Conformational change due to reduction of cytochrome-c oxidase in lauryl maltoside: picosecond time-resolved tryptophan fluorescence studies on the native and heat modified enzyme

Electrochemical method assisted immobilization and orientation of myoglobin into biomimetic brij 56 film and its direct electrochemistry study

A simple cyclic voltammetric method was applied to assemble and orient a model protein, namely, myoglobin (Mb), into a biocompatible Brij 56 film. Ultraviolet-visible and circular dichroism spectra indicated that Mb in Brij 56 matrix preserved its secondary structure. Fourier transform infrared spectra confirmed the formation of hydrogen bonds between Mb and Brij 56. These hydrogen bonds acted as the electron tunnel to transfer electrons from Mb's active sites to the underlying glassy carbon electrode. Effective direct electron transfer of Mb was realized with the presence of a couple of quasi-reversible and well-defined redox peaks at -310 mV (vs standard calomel electrode) in the studied potential range. The peaks were attributed to the redox couple of heme Fe(II)/Fe(III) of the well-oriented Mb in Brij 56 matrix. The surface coverage and the electron transfer rate (ks) of Mb immobilized into the Brij 56 film was ∼4.9×10(-11) mol cm(-2) and 72.6±3.0 s(-1), respectively. An excellent electrocatalytic response of the immobilized Mb toward nitrite in the absence of electron transfer mediators was observed. These results emphasized that the biomimetic Brij 56 could be used as an attractive material for immobilizing proteins and constructing biosensors.

Active site structure of the aa3 quinol oxidase of Acidianus ambivalens

The membrane bound aa(3)-type quinol:oxygen oxidoreductase from the hyperthermophilic archaeon, Acidianus ambivalens, which thrives at a pH of 2.5 and a temperature of 80 degrees C, has several unique structural and functional features as compared to the other members of the heme-copper oxygen reductase superfamily, but shares the common redox-coupled, proton-pumping function. To better understand the properties of the heme a(3)-Cu(B) catalytic site, a resonance Raman spectroscopic study of the enzyme under a variety of conditions and in the presence of various ligands was carried out. Assignments of several heme vibrational modes as well as iron-ligand stretching modes are made to serve as a basis for comparing the structure of the enzyme to that of other oxygen reductases. The CO-bound oxidase has conformations that are similar to those of other oxygen reductases. However, the addition of CO to the resting enzyme does not generate a mixed valence species as in the bovine aa(3) enzyme. The cyanide complex of the oxidized enzyme of A. ambivalens does not display the high stability of its bovine counterpart, and a redox titration demonstrates that there is an extensive heme-heme interaction reflected in the midpoint potentials of the cyanide adduct. The A. ambivalens oxygen reductase is very stable under acidic conditions, but it undergoes an earlier alkaline transition than the bovine enzyme. The A. ambivalens enzyme exhibits a redox-linked reversible conformational transition in the heme a(3)-Cu(B) center. The pH dependence and H/D exchange demonstrate that the conformational transition is associated with proton movements involving a group or groups with a pK(a) of approximately 3.8. The observed reversibility and involvement of protons in the redox-coupled conformational transition support the proton translocation model presented earlier. The implications of such conformational changes are discussed in relation to general redox-coupled proton pumping mechanisms in the heme-copper oxygen reductases.

NMR studies on interaction of lauryl maltoside with cytochrome c oxidase: a model for surfactant interaction with the membrane protein

Interaction of lauryl maltoside (LM) surfactant with bovine heart cytochrome c oxidase (CcO) has been studied by NMR techniques. Detailed 2-D (1)H and (13)C NMR techniques were used to assign the NMR signals of the surfactant nuclei. Paramagnetic dipolar shift of the surfactant (13)C NMR signals were used to identify the atoms close to the enzyme. The diamagnetic carbon monoxide complex of CcO did not cause any shift in the surfactant NMR spectra suggesting that the paramagnetic centres of the native CcO cause the shifts by dipolar interactions. The results showed that the polar head groups of the surfactant comprised of two maltoside rings are more affected, while the hydrophobic tail groups did not show any significant change on binding of the surfactant to the enzyme. This indicated that surfactant head groups possibly bind to the enzyme surface and the hydrophobic tail of the surfactant forms micelles and remains away from the enzyme. Based on the results, we propose that the membrane bound enzyme is possibly stabilised in aqueous solution by association with the micelles of the neutral surfactant so that the polar heads of the micelles bind to the polar surface of the enzyme. These micelles might form a 'belt like' structure around the enzyme helping it to remain monodispersed in the active form.

pH-dependent structural changes at the Heme-Copper binuclear center of cytochrome c oxidase

The resonance Raman spectra of the aa3 cytochrome c oxidase from Rhodobacter sphaeroides reveal pH-dependent structural changes in the binuclear site at room temperature. The binuclear site, which is the catalytic center of the enzyme, possesses two conformations at neutral pH, assessed from their distinctly different Fe-CO stretching modes in the resonance Raman spectra of the CO complex of the fully reduced enzyme. The two conformations (alpha and beta) interconvert reversibly in the pH 6-9 range with a pKa of 7.4, consistent with Fourier transform infrared spectroscopy measurements done at cryogenic temperatures (D.M. Mitchell, J.P. Sapleigh, A.M.Archer, J.O. Alben, and R.B.Gennis, 1996, Biochemistry 35:9446-9450). It is postulated that the different structures result from a change in the position of the Cu(B) atom with respect to the CO due to the presence of one or more ionizable groups in the vicinity of the binuclear center. The conserved tyrosine residue (Tyr-288 in R. sphaeroides, Tyr-244 in the bovine enzyme) that is adjacent to the oxygen-binding pocket or one of the histidines that coordinate Cu(B) are possible candidates. The existence of an equilibrium between the two conformers at physiological pH and room temperature suggests that the conformers may be functionally involved in enzymatic activity.

Direct electrochemistry of heme proteins: effect of electrode surface modification by neutral surfactants

Direct electrochemical studies on horse heart myoglobin and horseradish peroxidase (HRP) have been carried out using tin-doped indium oxide (ITO) and surfactant modified glassy carbon working electrodes. These proteins show very slow electron transfer kinetics at metal or untreated electrodes. Moreover, small amounts of surface-active impurity were drastically affects the electrode reaction of these proteins. The results showed that modification of the electrode surface with neutral surfactants significantly improves the electrochemical response of myoglobin as well as of HRP. The electrode response was found to depend on the structure of the surfactants. The amount of surfactant required per unit area of the electrode surface to promote the maximum electron transfer rate constants was found to be constant. This indicated that the surfactant molecules interacted with the electrode surface in a specific manner and anchored the protein molecules to align in the suitable orientation. The hydrophobicity of the surfactants rather than their charge was found to be crucial in promoting the electrode response of these proteins at the glassy carbon electrode.

Redox-linked conformational changes in bovine heart cytochrome c oxidase: picosecond time-resolved fluorescence studies of cyanide complex

Picosecond time-resolved fluorescence studies are carried out on cyanide-inhibited and heat-modified cytochrome c oxidase in aqueous lauryl maltoside surfactant solution, as well as in an aqueous vesicle, to understand the conformational changes associated with electron transfer and proton pumping activity of the enzyme. The tryptophan fluorescence decay profiles follow a four exponential model, which also matches the lifetime maxima obtained in a maximum entropy method analysis. The fast lifetime components are highly affected by the reduction and chemical modification of the enzyme. Changes in these lifetime components are related to the conformational changes in the vicinity of the heme centers of the enzyme. The cyanide-inhibited enzyme in the oxidized form shows a fluorescence decay profile similar to that of the native oxidized form, indicating that the conformational changes due to cyanide binding are very small. However, reduction of the cyanide-inhibited enzyme that leaves cyanide bound heme alpha3 oxidized causes a large increase in the fluorescence lifetimes, which indicates very significant conformational changes due to electron transfer to the dinuclear Cu(A) and heme alpha centers. A comparison of the tryptophan fluorescence decay of various other modified forms of the enzyme leads us to propose that the possible site of conformational coupling is located near heme alpha instead of the binuclear heme alpha3-Cu(B) center.

Steady-state and picosecond-time-resolved fluorescence studies on the recombinant heme domain of Bacillus megaterium cytochrome P-450

The conformational changes associated with the interaction of sodium laurate with the recombinant heme domain for cytochrome P-450BM3 have been investigated by steady-state and picosecond-time-resolved fluorescence spectroscopy. The steady-state quenching experiments show that while all the five tryptophan residues are accessible to acrylamide in the free enzyme as well as the enzyme x substrate complex, the number of tryptophan residues accessible to ionic quenchers decreases on interaction of the substrate with the enzyme. This indicates that some of the tryptophan residues move towards the core of the protein on interaction with the substrate. The number of tryptophan residues accessible to the solvent as determined by the calculation of the solvent-accessible area for the free enzyme agrees with the values obtained by the quenching experiments. The time-resolved fluorescence studies carried out by means of the time-correlated single-photon-counting technique show that the fluorescence-decay curve is best fitted to a three-exponential model (0.2, 1.0 and 5.4 ns). Lifetime distributions, as recovered by the maximum-entropy method, agree with the discrete exponential model. The binding of the substrate does not lead to any significant change in the lifetime components of the enzyme, indicating that the tryptophan residues are possibly away from the substrate-binding domain. The decay-associated emission spectra and the magnitudes of amplitude of different lifetimes indicate that the shortest lifetime component (tau1) originates from the three tryptophan residues that are completely or partially accessible to the solvent, and tau2 originates from the tryptophan residues that are buried in the core of the enzyme and not accessible to the solvent. X-ray crystallographic data and solvent-acessible-area calculations have been used to identify these residues.