The radical chemistry of milk xanthine oxidase as studied by radiation chemistry techniques

The kinetics of electron transfer within the molybdoflavoenzyme xanthine oxidase has been investigated using the technique of pulse radiolysis. Subsequent to one-electron reduction of native enzyme at 20 degrees C in 20 mM pyrophosphate buffer, pH 8.5, using the CO-.2 species as reductant, a spectral change is observed having a rate constant of approximately 290 s-1. From its wavelength dependence, this spectral change is assigned to the transfer of an electron from flavin semiquinone (formed on reaction with the CO2-. species) to one of the iron-sulfur centers of the enzyme in an intramolecular equilibration process. The value for this rate constant agrees well with the 330 s-1 observed in previous stopped-flow pH-jump experiments carried out at 25 degrees C (Hille, R., and Massey, V. (1986) J. Biol. Chem. 261, 1241-1247). Experimental results with fully reduced enzyme reacting with the radiolytically generated N.3 species also support the conclusion that the equilibration of reducing equivalents among the oxidation-reduction centers of xanthine oxidase is a rapid process. Evidence is also found that xanthine oxidase possesses an unusually reactive disulfide bond that is reduced rapidly by radiolytically generated radicals. The ramifications of the present results with regard to the interpretation of experiments involving chemically reactive radical species, generated either by photolysis or radiolysis, are discussed.     

The presence of a reducible disulfide bond in milk xanthine oxidase

The stoichiometry of reducing equivalents per protomer for the complex molybdoflavoprotein xanthine oxidase has been re-examined by reductive titrations with sodium dithionite and anaerobic reoxidation with cytochrome c and phenazine methosulfate of dithionite- or photo-reduced enzyme. It is found that 8.0 +/- 0.1 reducing equivalents are taken up (or given up) by the enzyme, a value of 2 eq greater than expected on the basis of the known oxidation-reduction centers in the enzyme. The reaction of reduced xanthine oxidase with [14C]iodoacetate indicates that, in the reduced form of the enzyme, additional cysteine residues are available for reaction. These results, in conjunction with the observation that reaction of oxidized enzyme with sulfite results in the appearance of an additional equivalent of thiol capable of reacting with 5,5'-dithiobis-(2-nitrobenzoic acid) or iodoacetate, indicate the presence of a disulfide linkage in the enzyme that can be reduced by dithionite or photochemically employing EDTA and 5-deazaflavin. Neither xanthine nor lumazine, however, is capable of reducing this oxidation-reduction center, suggesting that the disulfide does not play a role in the catalytic reactions of the enzyme. These results resolve discrepancies in the literature which indicated that greater than 6 reducing equivalents were consistently needed to bring about the complete reduction of xanthine oxidase.     

The reaction of reduced xanthine oxidase with oxygen. Kinetics of peroxide and superoxide formation

Product formation during the oxidation of xanthine oxidase has been examined directly by using cytochrome c peroxidase as a trapping agent for hydrogen peroxide and the reduction of cytochrome c as a measure of superoxide formation. When fully reduced enzyme is mixed with high concentrations of oxygen, 2 molecules of H2O2/flavin are produced rapidly, while 1 molecule of O2-/flavin is produced rapidly and another produced much more slowly. Time courses for superoxide formation and those for the absorbance changes due to enzyme oxidation were fitted successfully to the mechanism proposed earlier (Olson, J. S., Ballou, D. P., Palmer, G., and Massey, V. (1974) J. Biol. Chem. 249, 4363-4382). In this scheme, each oxidative step is initiated by the very rapid and reversible formation of an oxygen.FADH2 complex (the apparent KD = 2.2 X 10(-4) M at 20 degrees C, pH 8.3). In the cases of 6- and 4-electron-reduced enzyme, 2 electrons are transferred rapidly (ke = 60 s-1) to generate hydrogen peroxide and partially oxidized xanthine oxidase. In the case of the 2-electron-reduced enzyme, only 1 electron is transferred rapidly and superoxide is produced. The remaining electron remains in the iron-sulfur centers and is removed slowly by a second order process (ks = 1 X 10(4) M-1 s-1). When the pH is decreased from 9.9 to 6.2, both the apparent KD for oxygen binding and the rapid rate of electron transfer are decreased about 20-fold. This result is suggestive of uncompetitive inhibition and implies that proton binding to the enzyme-flavin active site affects primarily the rate of electron transfer, not the formation of the initial oxygen complex.     

Studies of the primary oxygen intermediate in the reaction of fully reduced cytochrome oxidase.

The formation and disappearance of a photosensitive species during the reaction of reduced cytochrome c oxidase (putatively a3II.O2), EC 1.9.3.1, has been followed by (a) mixing a3II.CO with O2 in a stopped flow apparatus; (b) initiating the oxygen-oxidase reaction by removing CO with a laser flash; (c) probing the reaction mixture for photosensitivity with a second laser flash. Photosensitivity appears in the reaction mixture after the first laser flash, reaches a maximum after 50-60 microseconds ([O2] greater than 100 microM), and disappears in a further 50-100 microseconds. The kinetics can be represented by the scheme [formula: see text]. In species B, O2 is associated with the protein, possibly CuB, but not with the heme. Species C is the photosensitive a3II.O2 complex, and in D, a3 iron has been oxidized. The formation of species C is responsible for the rapid phase of absorbance change in the oxidase-oxygen reaction. The rate of reaction with oxygen approaches the limit of 35,000 s-1 at high oxygen. Nitric oxide, however, reacts with FeII oxidase with a rate of 1 x 10(8) M-1 s-1, which is accurately maintained up to an observed rate of 10(5) s-1. In flash photolysis experiments, approximately half of the photodissociated nitric oxidase recombines in a biphasic geminate reaction with rates of 1 x 10(8) s-1 and 1 x 10(7) s-1.     

Electron transfer in milk xanthine oxidase as studied by pulse radiolysis

Electron transfer within milk xanthine oxidase has been examined by the technique of pulse radiolysis. Radiolytically generated N-methylnicotinamide radical or 5-deazalumiflavin radical has been used to rapidly and selectively introduce reducing equivalents into the enzyme so that subsequent equilibration among the four redox-active centers of the enzyme (a molybdenum center, two iron-sulfur centers, and FAD) could be monitored spectrophotometrically. Experiments have been performed at pH 6 and 8.5, and a comprehensive scheme describing electron equilibration within the enzyme at both pH values has been developed. All rate constants ascribed to equilibration between specific pairs of centers in the enzyme are found to be rapid relative to enzyme turnover under the same conditions. Electron equilibration between the molybdenum center and one of the iron-sulfur centers of the enzyme (tentatively assigned Fe/S I) is particularly rapid, with a pH-independent first-order rate constant of approximately 8.5 x 10(3) s-1. The results unambiguously demonstrate the role of the iron-sulfur centers of xanthine oxidase in mediating electron transfer between the molybdenum and flavin centers of the enzyme.     

Molecular basis of the Bohr effect in arthropod hemocyanin

Flash photolysis and K-edge x-ray absorption spectroscopy (XAS) were used to investigate the functional and structural effects of pH on the oxygen affinity of three homologous arthropod hemocyanins (Hcs). Flash photolysis measurements showed that the well-characterized pH dependence of oxygen affinity (Bohr effect) is attributable to changes in the oxygen binding rate constant, k(on), rather than changes in k(off). In parallel, coordination geometry of copper in Hc was evaluated as a function of pH by XAS. It was found that the geometry of copper in the oxygenated protein is unchanged at all pH values investigated, while significant changes were observed for the deoxygenated protein as a function of pH. The interpretation of these changes was based on previously described correlations between spectral lineshape and coordination geometry obtained for model compounds of known structure (Blackburn, N. J., Strange, R. W., Reedijk, J., Volbeda, A., Farooq, A., Karlin, K. D., and Zubieta, J. (1989) Inorg. Chem., 28, 1349-1357). A pH-dependent change in the geometry of cuprous copper in the active site of deoxyHc, from pseudotetrahedral toward trigonal was assigned from the observed intensity dependence of the 1s --> 4p(z) transition in x-ray absorption near edge structure (XANES) spectra. The structural alteration correlated well with increase in oxygen affinity at alkaline pH determined in flash photolysis experiments. These results suggest that the oxygen binding rate in deoxyHc depends on the coordination geometry of Cu(I) and suggest a structural origin for the Bohr effect in arthropod Hcs.     

Kinetic comparison of reduction and intramolecular electron transfer in milk xanthine oxidase and chicken liver xanthine dehydrogenase by laser flash photolysis

A comparative study using laser flash photolysis of the kinetics of reduction and intramolecular electron transfer among the redox centers of chicken liver xanthine dehydrogenase and of bovine milk xanthine oxidase is described. The photogenerated reductant, 5-deazariboflavin semiquinone, reacts with the dehydrogenase (presumably at the Mo center) in a second-order manner, with a rate constant (k = 6 x 10(7) M-1 s-1) similar to that observed with the oxidase [k = 3 x 10(7) M-1 s-1; Bhattacharyya et al. (1983) Biochemistry 22, 5270-5279]. In the case of the dehydrogenase, neutral FAD radical formation is found to occur by intramolecular electron transfer (kobs = 1600 s-1), presumably from the Mo center, whereas with the oxidase the flavin radical forms via a bimolecular process involving direct reduction by the deazaflavin semiquinone (k = 2 x 10(8) M-1 s-1). Biphasic rates of Fe/S center reduction are observed with both enzymes, which are due to intramolecular electron transfer (kobs approximately 100 s-1 and kobs = 8-11 s-1). Intramolecular oxidation of the FAD radical in each enzyme occurs with a rate constant comparable to that of the rapid phase of Fe/S center reduction. The methylviologen radical, generated by the reaction of the oxidized viologen with 5-deazariboflavin semiquinone, reacts with both the dehydrogenase and the oxidase in a second-order manner (k = 7 x 10(5) M-1 s-1 and 4 x 10(6) M-1 s-1, respectively). Alkylation of the FAD centers results in substantial alterations in the kinetics of the reaction of the viologen radical with the oxidase but not with the dehydrogenase. These results suggest that the viologen radical reacts directly with the FAD center in the oxidase but not in the dehydrogenase, as is the case with the deazaflavin radical. The data support the conclusion that the environments of the FAD centers differ in the two enzymes, which is in accord with other studies addressing this problem from a different perspective [Massey et al. (1989) J. Biol. Chem. 264, 10567-10573]. In contrast, the rate constants for intramolecular electron transfer among the Mo, FAD, and Fe/S centers in the two enzymes (where they can be determined) are quite similar.     

Coulometric and potentiometric evaluation of the redox components of cytochrome c oxidase in situ

A new coulometric-potentiometric titration cuvette is described which permits accurate measurements of oxidation-reduction components in membranous systems. This cuvette has been utilized to measure the properties of cytochrome c oxidase in intact membranes of pigeon breast muscle mitochondria. The reducing equivalents accepted and donated by the portion of the respiratory chain with half-reduction potentials greater than 200 mV are equal to those required for the known components (cytochrome a3 and the high-potential copper plus cytochrome a, 'visible copper', cytochrome c1, cytochrome c, and the Rieske iron-sulfur protein). Titrations in the presence of CO show that formation of the reduced cytochrome a3-CO complex requires two reducing equivalents per cytochrome a3 (coulometric titration). Potentiometric titrations indicate (Lindsay, J.G., Owen, C.S. and Wilson, D.F. (1975) Arch. Biochem. Biophys. 169, 492--505) that both cytochromes a3 and the high-potential copper must be reduced in order to form the CO complex (n = 2.0 with a CO concentration-dependent half-reduction potential, Em). By contrast, titrations in the presence of azide show that the Em value of the high-potential copper is unchanged by the presence of azide and thus azide binds with nearly equal affinity whether the copper is reduced or oxidized.     

Chicken liver sulfite oxidase. Kinetics of reduction by laser-photoreduced flavins and intramolecular electron transfer

Laser flash photolysis was used to study the reaction of photoproduced 5-deazariboflavin (dRFH.), lumiflavin (LFH.), and riboflavin (RFH.) semiquinone radicals with the redox centers of purified chicken liver sulfite oxidase. Kinetic studies of the native enzyme with dRFH. yielded a second-order rate constant of 4.0 X 10(8) M-1 s-1 for direct reduction of the heme and a first-order rate constant of 310 s-1 for intramolecular electron transfer from the Mo center to the heme. The reaction with LFH. gave a second-order rate constant of 2.9 X 10(7) M-1 s-1 for heme reduction. Reoxidation of the reduced heme due to intramolecular electron transfer to the Mo center gave a first-order rate constant of 155 s-1. The direction of intramolecular electron transfer using dRFH. and LFH. was independent of the buffer used for the experiment. The different first-order rate constants observed for intramolecular electron transfer using dRFH. and LFH. are proposed to result from chemical differences at the Mo site. Flash photolysis studies with cyanide-inactivated sulfite oxidase using dRFH. and LFH. resulted in second-order reduction of the heme center with rate constants identical with those obtained with the native enzyme, whereas the first-order intramolecular electron-transfer processes seen with the native enzyme were absent. The isolated heme peptide of sulfite oxidase gave only second-order kinetics upon laser photolysis and confirmed that the first-order processes observed with the native enzyme involve the Mo site.(ABSTRACT TRUNCATED AT 250 WORDS)     

The genetic control of molybdoflavoproteins in Aspergillus nidulans. A xanthine dehydrogenase I half-molecule in cnx- mutant strains of Aspergillus nidulans.

The cnx- group of mutants of Aspergillus nidulans lacks xanthine dehydrogenase (xanthine: NAD+ oxidoreductase, EC 1.2.1.37) and nitrate reductase (EC 1.6.6.3) activities and are thought to be defective in the synthesis of a molybdenum-containing cofactor, 'cnx', common to xanthine dehydrogenase and nitrate reductase [Pateman, J.A., Rever, B.M., Cove, D.J. and Roberts, D.B. (1964) Nature (Lond.) 201, 58-60]. The cnx cofactor has a role in maintaining the aggregated multimeric structure of nitrate reductase [MacDonald, D.W., Cove, D.J. and Coddington, A. (1974) Mol. Gen. Genet. 128, 187-199]. We report here that, in cnx- mutants grown under conditions inducing xanthine dehydrogenase I, a species cross-reacting with antisera to the native enzyme and of half its molecular weight is present, together with cross-reacting molecules of similar molecular weight to the native enzyme. This suggests that the cnx cofactor has a role in maintaining the aggregated structure of xanthine dehydrogenase I. Both cross-reacting species are capable of passing reducing equivalents from NADH to a tetrazolium salt, showing that the cnx cofactor is not necessary for enzymic activity towards NADH.     

Transient kinetics of subunit-III-depleted cytochrome c oxidase.

Cytochrome c oxidase from ox heart was depleted of subunit III and its transient kinetic properties studied by stopped-flow and flash photolysis. It was found that the overall mechanism of electron transfer is very similar for subunit-III-depleted and native oxidase, although significant differences in some kinetic parameters have been detected. These include the second-order rate constant for cytochrome c oxidation and the rate-limiting step of the overall process. Moreover, at low cytochrome c/oxidase ratios (where the number of reducing equivalents is insufficient), the rate of reoxidation of cytochrome a was found to be very slow, even in air, and in fact for the subunit-III-depleted enzyme is even slower than for the native oxidase. The stability of reduced cytochrome a excludes the likelihood that removal of subunit III leads to a new O2-binding site, and the result may be relevant to the lowered vectorial H+/e- stoichiometry. The subunit-III-depleted oxidase can be pulsed under appropriate conditions and its combination with CO is unchanged, as shown by kinetic experiments and difference spectroscopy.     

Electron transfer within xanthine oxidase: a solvent kinetic isotope effect study

Solvent kinetic isotope effect studies of electron transfer within xanthine oxidase have been performed, using a stopped-flow pH-jump technique to perturb the distribution of reducing equivalents within partially reduced enzyme and follow the kinetics of reequilibration spectrophotometrically. It is found that the rate constant for electron transfer between the flavin and one of the iron-sulfur centers of the enzyme observed when the pH is jumped from 10 to 6 decreases from 173 to 25 s-1 on going from H2O to D2O, giving an observed solvent kinetic isotope effect of 6.9. An effect of comparable magnitude is observed for the pH jump in the opposite direction, the rate constant decreasing from 395 to 56 s-1. The solvent kinetic isotope effect on kobs is found to be directly proportional to the mole fraction of D2O in the reaction mix for the pH jump in each direction, consistent with the effect arising from a single exchangeable proton. Calculations of the microscopic rate constants for electron transfer between the flavin and the iron-sulfur center indicate that the intrinsic solvent kinetic isotope effect for electron transfer from the neutral flavin semiquinone to the iron-sulfur center designated Fe/S I is substantially greater than for electron transfer in the opposite direction and that the observed solvent kinetic isotope effect is a weighted averaged of the intrinsic isotope effects for the forward and reverse microscopic electron-transfer steps.(ABSTRACT TRUNCATED AT 250 WORDS)     

Changes in the apparent quantum efficiency for photolysis of Hb(CO)1.

Recent studies suggest that the allosteric state of the protein surrounding the hemes in hemoglobin affects both geminate recombination of CO and the apparent quantum efficiency (AQE) for photolysis (Rohlfs, R.J., J.S. Olson, and Q.H. Gibson, 1988, J. Biol. Chem. 263: 1803-1813. We report combined flow/flash experiments in which the AQE for photolysis of Hb(CO)1 was measured as a function of time delay after its formation. Experiments were carried out at 20 degrees C in 0.1 M phosphate buffer at pH 7.0 with CO saturations of 10% or less. The AQE was observed to decrease from a value close to 1.0 at short times to approximately 0.6 after 2 s. The fundamental photolysis step for carboxyhemoglobin is known to have a quantum efficiency of nearly 1.0, whereas the lower AQE values we observe result from competition between rapid geminate recombination and a rapid reaction step leading to escape of the CO to the solution phase. Changes in AQE values reflect changes in these rapid reaction steps which presumably result from conformational change in Hb(CO)1. The change in AQE is consistent with conversion of one or more hemes to an R-like state but these changes could not be even approximately described in terms of a simple two-state allosteric model.     

Effect of solution viscosity on intramolecular electron transfer in sulfite oxidase

Our previous studies have shown that the rate constant for intramolecular electron transfer (IET) between the heme and molybdenum centers of chicken liver sulfite oxidase varies from approximately 20 to 1400 s(-1) depending upon reaction conditions [Pacheco, A., Hazzard, J. T., Tollin, G., and Enemark, J. H. (1999) J. Biol. Inorg. Chem. 4, 390-401]. These two centers are linked by a flexible polypeptide loop, suggesting that conformational changes, which alter the Mo-Fe distance, may play an important role in the observed IET rates. In this study, we have investigated IET in sulfite oxidase using laser flash photolysis as a function of solution viscosity. The solution viscosity was varied over the range of 1.0-2.0 cP by addition of either polyethylene glycol 400 or sucrose. In the presence of either viscosogen, an appreciable decrease in the IET rate constant value is observed with an increase in the solvent viscosity. The IET rate constant exhibits a linear dependence on the negative 0.7th power of the viscosity. Steady-state kinetics and EPR experiments are consistent with the interpretation that viscosity, and not other properties of the added viscosogens, is responsible for the dependence of IET rates on the solvent composition. The results are consistent with the role of conformational changes on IET in sulfite oxidase, which helps to clarify the inconsistency between the large rate constant for IET between the Mo and Fe centers and the long distance (approximately 32 A) between these two metal centers observed in the crystal structure [Kisker, C., Schindelin, H., Pacheco, A., Wehbi, W., Garnett, R. M., Rajagopalan, K. V., Enemark, J. H., and Rees, D. C. (1997) Cell 91, 973-983].     

Reaction of oxygen with cytochrome c oxidase from Paracoccus denitrificans

The reaction of reduced cytochrome c oxidase (EC 1.9.3.1) from Paracoccus denitrificans (American Type Culture Collection 13543) with dioxygen has been followed by laser flash photolysis of the CO derivative. In detergent-stabilized solutions the reaction showed at least two distinct kinetic components, the faster of which was oxygen concentration dependent and had a rate of approximately 60 X 10(6) M-1 s-1. The slower reaction was independent of oxygen concentration and had a rate of 9 X 10(2) s-1. These rates are about 1.5 times greater than comparable rates for ox heart oxidase reported by C. Greenwood and Q. H. Gibson (J. Biol. Chem. (1967) 242, 1782-1787). The kinetic components have markedly different optical spectra which agree precisely in form with those for ox heart enzyme (Greenwood, C., and Gibson, Q. H. (1967) J. Biol. Chem. 242, 1782-1787) but are shifted by 2 nm toward the red. In phospholipid vesicles, the spectral contribution of the faster component was augmented. The dissociation constant for CO at 20 degrees C is 1.6 microM, 6 times greater than for the ox heart enzyme. The bacterial enzyme binds one CO per 2 heme a. The enzyme has an absorption band at 830 nm in the oxidized form similar to that of the ox heart enzyme.     

Coulometric and potentiometric evaluation of the redox components of cytochrome c oxidase in situ

A new coulometric-potentiometric titration cuvette is described which permits accurate measurements of oxidation-reduction components in membranous systems. This cuvette has been utilized to measure the properties of cytochrome c oxidase in intact membranes of pigeon breast muscle mitochondria. The reducing equivalents accepted and donated by the portion of the respiratory chain with half-reduction potentials greater than 200 mV are equal to those required for the known components (cytochrome a₃ and the high-potential copper plus cytochrome a, ‘visible copper’, cytochrome c₁, cytochrome c, and the Rieske iron-sulfur protein). Titrations in the presence of CO show that formation of the reduced cytochrome a₃-CO complex requires two reducing equivalents per cytochrome a₃ (coulometric titration). Potentiometric titrations indicate (Lindsay, J.G., Owen, C.S. and Wilson, D.F. (1975) Arch. Biochem. Biophys. 169, 492–505) that both cytochromes a₃ and the high-potential copper must be reduced in order to form the CO complex (n=2.0 with a CO concentration-dependent half-reduction potential, E_m). By contrast, titrations in the presence of azide show that the E_m value of the high-potential copper is unchanged by the presence of azide and thus azide binds with nearly equal affinity whether the copper is reduced or oxidized.     

Kinetic isotope effect studies on milk xanthine oxidase and on chicken liver xanthine dehydrogenase

The effect of isotopic substitution of the 8-H of xanthine (with 2H and 3H) on the rate of oxidation by bovine xanthine oxidase and by chicken xanthine dehydrogenase has been measured. V/K isotope effects were determined from competition experiments. No difference in H/T(V/K) values was observed between xanthine oxidase (3.59 +/- 0.1) and xanthine dehydrogenase (3.60 +/- 0.09). Xanthine dehydrogenase exhibited a larger T/D(V/K) value (0.616 +/- 0.028) than that observed for xanthine oxidase (0.551 +/- 0.016). Observed H/T(V/K) values for either enzyme are less than those H/T(V/K) values calculated with D/T(V/K) data. These discrepancies are suggested to arise from the presence of a rate-limiting step(s) prior to the irreversible C-H bond cleavage step in the mechanistic pathways of both enzymes. These kinetic complexities preclude examination of whether tunneling contributes to the reaction coordinate for the H-transfer step in each enzyme. No observable exchange of tritium with solvent is observed during the anaerobic incubation of [8-3H]xanthine with either enzyme, which suggests the reverse commitment to catalysis (Cr) is essentially zero. With the assumption of adherence to reduced mass relationships, the intrinsic deuterium isotope effect (Dk) for xanthine oxidation is calculated to be 7.4 +/- 0.7 for xanthine oxidase and 4.2 +/- 0.2 for xanthine dehydrogenase. By use of these values and steady-state kinetic data, the minimal rate for the hydrogen-transfer step is calculated to be approximately 75-fold faster than kcat for xanthine oxidase and approximately 10-fold faster than kcat for xanthine dehydrogenase. This calculated rate is consistent with data obtained by rapid-quench experiments with XO. A stoichiometry of 1.0 +/- 0.3 mol of uric acid/mol of functional enzyme is formed within the mixing time of the instrument (5-10 ms). The kinetic isotope effect data also permitted the calculation of the Kd values [Klinman, J. P., & Mathews, R. G. (1985) J. Am. Chem. Soc. 107, 1058-1060] for substrate dissociation, including all reversible steps prior to C-H bond cleavage. Values calculated for each enzyme (Kd = 120 microM) were found to be identical within experimental uncertainty.     

The reaction of Pseudomonas aeruginosa cytochrome c oxidase with carbon monoxide.

The binding of CO to ascorbate-reduced Pseudomonas cytochrome oxidase was investigated by static-titration, stopped-flow and flash-photolytic techniques. Static-titration data indicated that the binding process was non-stoicheiometric, with a Hill number of 1.44. Stopped-flow kinetics obtained on the binding of CO to reduced Pseudomonas cytochrome oxidase were biphasic in form; the faster rate exhibited a linear dependence on CO concentration with a second-order rate constant of 2 X 10(4) M-1-s-1, whereas the slower reaction rapidly reached a pseudo-first-order rate limit at approx. 1s-1. The relative proportions of the two phases observed in stopped-flow experiments also showed a dependency on CO concentration, the slower phase increasing as the CO concentration decreased. The kinetics of CO recombination after flash-photolytic dissociation of the reduced Pseudomonas cytochrome oxidase-CO complex were also biphasic in character, both phases showing a linear pseudo-first-order rate dependence on CO concentration. The second-order rate constants were determined as 3.6 X 10(4)M-1-s-1 and 1.6 X 10(4)M-1-s-1 respectively. Again the relative proportions of the two phases varied with CO concentration, the slower phase predominating at low CO concentrations. CO dissociation from the enzyme-CO complex measured in the presence of O2 and NO indicated the presence of two rates, of the order of 0.03s-1 and 0.15s-1. When sodium dithionite was used as a reducing agent for the Pseudomonas cytochrome oxidase, the CO-combination kinetics observed by both stopped flow and flash photolysis were extremely complex and not able to be simply analysed.     

Time-resolved magnetic circular dichroism spectroscopy of photolyzed carbonmonoxy cytochrome c oxidase (cytochrome aa3).

Nanosecond time-resolved magnetic circular dichroism (TRMCD) and time-resolved natural circular dichroism (TRCD) measurements of photolysis products of the CO complex of eukaryotic cytochrome c oxidase (CcO-CO) are presented. TRMCD spectra obtained at 100 ns and 10 microseconds after photolysis are diagnostic of pentacoordinate cytochrome a3Fe2+, as would be expected for simple photodissociation. Other time-resolved spectroscopies (UV-visible and resonance Raman), however, show evidence for unusual Fea3(2+) coordination after CO photolysis (Woodruff, W. H., O. Einarsdóttir, R. B. Dyer, K. A. Bagley, G. Palmer, S. J. Atherton, R. A. Goldbeck, T. D. Dawes, and D. S. Kliger. 1991. Proc. Nat. Acad. Sci. U.S.A. 88:2588-2592). Furthermore, time-resolved IR experiments have shown that photodissociated CO binds to CuB+ prior to recombining with Fea3(2+) (Dyer, R. B., O. Einarsdóttir, P. M. Killough, J. J. López-Garriga, and W. H. Woodruff. 1989. J. Am. Chem. Soc. 111:7657-7659). A model of the CcO-CO photolysis cycle which is consistent with all of the spectroscopic results is presented. A novel feature of this model is the coordination of a ligand endogenous to the protein to the Fe axial site vacated by the photolyzed CO and the simultaneous breaking of the Fe-imidazole(histidine) bond.     

Protein-protein interactions of cytochrome oxidase in inner mitochondrial membranes. The effect of liposome fusion on protein rotational mobility

Rotational diffusion of cytochrome oxidase in the inner membrane of rat liver mitochondria was measured by detecting the decay of absorption anisotropy after photolysis of the heme a3.CO complex by a vertically polarized laser flash. As in previous experiments with beef heart mitochondria (Kawato, S., Sigel, E., Carafoli, E., and Cherry, R. J. (1980) J. Biol. Chem. 255, 5508-5510), co-existence of rotating cytochrome oxidase (mean rotational relaxation time, phi, of 700 to 1400 microseconds) and immobilized cytochrome oxidase (phi greater than 20 ms) was observed in mitochondria and mitoplasts. The effect of lipid/protein ratio by weight (L/P) on the relative proportions of mobile and immobile cytochrome oxidase was investigated following the fusion of soybean phospholipid vesicles with mitoplasts. The fusion procedure yielded four separate fractions upon sucrose density gradient centrifugation with L/P as follows: 0.3 in Pellet, 0.7 in Band 3, 1.5 in Band 2, and 3.0 in Band 1. The percentage of rotationally mobile cytochrome oxidase (phi = 700 to 1000 microseconds) in each of the different bands was found to be 16% in Pellet, 25% in Band 3, 47% in Band 2, and 76% in Band 1 at 37 degrees C. The dependence of the amount of mobile cytochrome oxidase on L/P indicates that the fraction of aggregated protein progressively decreases with decreasing concentration of proteins in the membrane. Thus, the large immobile fraction of cytochrome oxidase in mitochondrial inner membranes can be explained by nonspecific protein aggregation which is a consequence of the low L/P. The decrease in the mobile fraction in Pellet compared with mitoplasts was shown to be due to the pH 6.5 incubation used for fusion.