q/D44N mutant showed a similar interaction surface as wild-type plastocyanin. The interaction surface in the E59K/E60Q mutant was distinctly different from wild type. It is concluded that all four charged residues contribute to the affinity and that residues E59 and E60 have an additional role in fine tuning the orientation of the proteins in the complex.     

Models for the complexes formed between cytochrome b5 and the subunits of methemoglobin

Computer graphics-generated models for the electron transfer complexes formed between cytochrome b5 and the subunits of methemoglobin are proposed. For both complexes, the orientation allowing optimal hydrogen bonding involves interaction between negatively charged residues on cytochrome b5 and positively charged residues on methemoglobin. In each complex, the heme groups of the interacting species are coplanar with the edges of the heme groups separated by 7-8 A and with the iron atoms 16 A apart. For the alpha-chain X cytochrome b5 complex, alpha-chain residues 56 (Lys), 60 (Lys), and 90 (Lys) interact with cytochrome b5 residues 44 (Glu), 43 (Glu), and 60 (Asp) respectively. A fourth hydrogen bond involves alpha-61 (Lys) bridging between a heme propionate from cytochrome b5 and a heme propionate from the alpha-chain. The contacts present in the beta-chain X cytochrome b5 complex involve hydrogen-bonding between beta-chain lysyl residues 59, 61, 65, and 95, and cytochrome b5 residues 48 (Glu), 44 (Glu), 43 (Glu), and 60 (Asp) respectively. An additional hydrogen bond can be formed by bridging of the epsilon-amino group of beta-66 (Lys) between a heme propionate from cytochrome b5 and a beta-chain heme propionate. In each complex, two nonionic interactions, one on each side of the heme groups, are also suggested. These interactions appear to effectively exclude external water molecules from the center of the protein-protein interaction domain. Comparison of the proposed binding loci for cytochrome b5 on the methemoglobin subunits with those proposed on cytochrome c reveals considerable structural homology between the cytochrome b5 binding sites.     

Electrostatic properties of cytochrome f: implications for docking with plastocyanin.

The electrostatic properties of cytochrome f (cyt f), a member of the cytochrome b6f complex and reaction partner with plastocyanin (PC) in photosynthetic electron transport, are qualitatively studied with the goal of determining the mechanism of electron transfer between cyt f and PC. A crystal structure for cyt f was analyzed with the software package GRASP, revealing a large region of positive potential generated by a patch of positively charged residues (including K58, K65, K66, K122, K185, K187, and R209) and reinforced by the iron center of the heme. This positive field attracts the negative charges of the two acidic patches on the mobile electron carrier PC. Three docked complexes are obtained for the two proteins, based on electrostatic or hydrophobic interactions or both and on steric fits by manual docking methods. The first of these three complexes shows strong electrostatic interactions between K187 on cyt f and D44 on PC and between E59 on PC and K58 on cyt f. Two other manually docked complexes are proposed, implicating H87 on PC as the electron-accepting site from the iron center of cyt f through Y1. The second complex maintains the D44/K187 cross-link (but not the E59/K58 link) while increasing hydrophobic interactions between PC and cyt f. Hydrophobic interactions are increased still further in the third complex, whereas the link between K187 on cyt f and D44 on PC is broken. The proposed reaction mechanism, therefore, involves an initial electrostatic docking complex that gives rise to a nonpolar attraction between the regions surrounding H87 on PC and Y1 on cyt f, providing for an electron-transfer active complex.     

The identification of cation-binding domains on the surface of microsomal cytochrome b5 using 1H-NMR paramagnetic difference spectroscopy.

One-dimensional and two-dimensional 1H-NMR methods and paramagnetic difference spectroscopy have defined cation binding domains on the surface of the tryptic fragment of microsomal cytochrome b5. The addition of tris(ethylenediamine) chromium(III) [Cr(en)3(3+)] to solutions of ferricytochrome b5 reveals at least three distinct sites on the surface of the protein to which highly charged cations may bind (20 mM phosphate pH 7.0, T = 300 K). Surprisingly only one of these sites is located close to the haem edge region of the protein, whilst the remaining two sites are more remote. Site I contains the exposed haem C13 propionate and a series of carboxylate residues that includes glutamates 37, 38, 43, 44, and 48. Sites II and III are located away from the haem edge region and are delineated by the broadening of aromatic resonances of histidines 26 and 80. Further investigation of the interaction between Cr(en)3(3+) and cytochrome b5 using two-dimensional double-quantum-filtered correlated spectroscopy shows that resonances assigned to Glu59, Asp60, Glu79, Asp82 and Asp83 are broadened with the distribution of these charged side chains correlating with the relaxation broadening observed from one-dimensional experiments. In a binary complex with ferricytochrome c, Cr(en3(3+) broadens many cytochrome b45 resonances including the haem propionates, His26, Ala54, Thr55 and His80. Although the pattern of line-broadening of resonances at sites II and III is unaltered by complex formation, cytochrome c shields residues at site I, the haem edge site. The results indicate that the interaction between cytochrome b5 and c in a binary complex involves multiple protein configurations.     

Identification of the interactions between cytochrome P450 2E1 and cytochrome b5 by mass spectrometry and site-directed mutagenesis

The reaction cycles of cytochrome P450s (P450) require input of two electrons. Electrostatic interactions are considered important driving forces in the association of P450s with their redox partners, which in turn facilitates the transfer of the two electrons. In this study, the cross-linking reagent, 1-ethyl-3-[3-dimethylaminopropyl]carbodiimide hydrochloride (EDC), was used to covalently link cytochrome P450 2E1 (CYP2E1) with cytochrome b(5) (b(5)) through the formation of specific amide bonds between complementary charged residue pairs. Cross-linked peptides in the resulting protein complex were distinguished from non-cross-linked peptides using an (18)O-labeling method on the basis that cross-linked peptides incorporate twice as many (18)O atoms as non-cross-linked peptides during proteolysis conducted in (18)O-water. Subsequent tandem mass spectrometric (MS/MS) analysis of the selected cross-linked peptide candidates led to the identification of two intermolecular cross-links, Lys(428)(CYP2E1)-Asp(53)(b(5)) and Lys(434)(CYP2E1)-Glu(56)(b(5)), which provides the first direct experimental evidence for the interacting orientations of a microsomal P450 and its redox partner. The biological importance of the two ion pairs for the CYP2E1-b(5) interaction, and the stimulatory effect of b(5), was confirmed by site-directed mutagenesis. Based on the characterized cross-links, a CYP2E1-b(5) complex model was constructed, leading to improved insights into the protein interaction. The described method is potentially useful for mapping the interactions of various P450 isoforms and their redox partners, because the method is relatively rapid and sensitive, and is capable of suggesting not only protein interacting regions, but also interacting orientations.     

Mapping the electron transfer interface between cytochrome b5 and cytochrome c

To characterize the cytochrome b(5) (Cyt b(5))-cytochrome c (Cyt c) interactions during electron transfer, variants of Cyt b(5) have been employed to assess the contributions of electrostatic interactions (substitution of surface charged residues Glu44, Glu48, Glu56, and Asp60 and heme propionate), hydrophobic interactions, and the thermodynamic driving forces (substitutions for hydrophobic residues in heme pocket residues Phe35, Pro40, Val45, Phe58, and Val61). The electrostatic interactions play an important role in maintaining the stability and specificity of the Cyt b(5)-Cyt c complex that is formed. There is no essential effect on the intraprotein complex electron transfer even if most of the involved negatively charged residues on the surface of Cyt b(5) have been removed. The results support a dynamic docking paradigm for Cyt b(5)-Cyt c interactions. The orientation that is optimal for binding may not be optimal form for electron transfer. Substitution of hydrophobic residues does not have a significant effect on the binding between Cyt b(5) and Cyt c; rather, it regulates the electron transfer rates via changes in the driving force. Combining the electron transfer studies of the Cyt b(5)-Cyt c system and the Cyt b(5)-Zn-Cyt c system, we obtain the reorganization energy (0.6 eV) at an ionic strength of 150 mM.     

Rational design of viscosity reducing mutants of a monoclonal antibody: Hydrophobic versus electrostatic inter-molecular interactions

High viscosity of monoclonal antibody formulations at concentrations ≥100 mg/mL can impede their development as products suitable for subcutaneous delivery. The effects of hydrophobic and electrostatic intermolecular interactions on the solution behavior of MAB 1, which becomes unacceptably viscous at high concentrations, was studied by testing 5 single point mutants. The mutations were designed to reduce viscosity by disrupting either an aggregation prone region (APR), which also participates in 2 hydrophobic surface patches, or a negatively charged surface patch in the variable region. The disruption of an APR that lies at the interface of light and heavy chain variable domains, V_H and V_L, via L45K mutation destabilized MAB 1 and abolished antigen binding. However, mutation at the preceding residue (V44K), which also lies in the same APR, increased apparent solubility and reduced viscosity of MAB 1 without sacrificing antigen binding or thermal stability. Neutralizing the negatively charged surface patch (E59Y) also increased apparent solubility and reduced viscosity of MAB 1, but charge reversal at the same position (E59K/R) caused destabilization, decreased solubility and led to difficulties in sample manipulation that precluded their viscosity measurements at high concentrations. Both V44K and E59Y mutations showed similar increase in apparent solubility. However, the viscosity profile of E59Y was considerably better than that of the V44K, providing evidence that inter-molecular interactions in MAB 1 are electrostatically driven. In conclusion, neutralizing negatively charged surface patches may be more beneficial toward reducing viscosity of highly concentrated antibody solutions than charge reversal or aggregation prone motif disruption.     

Rational design of viscosity reducing mutants of a monoclonal antibody: Hydrophobic versus electrostatic inter-molecular interactions

High viscosity of monoclonal antibody formulations at concentrations ≥100 mg/mL can impede their development as products suitable for subcutaneous delivery. The effects of hydrophobic and electrostatic intermolecular interactions on the solution behavior of MAB 1, which becomes unacceptably viscous at high concentrations, was studied by testing 5 single point mutants. The mutations were designed to reduce viscosity by disrupting either an aggregation prone region (APR), which also participates in 2 hydrophobic surface patches, or a negatively charged surface patch in the variable region. The disruption of an APR that lies at the interface of light and heavy chain variable domains, V_H and V_L, via L45K mutation destabilized MAB 1 and abolished antigen binding. However, mutation at the preceding residue (V44K), which also lies in the same APR, increased apparent solubility and reduced viscosity of MAB 1 without sacrificing antigen binding or thermal stability. Neutralizing the negatively charged surface patch (E59Y) also increased apparent solubility and reduced viscosity of MAB 1, but charge reversal at the same position (E59K/R) caused destabilization, decreased solubility and led to difficulties in sample manipulation that precluded their viscosity measurements at high concentrations. Both V44K and E59Y mutations showed similar increase in apparent solubility. However, the viscosity profile of E59Y was considerably better than that of the V44K, providing evidence that inter-molecular interactions in MAB 1 are electrostatically driven. In conclusion, neutralizing negatively charged surface patches may be more beneficial toward reducing viscosity of highly concentrated antibody solutions than charge reversal or aggregation prone motif disruption.     

Myoglobin and cytochrome b5: a nuclear magnetic resonance study of a highly dynamic protein complex

The transient complex of bovine myoglobin and cytochrome b(5) has been investigated using a combination of NMR chemical shift mapping, (15)N relaxation data, and protein docking simulations. Chemical shift perturbations observed for cytochrome b(5) amide resonances upon complex formation with either metmyoglobin (Fe(III)) or carbon monoxide-bound myoglobin (Fe(II)) are more than 10-fold smaller than in other transient redox protein complexes. From (15)N relaxation experiments, an increase in the overall correlation time of cytochrome b(5) in the presence of myoglobin is observed, confirming that complex formation is occurring. The chemical shift perturbations of proton and nitrogen amide nuclei as well as heme protons of cytochrome b(5) titrate with increasing myoglobin concentrations, also demonstrating the formation of a weak complex with a K(a) in the inverse millimolar range. The perturbed residues map over a wide surface area of cytochrome b(5), with patches of residues located around the exposed heme 6-propionate as well as at the back of the protein. The nature of the affected residues is mostly negatively charged contrary to perturbed residues in other transient complexes, which are mainly hydrophobic or polar. Protein docking simulations using the NMR data as constraints show several docking geometries both close to and far away from the exposed heme propionates of myoglobin. Overall, the data support the emerging view that this complex consists of a dynamic ensemble of orientations in which each protein constantly diffuses over the surface of the other. The characteristic NMR features may serve as a structural tool for the identification of such dynamic complexes.     

Time-resolved fluorescence and circular dichroism of porphyrin cytochrome c and Zn-porphyrin cytochrome c incorporated in reversed micelles

Interactions between fluorescent horse heart cytochrome c derivatives (e. g. porphyrin cytochrome c and Zn-porphyrin cytochrome c) with surfactant interfaces in reversed micellar solutions have been studied, using different spectroscopic techniques. Anionic [sodium bis(2-ethylhexyl)sulfosuccinate, AOT] and cationic (cetyltrime-thylammonium bromide, CTAB) surfactant solutions have been used in order to investigate the effects of charge interactions between proteins and interfaces. Circular dichroism reveals that much of the protein secondary structure is lost in AOT-reversed micelles, especially when the molar water/surfactant ratio, wo, is high (wo = 40), whereas in CTAB-reversed micelles secondary structure seems to be preserved. Time-resolved fluorescence measurements of the porphyrin in the cytochrome c molecule yields information about the changes in structure and the dynamics of the protein upon interaction with surfactant assemblies both in aqueous and in hydrocarbon solutions. With AOT as surfactant a strong interaction between protein and interface can be observed. The effects found in aqueous AOT solution are of the same kind as in hydrocarbon solution. In the CTAB systems the interactions between protein and surfactant are much less pronounced. The measured effects on the fluorescence properties of the proteins are different in aqueous and hydrocarbon solutions. In general, the observations can be explained by an electrostatic attraction between the overall positively charged protein molecules and the anionic AOT interface. Electrostatic attraction can also occur between the cytochrome c derivatives and CTAB because there is a negatively charged zone on the surface of the proteins. From the fluorescence anisotropy decays it can be concluded that in the CTAB-reversed micellar system these interactions are not important, whereas in an aqueous CTAB solution the proteins interact with surfactant molecules.     

Phospholipid membranes affect tertiary structure of the soluble cytochrome b5 heme-binding domain

The influence of charged phospholipid membranes on the conformational state of the water-soluble fragment of cytochrome b5 has been investigated by a variety of techniques at neutral pH. The results of this work provide the first evidence that aqueous solutions with high phospholipid/protein molar ratios (pH 7.2) induce the cytochrome to undergo a structural transition from the native conformation to an intermediate state with molten-globule like properties that occur in the presence of an artificial membrane surface and that leads to binding of the protein to the membrane. At other phospholipid/protein ratios, equilibrium was observed between cytochrome free in solution and cytochrome bound to the surface of vesicles. Inhibition of protein binding to the vesicles with increasing ionic strength indicated for the most part an electrostatic contribution to the stability of cytochrome b5-vesicle interactions at pH 7.2. The possible physiological role of membrane-induced conformational change in the structure of cytochrome b5 upon the interaction with its redox partners is discussed.     

Evidence for the intertwined dimer of the cytochrome bc(1) complex in solution

To confirm that the cytochrome bc(1) complex exists as a dimer with intertwining Rieske iron-sulfur proteins in solution, four Rhodobacter sphaeroides mutants expressing His-tagged cytochrome bc(1) complexes containing two pairs of cysteine substitutions, one in the interface between the head domain of iron-sulfur protein (ISP) and cytochrome b and the other between the tail domain of ISP and cytochrome b, were generated and characterized. They are: K70C(ISP)/A185C(cytb).P33C(ISP)/G89C(cytb), K70C(ISP)/A185C(cytb).P33C(ISP)/M92C (cytb), K70C (ISP)/A185C(cytb).L34C(ISP)/V64C(cytb), and K70C(ISP)/A185C(cytb).N36C(ISP)/G89C(cytb). The K70C(ISP)/A185C(cytb) cysteine pair cross-links the head domain of ISP and cytochrome b; the P33C(ISP)/G89C(cytb), P33C(ISP)/M92C (cytb), L34C(ISP)/V64C(cytb), and N36C(ISP)/G89C(cytb) cysteine pairs cross-link the tail domain of ISP and cytochrome b. An adduct protein with an apparent molecular mass of 128 kDa containing two cytochrome b and two ISP proteins is detected in the K70C(ISP)/A185C(cytb).P33C(ISP)/G89C(cytb) and K70C(ISP)/A185C(cytb).N36C(ISP)/G89C(cytb) mutant complexes, confirming that the bc(1) complex exists as a dimer with intertwining ISPs. The loss of activity in these two double-cysteine-pair mutant complexes was attributed to the disulfide bond between the head domain of ISP and cytochrome b and not the one between the tail domain of ISP and cytochrome b.     

Solution structure of cytochrome b(5) mutant (E44/48/56A/D60A) and its interaction with cytochrome c.

Using 1617 meaningful NOEs with 188 pseudocontact shifts, a family of 35 conformers of oxidized bovine microsomal cytochrome b5 mutant (E44/48/56A/D60A) has been obtained and is characterized by good resolution (rmsd to the mean structure are 0.047 +/- 0.007 nm and 0.095 +/- 0.008 nm for backbone and heavy atoms, respectively). The solution structure of the mutant, when compared with the X-ray structure of wild-type cytochrome b(5), has no significant changes in the whole folding and secondary structure. The binding between cytochrome b(5) and cytochrome c shows that the association constant of the mutant-cytochrome c complex is much lower than the one for wild-type complex (2.2 x 10(4) M(-1) vs. 5.1 x 10(3) M(-1)). The result suggests the four acidic residues have substantial effects on the formation of the complex between cytochrome b(5) and cytochrome c, and therefore it is concluded reasonably that the electrostatic interaction plays an important role in maintaining the stability and specificity of the complex formed. The competition between the ferricytochrome b(5) mutant and [Cr(oxalate)(3)](3-) for ferricytochrome c shows that site III of cytochrome c, which is a strong binding site to wild-type cytochrome b(5), still binds to the mutant with relatively weaker strength. Our results indicate that certain bonding geometries do occur in the interaction between the present mutant and cytochrome c and these geometries, which should be quite different from the ones of the Salemme and Northrup models.     

NMR studies of the association of cytochrome b5 with cytochrome c

In an effort to gain greater insight into the molecular mechanism of the electron-transfer reactions of cytochrome b(5), the bovine cytochrome b(5)-horse cytochrome c complex has been investigated by high-resolution multidimensional NMR spectroscopy using (13)C, (15)N-labeled cytochrome b(5) expressed from a synthetic gene. Chemical shifts of the backbone (15)N, (1)H, and (13)C resonances for 81 of the 82 residues of [U-90% (13)C,U-90% (15)N]-ferrous cytochrome b(5) in a 1:1 complex with ferrous cytochrome c were compared with those of ferrous cytochrome b(5) in the absence of cytochrome c. A total of 51% of these residues showed small, but significant, changes in chemical shifts (the largest shifts were 0.1 ppm for the amide (1)H, 1.15 for (13)C(alpha), 1.03 ppm for the amide (15)N, and 0.15 ppm for the (1)H(alpha) resonances). Some of the residues exhibiting chemical shift changes are located in a region that has been implicated as the binding surface to cyt c [Salemme, F. R. (1976) J. Mol. Biol. 10, 563-568]. Surprisingly, many of the residues with changes are not located on this surface. Instead, they are located within and around a cleft observed to form in a molecular dynamics study of cytochrome b(5) [Storch, E. M., and Daggett, V. (1995) Biochemistry 34, 9682-9693](.) The rim of this cleft can readily accommodate cytochrome c. Molecular dynamics simulations of the Salemme and cleft complexes were performed for 2 ns and both complexes were stable.     

Solubilizing water involved in protein extraction using reversed micelles

The extraction of protein using reversed micelles was investigated in relation to the amount of solubilizing water in the reversed micellar organic phase. The minimal concentration of amphiphilic molecule di-2-ethylhexyl sodium sulfosuccinate (C(20)H(37)O(7)Na) (AOT) required for 100% cytochrome c extraction was recognized. This critical AOT concentration increased with protein concentration in the aqueous phase. On this minimal AOT condition, the molar ratio of solubilizing water to extracted protein was found to be a constant of 3500 under C(KCI) = 1.0 x 10(2) mol . m(-3) in this system. This ratio means the hydrophillic surroundings required for extracting one protein molecule into the micellar organic phase under the suitable pH and salt concentration for the forward extraction. In this regard, AOT molecules seemed to take the part of water solubilizing agent in the reversed micellar extraction. This role of AOT is important to extract protein under the suitable pH and salt concentration. The amount of solubilizing water in the protein-containing system was larger than in the protein-free system. This difference shows that the water molecules accompany the extracted protein into the reversed micellar organic phase at constant ratio 2200 under C(KCI) = 1.0 x 10(2) mol . m(-3), i.e., accompanying water molecules per one extracted protein. The minimal AOT concentration increased with ionic strength. On this minimal AOT condition, the molar ratio of solubilizing water to extracted protein also increased with ionic strength, so that in higher ionic strength, more solubilizing water was required. Then more AOT was required to provide the hydrophillic surroundings for protein. The pH affected the minimal AOT concentration required for 100% protein extraction.     

Conformational states of the water-soluble fragment of cytochrome b5. I. pH-induced denaturation

Cytochrome b5 is a membrane protein that comprises two fragments: one is water-soluble and heme-containing, and the other is hydrophobic and membrane-embedded. The function of electron transfer is performed by the former whose crystal structure is known; however, its conformational states when in the membrane field and interacting with other proteins are still to be studied. Previously, we proposed water-alcohol mixtures for modeling the effect of membrane surface on proteins, and used this approach to study the conformational behavior of positively charged cytochrome c as well as relatively neutral retinol-binding protein also functioning in the field of negatively charged membrane. The current study describes the conformational behavior of the negatively charged water-soluble fragment of cytochrome b5 as dependent on pH. Decreasing pH was shown to transform the fragment state from native to intermediate, similar to the molten globule reported earlier for other proteins in aqueous solutions: at pH 3.0, the fragment preserved a pronounced secondary structure and compactness but lost its rigid tertiary structure. A possible role of this intermediate state in cytochrome b5 functioning is discussed.     

Mutational and structural analysis of the nitrate reductase heme domain of Nicotiana plumbaginifolia.

We have analyzed four Nicotiana plumbaginifolia null mutants presumably affected in the heme domain of nitrate reductase. The DNA sequence of this domain has been determined for each mutant and for the wild type. Two mutations were identified as single base changes leading to, respectively, the substitution of a histidine residue by an asparagine (mutant E56) and to the appearance of an ochre stop codon (mutant E64). Based on the amino acid sequence homology between the nitrate reductase heme domain and mammalian cytochrome b5, we have predicted the three-dimensional structure of this domain. This showed that the nitrate reductase heme domain is structurally very similar to cytochrome b5 and it also confirmed that the residue involved in E56 mutation is one of the two heme-binding histidines. The two other mutations (mutants A1 and K21) were found to be, respectively, -1 and +1 frameshift mutations resulting in the appearance of an opal stop codon. These sequence data confirmed previous genetic and biochemical hypotheses on nitrate reductase-deficient mutants. Northern blot analysis of these mutants indicated that mutant E56 overexpressed the nitrate reductase mRNA, whereas the nonsense mutations present in the other mutants led to reduced levels of nitrate reductase mRNA.