Structure of an electron transfer complex. I. Covalent cross-linking of cytochrome c peroxidase and cytochrome c

Solvent Isotope Effects on Interfacial Protein Electron Transfer in Crystals and Electrode Films

D(2)O-grown crystals of yeast zinc porphyrin substituted cytochrome c peroxidase (ZnCcP) in complex with yeast iso-1-cytochrome c (yCc) diffract to higher resolution (1.7 A) and pack differently than H(2)O-grown crystals (2.4-3.0 A). Two ZnCcP's bind the same yCc (porphyrin-to-porphyrin separations of 19 and 29 A), with one ZnCcP interacting through the same interface found in the H(2)O crystals. The triplet excited-state of at least one of the two unique ZnCcP's is quenched by electron transfer (ET) to Fe(III)yCc (k(e) = 220 s(-1)). Measurement of thermal recombination ET between Fe(II)yCc and ZnCcP+ in the D(2)O-treated crystals has both slow and fast components that differ by 2 orders of magnitude (k(eb)(1) = 2200 s(-1), k(eb)(2) = 30 s(-1)). Back ET in H(2)O-grown crystals is too fast for observation, but soaking H(2)O-grown crystals in D(2)O for hours generates slower back ET, with kinetics similar to those of the D(2)O-grown crystals (k(eb)(1) = 7000 s(-1), k(eb)(2) = 100 s(-1)). Protein-film voltammetry of yCc adsorbed to mixed alkanethiol monolayers on gold electrodes shows slower ET for D(2)O-grown yCc films than for H(2)O-grown films (k(H) = 800 s(-1); k(D) = 540 s(-1) at 20 degrees C). Soaking H(2)O- or D(2)O-grown films in the counter solvent produces an immediate inverse isotope effect that diminishes over hours until the ET rate reaches that found in the counter solvent. Thus, D(2)O substitution perturbs interactions and ET between yCc and either CcP or electrode films. The effects derive from slow exchanging protons or solvent molecules that in the crystal produce only small structural changes.

Interactions between yeast iso-1-cytochrome c and its peroxidase

Isothermal titration calorimetry was used to study the formation of 19 complexes involving yeast iso-1-ferricytochrome c (Cc) and ferricytochrome c peroxidase (CcP). The complexes comprised combinations of the wild-type proteins, six CcP variants, and three Cc variants. Sixteen protein combinations were designed to probe the crystallographically defined interface between Cc and CcP. The data show that the high-affinity sites on Cc and CcP coincide with the crystallographically defined sites. Changing charged residues to alanine increases the enthalpy of complex formation by a constant amount, but the decrease in stability depends on the location of the amino acid substitution. Deleting methyl groups has a small effect on the binding enthalpy and a larger deleterious effect on the binding free energy, consistent with model studies of the hydrophobic effect, and showing that nonpolar interactions also stabilize the complex. Double-mutant cycles were used to determine the coupling energies for nine Cc-CcP residue pairs. Comparing these energies to the crystal structure of the complex leads to the conclusion that many of the substitutions induce a rearrangement of the complex.

The structure of an electron transfer complex containing a cytochrome c and a peroxidase

Efficient biological electron transfer may require a fluid association of redox partners. Two noncrystallographic methods (a new molecular docking program and 1H NMR spectroscopy) have been used to study the electron transfer complex formed between the cytochrome c peroxidase (CCP) of Paracoccus denitrificans and cytochromes c. For the natural redox partner, cytochrome c550, the results are consistent with a complex in which the heme of a single cytochrome lies above the exposed electron-transferring heme of the peroxidase. In contrast, two molecules of the nonphysiological but kinetically competent horse cytochrome bind between the two hemes of the peroxidase. These dramatically different patterns are consistent with a redox active surface on the peroxidase that may accommodate more than one cytochrome and allow lateral mobility.

Cytochrome c/cytochrome c peroxidase complex: effect of binding-site mutations on the thermodynamics of complex formation

The cytochrome c/cytochrome c peroxidase system has been extensively investigated as a model for long-range electron transfer in biology. Two models for the structure of the one-to-one cytochrome c/cytochrome c peroxidase complex in solution exist: one is based upon computer docking of the two proteins and the second is based upon the structure of the complex in the crystalline state. Titration calorimetry is used to investigate the interaction of horse ferricytochrome c with baker's yeast cytochrome c peroxidase and with six cytochrome c peroxidase mutants. Five of the six peroxidase mutants eliminate a negative charge in the cytochrome c binding site by replacing a side-chain carboxylate with an amide. The sixth mutation replaces a surface alanine residue with phenylalanine. The binding affinity between cytochrome c and the cytochrome c peroxidase mutants varies from no significant change in comparison to the wild-type enzyme to a 4-fold decrease in the equilibrium association constant. The pattern of decreasing cytochrome c binding affinity for the cytochrome c peroxidase mutants is consistent with the cytochrome c binding domain defined by X-ray crystallography [Pelletier, H., & Kraut, J. (1992) Science 258, 1748-1755]. For those mutants which have lower affinity for cytochrome c, the lower affinity is due to a decrease in the entropy change upon complex formation, consistent with the difference in hydration of carboxylate and amide groups.

Probing the cytochrome c peroxidase-cytochrome c electron transfer reaction using site specific cross-linking

Engineered cysteine residues in yeast cytochrome c peroxidase (CCP) and yeast iso-1-cytochrome c have been used to generate site specifically cross-linked peroxidase-cytochrome c complexes for the purpose of probing interaction domains and the intramolecular electron transfer reaction. Complex 2 was designed earlier [Pappa, H.S., & Poulos, T.L. (1995) Biochemistry 34, 6573-6580] to mimic the known crystal structure of the peroxidase-cytochrome c noncovalent complex [Pelletier, H., & Kraut, J. (1992) Science 258, 1748-1755]. Complex 3 was designed such that cytochrome c is tethered to a region of the peroxidase near Asp148 which has been suggested to be a second site of interaction between the peroxidase and cytochrome c. Using stopped flow methods, the rate at which the ferrocytochrome c covalently attached to the peroxidase transfers an electron to peroxidase compound I is estimated to be approximately 0.5-1 s-1 in complex 3 and approximately 800 s-1 in complex 2. In both complexes the Trp191 radical and not the Fe4+=O oxyferryl center of compound I is reduced. Conversion of Trp191 to Phe slows electron transfer about 10(3) in complex 2. Steady state kinetic measurements show that complex 3 behaves like the wild type enzyme when either horse heart or yeast ferrocytochrome c is used as an exogenous substrate, indicating that the region blocked in complex 3 is not a functionally important interaction site. In contrast, complex 2 is inactive toward horse heart ferrocytochrome c at all ionic strengths tested and yeast ferrocytochrome c at high ionic strengths. Only at low ionic strengths and low concentrations of yeast ferrocytochrome c does complex 2 give wild type enzyme activity. This observation indicates that in complex 2 the primary site of interaction of CCP with horse heart and yeast ferrocytochrome c at high ionic strengths is blocked. The relevance of these results to the pathway versus distance models of electron transfer and to the interaction domains between peroxidase and cytochrome c is discussed.

Species-specific differences in covalently crosslinked complexes of yeast cytochrome c peroxidase with horse and yeast iso-1 ferricytochromes c

1. The results of chemically crosslinking yeast cytochrome c peroxidase with both horse and yeast iso-1 ferricytochromes c have been studied by a combination of gel electrophoresis and proton NMR spectroscopy. 2. The complexes were formed at a variety of potassium phosphate concentrations ranging from 10 to 300 mM using the water soluble crosslinking agent, EDC (1-ethyl-3-[3-(dimethylamino)propyl]-carbodiimide). 3. The primary crosslinking product in both cases is the 1:1 covalent complex, but, for each pair of partner proteins the yield of the 1:1 crosslinked complex varies with the salt concentration. 4. Furthermore, at low salt concentrations the yield of the 1:1 covalent complex involving horse cytochrome c is much larger than the yield of the 1:1 covalent complex formed with yeast iso-1 cytochrome c, whereas at high salt concentrations the situation is reversed. 5. Proton NMR spectroscopy, in combination with gel electrophoresis, provides evidence for the formation of different types of 1:1 complexes for the peroxidase/yeast cytochrome c pair and has been used to study the effect of changes in the solution ionic strength upon both the peroxidases/horse cytochrome c and the peroxidase/yeast cytochrome c complexes. 6. This work indicates that electrostatic interactions between proteins play a dominant role in formation of complexes between cytochrome c peroxidase and horse ferricytochrome c, whereas the hydrophobic effect plays a comparatively larger role in stabilizing complexes between cytochrome c peroxidase and yeast iso-1 ferricytochrome c.

Identification of the site of interaction between cytochrome c3 and ferredoxin using peptide mapping of the cross-linked complex

Structural studies carried out on a cross-linked complex between cytochrome c3 and ferredoxin I, both isolated from Desulfovibrio desulfuricans Norway, allowed the identification of the site of interaction between the two redox proteins. Staphylococcus aureus proteinase and chymotrypsin digestions led to characterization of peptides containing both cytochrome c3 and ferredoxin sequences. The cytochrome c3 sequences involved in the three isolated cross-linked peptides contained several lysine residues localized around the heme 4 crevice. This analysis stressed the peculiar role of lysines 100, 101, 103, 104 and 113, which could be considered as major cross-link sites, as opposed to the lysines 75, 79 and 82, which could be considered as minor cross-link sites. One cross-linked peptide, containing two ferredoxin sequences joined to one cytochrome c3 sequence, had been isolated, suggesting the possibility of more than one cross-link per covalent complex. All these results led to the identification of heme 4 of cytochrome c3 as the site of interaction for the ferredoxin I. This study confirms the proposal that could be deduced from the hypothetical structure of the complex built by computer graphics modelling (Cambillau, C., Frey, M., Mosse, J., Guerlesquin, F. and Bruschi, M. (1988) Proteins: struct., funct. genet. 4, 63-70).

Comparison of the kinetics of reduction and intramolecular electron transfer in electrostatic and covalent complexes of ferredoxin-NADP+ reductase and flavodoxin from Anabaena PCC 7119

The kinetics of reduction and intracomplex electron transfer in electrostatically stabilized and covalently crosslinked complexes between ferredoxin-NADP+ reductase (FNR) and flavodoxin (Fld) from the cyanobacterium Anabaena PCC 7119 were compared using laser flash photolysis. The second-order rate constant for reduction by 5-deazariboflavin semiquinone (dRfH) of FNR within the electrostatically stabilized complex at 10 mM ionic strength (4.0 X 10(8) M-1 s-1) was identical to that for free FNR. This suggests that the FAD cofactor of FNR is not sterically hindered upon complex formation. A lower limit of approximately 7000 s-1 was estimated for the first-order rate constant for intracomplex electron transfer from FNRred to Fldox under these conditions. In contrast, for the covalently crosslinked complex, a smaller second-order rate constant (2.1 X 10(8) M-1 s-1) was obtained for the reduction of FNR by dRfH within the complex, suggesting that some steric hindrance of the FAD cofactor of FNR occurs due to crosslinking. A limiting rate constant of 1000 s-1 for the intracomplex electron transfer reaction was obtained for the covalent complex, which was unaffected by changes in ionic strength. The substantially diminished limiting rate constant, relative to that of the electrostatic complex, may reflect either a suboptimal orientation of the redox cofactors within the covalent complex or a required structural reorganization preceding electron transfer which is not allowed once the proteins have been covalently linked. Thus, although the covalent complex is biochemically competent, it is not a quantitatively precise model for the catalytically relevant intermediate along the reaction pathway.

Stability of water-soluble carbodiimides in aqueous solution

A dimethylbarbituric acid reagent has been used to follow the kinetics of loss of two water-soluble carbodiimides, 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide (EDC) and the structurally related 1-ethyl-3-(4-azonia-4,4-dimethylpentyl) carbodiimide (EAC), in aqueous solution as a function of pH and added chemical reagents. In 50 mM 2-(N-morpholino)ethanesulfonic acid at 25 degrees C, EDC has t1/2 values of 37, 20, and 3.9 h at pH 7.0, 6.0, and 5.0, respectively, while the corresponding values for EAC are 12, 2.9, and 0.32 h. Iodide, bromide, or chloride, at 0.1 M, has very little or no effect on carbodiimide stability. However, 0.1 M glycine methyl ester or 0.1 M ethylenediamine causes a significant increase in the rate of loss of EAC and EDC, while the presence of 0.1 M phosphate, 0.1 M hydroxylamine, or 0.01 M ATP decreases the half-lives to less than or equal to 0.4 h at all pH values.

Crosslinking of cytochrome c and cytochrome b5 with a water-soluble carbodiimide. Reaction conditions, product analysis and critique of the technique

A water soluble carbodiimide, 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC), has been used to crosslink horse heart cytochrome c and trypsin-solubilized bovine liver microsomal cytochrome b5. The reaction was conducted under a variety of solution conditions, and the products were purified by a combination of gel filtration and ion-exchange chromatography. Under all conditions of pH, ionic strength, EDC/protein ratio and reaction time that were studied, multiple 1:1 crosslinked complexes were observed with no evidence of a single, dominant species. Acetate, which is often used as a quencher of such reactions, was found to increase the complexity of the reaction products, presumably through EDC-promoted coupling to cytochrome c. Hydroxylamine treatment of the crosslinked complexes, a procedure frequently used to reverse EDC modification of tyrosyl residues, did not reduce the number of crosslinked components observed. The cytochrome b5 heme group was readily extracted from each of the 1:1 crosslinked complexes by standard techniques, so the crosslinking of heme propionate 7 with Lys79 of cytochrome c that might have been anticipated on the basis of molecular graphics modeling [Salemme, F.R. (1976) J. Mol. Biol. 102, 563-568] was not evident from this analysis. Analysis of HPLC tryptic peptide maps produced from crosslinked complexes revealed reduced specificity of trypsin in hydrolysis of EDC-crosslinked protein-protein complexes and unsatisfactory resolution of crosslinked or branched peptides. Nevertheless, it was possible to demonstrate that residues 52-72 of cytochrome b5, a region predicted to be critical to interaction with cytochrome b5 [Salemme, F.R. (1976) J. Mol. Biol. 102, 563-568] was absent from all peptide maps of 1:1 cytochrome c.cytochrome b5 complexes. Based on these results and a review of the literature involving EDC crosslinking of electron transfer proteins, we conclude that the techniques available for specific protein hydrolysis and separation of crosslinked peptides are not adequate to permit routine unambiguous identification of crosslinking sites in carbodiimide-crosslinked complexes.

Site-directed mutagenesis of yeast cytochrome c peroxidase shows histidine 181 is not required for oxidation of ferrocytochrome c

The long-distance electron transfer observed in the complex formed between ferrocytochrome c and compound I, the peroxide-oxidized form of cytochrome c peroxidase (CCP), has been proposed to occur through the participation of His 181 of CCP and Phe 87 of yeast iso-1 cytochrome c [Poulos, T. L., & Kraut, J. (1980) J. Biol. Chem. 255, 10322-10330]. We have examined the role of His 181 of CCP in this process through characterization of a mutant CCP in which His 181 has been replaced by glycine through site-directed mutagenesis. Data from single-crystal X-ray diffraction studies, as well as the visible spectra of the mutant CCP and its 2-equiv oxidation product, compound I, show that at pH 6.0 the protein is not dramatically altered by the His 181----Gly mutation. The rate of peroxide-dependent oxidation of ferrocytochrome c by the mutant CCP is reduced only 2-fold relative to that of the parental CCP, under steady-state conditions. Transient kinetic measurements of the intracomplex electron transfer rate from ferrous cytochrome c to compound I indicate that the rate of electron transfer within the transiently formed complex at high ionic strength (mu = 114 mM, pH = 6) is also reduced by approximately 2-fold in the mutant CCP protein. The relatively minor effect of the loss of the imidazole side chain at position 181 on the kinetics of electron transfer in the CCP-cytochrome c complex precludes an obligatory participation of His 181 in electron transfer from ferrous cytochrome c to compound I.(ABSTRACT TRUNCATED AT 250 WORDS)

Kinetics of intracomplex electron transfer and of reduction of the components of covalent and noncovalent complexes of cytochrome c and cytochrome c peroxidase by free flavin semiquinones

The kinetics of reduction of free flavin semiquinones of the individual components of 1:1 covalent and electrostatic complexes of yeast ferric and ferryl cytochrome c peroxidase and ferric horse cytochrome c have been studied. Covalent cross-linking between the peroxidase and cytochrome c at low ionic strength results in a complex that has kinetic properties both similar to and different from those of the electrostatic complex. Whereas the cytochrome c heme exposure to exogenous reductants is similar in both complexes, the apparent electrostatic environment near the cytochrome c heme edge is markedly different. In the electrostatic complex, a net positive charge is present, whereas in the covalent complex, an essentially neutral electrostatic charge is found. Intracomplex electron transfer within the two complexes is also different. For the covalent complex, electron transfer from ferrous cytochrome c to the ferryl peroxidase has a rate constant of 1560 s-1, which is invariant with respect to changes in the ionic strength. The rate constant for intracomplex electron transfer within the electrostatic complex is highly ionic strength dependent. At mu = 8 mM a value of 750 s-1 has been obtained [Hazzard, J. T., Poulos, T. L., & Tollin, G. (1987) Biochemistry 26, 2836-2848], whereas at mu = 30 mM the value is 3300 s-1. This ionic strength dependency for the electrostatic complex has been interpreted in terms of the rearrangement of the two proteins comprising the complex to a more favorable orientation for electron transfer. In the case of the covalent complex, such reorientation is apparently impeded.(ABSTRACT TRUNCATED AT 250 WORDS)

Sulfite oxidase from chicken liver. Further characterization of the role of carboxyl groups in the reaction with cytochrome c

The mitochondrial enzyme sulfite oxidase catalyzes the oxidation of cytochrome c by sulfite. The reaction is inhibited when the enzyme is treated with N-cyclohexyl-N'-[2-(N-methylmorpholino)-ethyl]carbodiimide p-toluenesulfonate (CMC). Inhibition follows the conversion of two carboxyl groups to N-acylurea derivatives. The two groups are about equally reactive toward this inhibitor and blocking of either group abolishes electron transfer to cytochrome c. The rate of inactivation is almost the same in the presence of cytochrome c and under conditions where, on average, 89% of the enzyme is bound to cytochrome c. Therefore, the functional groups are not likely to be at the cytochrome c binding site. There are two equal and non-interacting cytochrome c binding sites per sulfite oxidase monomer. The Kd is 7.5 microM at pH 6.0 and low ionic strength. The data are difficult to reconcile with binding of cytochrome c to a cluster of acidic residues in the area of the heme b prosthetic group, as was envisaged for the cytochrome-b5--cytochrome c complex [Salemme, F.R. (1976) J. Mol. Biol. 102, 563-568]. An improved method for the purification of sulfite oxidase from chicken liver, using affinity chromatography on cytochrome c--Sepharose, is described.

Formation of electrostatically-stabilized complex at low ionic strength inhibits interprotein electron transfer between yeast cytochrome c and cytochrome c peroxidase

Electron transfer from yeast ferrous cytochrome c to H2O2-oxidized yeast cytochrome c peroxidase has been studied using flash photoreduction methods. At low ionic strength (mu less than 10 mM), where a strong complex is formed between cytochrome c and peroxidase, electron transfer occurs rather slowly (k approximately 200s-1). However, at high ionic strength where the electrostatic complex is largely dissociated, the observed first-order rate constant for peroxidase reduction increases significantly reaching a concentration independent limit of k approximately 1500 s-1. Thus, at least in some cases, formation of an electrostatically-stabilized complex can actually impede electron transfer between proteins.

Assignment of hyperfine-shifted resonances in yeast ferricytochrome c isozyme 2 using the proton pre-steady-state nuclear Overhauser effect

Yeast cytochrome c isozyme 2 is one of two cytochrome c isozymes which are found in yeast mitochondria. Unlike isozyme 1, which can dimerize in vitro due to a free sulfhydryl group at primary sequence position 102, isozyme 2 (Ala-102) is a monomer. The hyperfine proton NMR resonance pattern of ferric isozyme 2 is somewhat different from the horse and tuna ferricytochromes c. Thus, resonance assignments would help determine how similar the yeast, horse and tuna proteins actually are. In this work, many of the unassigned proton hyperfine resonances of the ferric protein have been assigned using the proton pre-steady-state nuclear Overhauser effect. Assigned resonances include those attributable to the heme 7-propionic acid alpha-CH2, His-18 alpha-CH and beta-CH2, Met-80 beta-CH2, and heme 4-beta-CH3 protons. The overall pattern of NOE connectivities is similar to the horse and tuna proteins. Combining shift and NOE patterns leads to the conclusion that the heme environment of yeast ferric isozyme 2 in solution is similar, but not identical, to the heme environment of horse and tuna cytochromes c.

The effect of complex formation upon the reduction rates of cytochrome c and cytochrome c peroxidase compound II

The effect of complex formation between ferricytochrome c and cytochrome c peroxidase (Ferrocytochrome-c:hydrogen peroxide oxidoreductase, EC 1.11.1.5) on the reduction of cytochrome c by N,N,N',N'-tetramethyl-p-phenylenediamine (TMPD), reduced N-methylphenazonium methosulfate (PMSH), and ascorbate has been determined at low ionic strength (pH 7) and 25 degrees C. Complex formation with the peroxidase enhances the rate of ferricytochrome c reduction by the neutral reductants TMPD and PMSH. Under all experimental conditions investigated, complex formation with cytochrome c peroxidase inhibits the ascorbate reduction of ferricytochrome c. This inhibition is due to the unfavorable electrostatic interactions between the ascorbate dianion and the negatively charged cytochrome c-cytochrome c peroxidase complex. Corrections for the electrostatic term by extrapolating the data to infinite ionic strength suggest that ascorbate can reduce cytochrome c peroxidase-bound cytochrome c faster than free cytochrome c. Reduction of cytochrome c peroxidase Compound II by dicyanobis(1,10-phenanthroline)iron(II) (Fe(phen)2(CN)2) is essentially unaffected by complex formation between the enzyme and ferricytochrome c at low ionic strength (pH 6) and 25 degrees C. However, reduction of Compound II by the negatively changed tetracyano-(1,10-phenanthroline)iron(II) (Fe(phen)(CN)4) is enhanced in the presence of ferricytochrome c. This enhancement is due to the more favorable electrostatic interactions between the reductant and cytochrome c-cytochrome c peroxidase Compound II complex then for Compound II itself. These studies indicate that complex formation between cytochrome c and cytochrome c peroxidase does not sterically block the electron-transfer pathways from these small nonphysiological reductants to the hemes in these two proteins.

A proton NMR study of the non-covalent complex of horse cytochrome c and yeast cytochrome-c peroxidase and its comparison with other interacting protein complexes

Cytochrome-c peroxidase (ferrocytochrome-c:hydrogen-peroxide oxidoreductase, EC 1.11.1.5) forms a noncovalent 1:1 complex with horse cytochrome c in low ionic strength solution that is detectable by proton NMR spectroscopy. When the entire proton hyperfine-shifted spectrum is considered only five hyperfine resonances exhibit unambiguously detectable shifts: the heme 8-CH3 and 3-CH3 resonances, single proton resonances near 19 ppm and -4 ppm and the methionine-80 methyl group. These shifts are very similar to those observed for the covalently crosslinked complex of cytochrome-c peroxidase and horse cytochrome c, but different from those reported for cytochrome c complexes with flavodoxin and cytochrome b5. By comparison with the shifts reported for lysine-13-modified cytochrome c we conclude that the results reported here support the Poulos-Kraut proposed structure for the molecular redox complex between cytochrome-c peroxidase and cytochrome c. These results indicate that the principal site of interaction with cytochrome-c peroxidase is the exposed heme edge of horse cytochrome c, in proximity to lysine-13 and the heme pyrrole II. The noncovalent cytochrome-c peroxidase-cytochrome c complex exists in the rapid-exchange time limit even at 500 mHz proton frequency. Our data provide an improved estimate of the minimum off-rate for exchanging cytochrome c as 1133 (+/- 120) s-1 at 23 degrees C.

A covalent complex between horse heart cytochrome c and yeast cytochrome c peroxidase: kinetic properties

The kinetic properties of a 1:1 covalent complex between horse-heart cytochrome c and yeast cytochrome c peroxidase (ferrocytochrome-c:hydrogen-peroxide oxidoreductase, EC 1.11.1.5) have been investigated by transient-state and steady-state kinetic techniques. Evidence for heterogeneity in the complex is presented. About 50% of the complex reacts with hydrogen peroxide with a rate 20-40% faster than that of native enzyme; 20% of the complex exists in a conformation which does not react with hydrogen peroxide but converts to the reactive form at a rate of 20 +/- 5 s-1; 30% of the complex does not react with hydrogen peroxide to form the oxidized enzyme intermediate, cytochrome c peroxidase Compound I. Intramolecular electron transfer between covalently bound ferrocytochrome c and an oxidized site in cytochrome c peroxidase Compound I is too fast to measure, but a lower limit of 600 s-1 can be estimated at 5 degrees C in a 10 mM potassium phosphate buffer at pH 7.5. Free ferrocytochrome c reduces cytochrome c peroxidase Compound I covalently bound to ferricytochrome c at a rate 10(-4) to 10(-5)-times slower than for free Compound I. The transient-state ferrocytochrome c reduction rates of Compound I covalently linked to ferricytochrome c are about 70-times too slow to account for the steady-state catalytic properties of the 1:1 covalent complex. This indicates that hydrogen peroxide can interact with the 1:1 complex at sites other than the heme of cytochrome c peroxidase, generating additional species capable of oxidizing free ferrocytochrome c.

Sulfite oxidase from chicken liver. The role of imidazole and carboxyl groups for the reaction with cytochrome c

Oxidation of sulfite to sulfate by sulfite oxidase is inhibited when the enzyme is treated with reagents known to modify imidazole and carboxyl groups. Modification inhibits the oxidation of sulfite by the physiological electron acceptor cytochrome c, but not by the artificial acceptor ferricyanide. This indicates interference with reaction steps that follow the oxidation of sulfite by the enzyme's molybdenum cofactor. Reaction with diethylpyrocarbonate modifies ten histidines per enzyme monomer. Loss of activity is concomitant to the modification of only a single histidine residue. Inactivation takes place at the same rate in free sulfite oxidase and in the sulfite-oxidase--cytochrome-c complex. Blocking of carboxyl groups with water-soluble carbodiimides inactivates the enzyme. But none of the enzyme's carboxyl groups seems to be essential in the sense that its modification fully abolishes activity. The pattern of inactivation by chemical modification of sulfite oxidase is quite similar to that observed previously for cytochrome c peroxidase from yeast [Bosshard, H. R., Bänziger, J., Hasler, T. and Poulos, T. L. (1984) J. Biol. Chem. 259, 5683-5690; Bechtold, R. and Bosshard, H. R. (1985) J. Biol. Chem. 260, 5191-5200]. The two enzymes have very different structures yet share cytochrome c as a common substrate of which they recognize the same electron-transfer domain around the exposed heme edge.

Evidence of binary complex formations between cytochrome P-450, cytochrome b5, and NADPH-cytochrome P-450 reductase of hepatic microsomes

Water-soluble carbodiimide-catalyzed cross-linking of purified cytochrome P-450 LM2, cytochrome b5, and NADPH-cytochrome P-450 reductase was used to identify stable complexes formed between these proteins. High yields of P-450-b5 and P-450 reductase-b5 dimers, and lower yields of P-450 reductase-LM2 dimers were obtained. Substitution of native b5 and P-450 reductase with fully amidinated derivatives showed that LM2 and b5 were cross-linked exclusively through their respective amino and carboxyl groups. However, there appeared to be two complexation sites on the reductase which cross-link to b5 through amino groups and to LM2 through carboxyl groups respectively. A heterotrimer could not be identified following incubation of all three proteins together with EDC.

The use of a water-soluble carbodiimide to study the interaction between Chromatium vinosum flavocytochrome c-552 and cytochrome c

The interaction between horse heart cytochrome c and Chromatium vinosum flavocytochrome c-552 was studied using the water-soluble reagent 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC). Treatment of flavocytochrome c-552 with EDC was found to inhibit the sulfide: cytochrome c reductase activity of the enzyme. SDS gel electrophoresis studies revealed that EDC treatment led to modification of carboxyl groups in both the Mr 21 000 heme peptide and the Mr 46 000 flavin peptide, and also to the formation of a cross-linked heme peptide dimer with an Mr value of 42 000. Both the inhibition of sulfide: cytochrome c reductase activity and the formation of the heme peptide dimer were decreased when the EDC modification was carried out in the presence of cytochrome c. In addition, two new cross-linked species with Mr values of 34 000 and 59 000 were formed. These were identified as cross-linked cytochrome c-heme peptide and cytochrome c-flavin peptide species, respectively. Neither of these species were formed in the presence of a cytochrome c derivative in which all of the lysine amino groups had been dimethylated, demonstrating that EDC had cross-linked lysine amino groups on native cytochrome c to carboxyl groups on the heme and flavin peptides. A complex between cytochrome c and flavocytochrome c-552 was required for cross-linking to occur, since ionic strengths above 100 mM inhibited cross-linking.