Kinetic and structural model for the binding of formate to the rapid form of cytochrome c oxidase

Characterisation of the Cyanate Inhibited State of Cytochrome c Oxidase

Heme-copper oxygen reductases are terminal respiratory enzymes, catalyzing the reduction of dioxygen to water and the translocation of protons across the membrane. Oxygen consumption is inhibited by various substances. Here we tested the relatively unknown inhibition of cytochrome c oxidase (CcO) with isocyanate. In contrast to other more common inhibitors like cyanide, inhibition with cyanate was accompanied with the rise of a metal to ligand charge transfer (MLCT) band around 638 nm. Increasing the cyanate concentration furthermore caused selective reduction of heme a. The presence of the CT band allowed for the first time to directly monitor the nature of the ligand via surface-enhanced resonance Raman (SERR) spectroscopy. Analysis of isotope sensitive SERR spectra in comparison with Density Functional Theory (DFT) calculations identified not only the cyanate monomer as an inhibiting ligand but suggested also presence of an uretdion ligand formed upon dimerization of two cyanate ions. It is therefore proposed that under high cyanate concentrations the catalytic site of CcO promotes cyanate dimerization. The two excess electrons that are supplied from the uretdion ligand lead to the observed physiologically inverse electron transfer from heme a₃ to heme a.

The dinuclear center of cytochrome bo3 from Escherichia coli

For the study of the dinuclear center of heme-copper oxidases cytochrome bo3 from Escherichia coli offers several advantages over the extensively characterized bovine cytochrome c oxidase. The availability of strains with enhanced levels of expression allows purification of the significant amounts of enzyme required for detailed spectroscopic studies. Cytochrome bo3 is readily prepared as the fast form, with a homogeneous dinuclear center which gives rise to characteristic broad EPR signals not seen in CcO. The absence of CuA and the incorporation of protohemes allows for a detailed interpretation of the MCD spectra arising from the dinuclear center heme o3. Careful analysis allows us to distinguish between small molecules that bind to heme o3, those which are ligands of CuB, and those which react to yield higher oxidation states of heme o3. Here we review results from our studies of the reactions of fast cytochrome bo3 with formate, fluoride, chloride, azide, cyanide, NO, and H2O2.

Mössbauer and Magnetization Studies of Heme-Copper-Bridged Assemblies Pertinent to Cytochrome c Oxidase

In order to elucidate the structural and electronic properties of the dioxygen-reducing site of cytochrome oxidases, we have studied heme-based molecular assemblies containing the bridge unit Fe(III)-X-Cu(II) with X = O(2)(-), OH(-), and HCO(2)(-). Here we describe the results of Mössbauer and magnetization studies of [(OEP)Fe-O-Cu(Me(6)tren)](1+) (3), [(OEP)Fe-(OH)-Cu(Me(5)dien)(OClO(3))](1+) (4), [(OEP)Fe-(O(2)CH)-Cu(Me(5)dien)(OClO(3))](1+) (5), and [(OEP)Fe{(O(2)CH)Cu(Me(6)tren)}(2)](3+) (6). On the basis of magnetization studies, the oxidized binuclear enzyme site has been reported to be strongly coupled, with an S = 2 ground state. For 3 and 4 the Mössbauer data were analyzed for strong antiferromagnetic coupling between a high-spin Fe(III) (S(1) = (5)/(2)) and Cu(II) (S(2) = (1)/(2)), resulting in an S = 2 ground state. The exchange coupling of hydroxo-bridged 4 is substantial, J = 170 cm(-)(1) (H(ex) = JS(1).S(2)), but is smaller than J >/= 200 cm(-)(1) reported for oxo-bridged 3. The collective evidence for synthetic complexes and the oxidized enzymes indicates that the Fe(III)-(OH)-Cu(II) bridge unit is probable for the latter in the resting state. The similarity of properties of formate-inhibited and "slow" cytochrome oxidase has suggested a carboxylate bridge in the binuclear site. Formate-bridged 5 maintains an anti-anti carboxylate bridge conformation and has J = +18 cm(-)(1). While the bridge conformation of 5 might not apply to a putative carboxylate bridge in the binuclear enzyme site, the order-of-magnitude difference between the J values of 3, 4, and 5 suggests that no stereochemically possible &mgr;-eta(1):eta(1) carboxylate bridge conformation is likely to approach the strong coupling (J >/= 200 cm(-)(1)) of the formate-inhibited and "slow" enzyme forms. If carboxylate does bridge, the &mgr;-eta(2) mode appears more likely. Complex 6, which is not biologically pertinent, displays weak ferromagnetic coupling between a central high-spin Fe(III) and two Cu(II) sites, -0.50 cm(-)(1) < J < -0.10 cm(-)(1).

Formate bound to cytochrome oxidase can be removed by cyanide and by reduction

Using 14C-radiolabeled formate we have found that the rapid form of oxidized cytochrome oxidase can bind up to 1 mol of formate. Treatment of this formate-ligated enzyme with excess cyanide releases 97% of the radiolabel while reduction of formate-labeled enzyme with NADH+ruthenium releases 80-85% of the radioactivity. These data are most simply interpreted by assuming that formate binds to the heme iron of cytochrome a3.

Molecular Assemblies Containing Unsupported [Fe(III)-(&mgr;(2):eta(2)-RCO(2))-Cu(II)] Bridges

Formate is an inhibitor of cytochrome oxidases and also effects conversion of the bovine heart enzyme from the "fast" to the "slow" cyanide-binding form. The molecular basis of these effects is unknown; one possibility is that formate inserts as a bridge into the binuclear heme a(3)-Cu(B) site, impeding the binding of dioxygen or cyanide. Consequently, Fe-Cu-carboxylate interactions are a matter of current interest. We have initiated an examination of such interactions by the synthesis of the first examples of [Fe(III)-(&mgr;(2):eta(2)-RCO(2))-Cu(II)] bridges, minimally represented by Fe(III)-L + Cu(II)-O(2)CR --> [Fe(III)-(RCO(2))-Cu(II)] + L. A series of Cu(II) precursor complexes and solvate forms have been prepared and their structures determined, including [Cu(Me(5)dien)(O(2)CH)](+) (3), [Cu(Me(5)dien)(O(2)CH)(MeOH)](+) (4), [Cu(Me(6)tren)(O(2)CH)](+) (5), and [Cu(Me(5)dien)(OAc)](+) (6). [4](ClO(4)) was obtained in monoclinic space group P2(1)/n with a = 8.166(3) Å, b = 15.119(5) Å, c = 15.070(4) Å, beta = 104.65(2) degrees, and Z = 4. [5](ClO(4))/[6](ClO(4)) crystallize in orthorhombic space groups Pnma/Pna2(1) with a = 16.788(2)/14.928(5) Å, b = 9.542(1)/9.341(4) Å, c = 12.911(1)/12.554(4) Å, and Z = 4/4. In all cases, the carboxylate ligand is terminal and is bound in a syn orientation. Also prepared for the purpose of structural comparison was [Fe(OEP)(O(2)CH)], which occurred in monoclinic space group P2(1)/c with a = 13.342(2) Å, b = 13.621(2) Å, c = 19.333(2) Å, beta = 106.12(2) degrees, and Z = 4. The desired bridges were stabilized in the assemblies [(OEP)Fe(O(2)CH)Cu(Me(5)dien)(OClO(3))](+) (9), [(OEP)Fe(OAc)Cu(Me(5)dien)](2+) (10), and {(OEP)Fe[(O(2)CH)Cu(Me(6)tren)](2)}(3+) (11), which were prepared by the reaction of 3, 6, and 5, respectively, with [Fe(OEP)(OClO(3))] in acetone or dichloromethane. [9](ClO(4))/[10](ClO(4))(2).CH(2)Cl(2) crystallize in triclinic space group P&onemacr; with a = 9.016(3)/13.777(3) Å, b = 15.377(5)/13.847(3) Å, c = 19.253(5)/17.608(4) Å, alpha = 78.12(3)/96.82(3) degrees, beta = 86.30(4)/108.06(3) degrees, gamma = 76.23(3)/114.32(3) degrees, and Z = 2/2. Each assembly contains a [Fe(III)-(RCO(2))-Cu(II)] bridge but with the differing orientations anti-anti (9) and syn-anti (10, 11). The compound [11](ClO(4))(2)(SbF(6)) occurs in orthorhombic space group Pbcn with a = 12.517(6) Å, b = 29.45(1) Å, c = 21.569(8) Å, and Z = 4. Complex 11 is trinuclear; the Fe(III) site has two axial formate ligands with bond distances indicative of a high-spin configuration. Structural features of 9-11 are discussed and are considered in relation to the possible insertion of formate into the binuclear sites of two oxidases whose structures were recently determined. The present results contribute to the series of molecular assemblies with the bridge groups [Fe(III)-X-Cu(II)], X = O(2)(-), OH(-), and RCO(2)(-), all with a common high-spin heme, thereby allowing an examination of electronic structure as dependent on the bridging atom or group and bridge structure. (Me(5)dien = 1,1,4,7,7-pentamethyldiethylenetriamine; Me(6)tren = tris(2-(dimethylamino)ethyl)amine; OEP = octaethylporphyrinate(2-).)

Kinetics and mechanism for the binding of HCN to cytochrome c oxidase

The kinetics of cyanide binding to cytochrome c oxidase were systematically studied as a function of [HCN], [oxidase], pH, ionic strength, temperature, type and concentration of solubilizing detergent, and monomer-dimer content of oxidase. On the basis of these results a minimum reaction mechanism is proposed in which the spectrally visible rapid and slow cyanide binding reactions are two consecutive first-order reactions, not parallel reactions with different conformers of cytochrome c oxidase. The fast reaction (k'obs) follows saturation type kinetics to form an HCN complex that subsequently undergoes a slow reaction (k'obs). The fast k'obs reaction is independent of ionic strength but is strongly dependent upon pH. Two pK values were evaluated from the bell-shaped rate versus pH profile; one is due to an ionizable group on the protein (pKa = 7.45), while the other is that of HCN (pKHCN = 9.15). Therefore, oxidase is reactive toward HCN only when the group on the protein is unprotonated. The slow k'obs reaction is not a reaction of oxidase with either CN- or HCN; in fact, the product formed by the fast k'obs reaction, the oxidase-HCN complex, still undergoes the slow k" process even if all of the excess KCN is removed. The apparent rate constant of the slower phase (k"obs) is independent of all the variations done in this study, and it probably corresponds to either a slow conformational change in the protein or a change in ligand coordination at one of the metal centers after HCN binds to the bimetallic center of oxidase. Based upon the bell-shaped pH dependence of the fast phase and the pH independence of the slow phase, the mechanism also predicts that a single conformer of cytochrome c oxidase can exhibit either monophasic or biphasic cyanide binding kinetics depending upon the pH. At either very low or very high pH, the two rates become comparable in magnitude, which makes the reaction appear to be monophasic even though both reactions still occur. The amount of monomeric or dimeric oxidase only slightly affects the magnitude of k'obs and k"obs values, and both processes are clearly present in both types of oxidase.

Interconversion of fast and slow forms of cytochrome bo from Escherichia coli

The fully oxidized fast form of cytochrome bo from Escherichia coli is shown to convert spontaneously to a slow form when stored at -20 degrees C in 50 mM potassium borate, pH 8.5, containing 0.5 mM potassium EDTA. Evidence for the conversion, and that the form produced is analogous to the slow form of bovine heart cytochrome c oxidase, comes from (a) decreases in the extents of fast (k = 1-2 x 10(3) M-1 s-1) H2O2 binding and fast (k = 20-30 M-1 s-1) cyanide binding; (b) changes in the optical spectrum that are like those induced by formate, i.e., a blue shift in the Soret absorption band, loss of absorbance in the alpha and beta bands, and a red shift in the "630 nm" charge-transfer band; (c) changes in the EPR spectrum that are like those induced by formate, i.e., disappearance of signals at g = 8.6 and g = 3.71, and appearance of signals at g approximately 13, g = 3.14, and g = 2.58; and (d) appearance of a slow phase of reduction of heme o by dithionite. The mutant enzyme E286Q also converts to a slow form under the same conditions, as shown by (a) a decrease in the extent of fast H2O2 binding; (b) changes in the optical spectrum like those seen with wild-type enzyme; and (c) changes in the EPR spectrum that are like those induced by formate, i.e., disappearance of signals at g = 7.3 and g = 3.6 and appearance of signals at g approximately 13, g = 3.18, and g = 2.59.(ABSTRACT TRUNCATED AT 250 WORDS)