Electron transfer turnover rate

For example, Willner and co-workers accomplished electrical communication between glucose oxidase (GOx) and a gold electrode using molecular-level assembly.5,6 Unprecedentedly effective electron transfer was observed from the FAD moiety to the electrode through the Au-nano particles (electron transfer turnover rate, 4500 s-1), derived from molecular conjugation and bioconjugation. 

The electron-transfer turnover rate of GOx with molecular oxygen as the electron acceptor corresponds to ca. 600 s at 25 °C. Using an activation energy of 7.2 kcal moU , the electron-transfer turnover rate of GOx at 35 °C is estimated to be ca. 900 s [149, 150]. A densely packed monolayer of GOx (ca. 1.7 X 10 mol cm ) that exhibits the theoretical electron-transfer turnover rate is... 

When the first electron transfer is rate limiting, it has been suggested that the thermodynamic argument for gating by substrate binding overlooked the effect of O2 binding. The reduction potential of the P450 enzymes (equation 7) are measured under anaerobic conditions. Under turnover conditions, O2 is present and rapidly binds to the heme Fe(II) center (equation 8). 

This would indicate that the reduction of the enzyme-substrate complex is the rate-limiting step of the overall reaction sequence. Two other observations are in accord with this assumption. Firstly, in a reconstituted system the turnover number can be increased up to 600 when the reductase concentration has reached a saturation value (Coon, M. J., priv. commun.). Secondly, NADH can also reduce cytochrome P450 via cytochrome bs in liver microsomes (Chap. 5.3) and this electron transfer adds to the rate observed with NADPH which would not be the case if any other reaction except electron transfer would be rate limiting. Since NADH with some substrates often exerts a synergistic effect, it may be that in those cases the second electron transfer becomes rate limiting. This possibility will be discussed in the next chapter. 

Voltammetry of Adsorbed Proteins, Fig. 3 Illustrative cyclic voltammograms from non-turnover processes measured at scan rates that are sufficiently high to preclude equilibration with the electrode potential, (a) Interfacial electron transfer is rate limiting when the peaks (black lines) are offset symmetrically from the midpoint potential of the redox... 

However, some data have been more difficult to incorporate into the mechanism shown in Figs. 8 and 9. As reported 21) in Section II,B the Fe protein can be reduced by two electrons to the [Fe4S4]° redox state. In this state the protein is apparently capable of passing two electrons to the MoFe protein during turnover, although it is not clear whether dissociation was required between electron transfers. More critically, it has been shown that the natural reductant flavodoxin hydroquinone 107) and the artificial reductant photoexcited eosin with NADH 108) are both capable of passing electrons to the complex between the oxidized Fe protein and the reduced MoFe protein, that is, with these reductants there appears to be no necessity for the complex to dissociate. Since complex dissociation is the rate-limiting step in the Lowe-Thorneley scheme, these observations could indicate a major flaw in the scheme. 

The half-wave potentials of (FTF4)Co2-mediated O2 reduction at pH 0-3 shifts by — 60 mV/pH [Durand et ah, 1983], which indicates that the turnover-determining part of the catalytic cycle contains a reversible electron transfer (ET) and a protonation, or two reversible ETs and two protonation steps. In contrast, if an irreversible ET step were present, the pH gradient would be 60/( + a) mV/pH, where n is the number of electrons transferred in redox equilibria prior to the irreversible ET and a is the transfer coefficient of the irreversible ET. The —60 mV/pH slope is identical to that manifested by simple Ee porphyrins (see Section 18.4.1). The turnover rate of ORR catalysis by (ETE4)Co2 was reported to be proportional to the bulk O2 concentration [Collman et ah, 1994], suggesting that the catalyst is not saturated with O2. 

Therefore, recent interest has been focused prevailingly on electrodes modified by a multilayer coverage, which can easily be achieved by using polymer films on electrodes. In this case, the mediated electron transfer to solution species can proceed inside the whole film (which actually behaves as a system with a homogeneous catalyst), and the necessary turnover rate is relatively lower than in a monolayer. 

Steady state kinetics and protein-protein binding measurements have also been reported for the interaction of these mutant cytochromes with bovine heart cytochrome c oxidase [120]. The binding of cytochrome c variants to the oxidase occurred with increasing values of Kj in the order He (3 x 10 Mol L ) < Leu = Gly < wild-type < Tyr < Ser (3 x 10 molL ). Steady-state kinetic analysis indicated that the rate of electron transfer with cytochrome c oxidase increased in the order Ser < He < Gly < Leu < Tyr < wild-type, an order notably different from that observed for a related analysis of the oxidation of these mutants by cytochrome c peroxidase [85]. This difference in order of mutant turnover by the oxidase and peroxidase may arise from differences in the mode of interaction of the cytochrome with these two enzymes. 

A single turnover study of the conversion of the heme-HO-1 complex to free biliverdin has elucidated the relative rates of the catalytic steps 129). This transient kinetic study indicates that the conversion of Fe heme to Fe verdoheme is biphasic. Electron transfer to the Fe -heme HO-1 complex occurred at a rate of 0.11 s at 4°C and 0.49 s at 25°C with a 0.1 1 ratio of NADPH-cytochrome P450 reductase to heme HO-l complex. Oxygen binding to the reduced iron was sufficiently rapid im-der the experimental conditions that the species actually monitored... 

A photoinduced electron relay system at solid-liquid interface is constructed also by utilizing polymer pendant Ru(bpy)2 +. The irradiation of a mixture of EDTA and water-insoluble polymer complex (Ru(PSt-bpy)(bpy) +, prepared by Eq. (15)) deposited as solid phase in methanol containing MV2+ induced MV 7 formation in the liquid phase 9). The rate of MV formation was 4 pM min-1. As shown in Fig. 14, photoinduced electron transfer occurs from EDTA in the solid to MV2+ in the liquid via Ru(bpy)2 +. The protons and Pt catalyst in the liquid phase brought about H2 evolution. One hour s irradiation of the system gave 9.32 pi H2 after standing 12 h and the turnover number of the Ru complex was 7.6 under this condition. The apparent rate constant of the electron transfer from Ru(bpy)2+ in the solid phase to MV2 + in the liquid was estimated to be higher than that of the entire solution system. The photochemical reduction and oxidation products, i.e., H2 and EDTAox were thus formed separately in different phases. Photoinduced electron relay did not occur in the system where a film of polymer pendant Ru complex separates two aqueous phases of EDTA and MV2 9) (see Fig. 15c). 

It has been considered that the high stability of the dye in a DSSC system could be obtained by the presence of I - ions as the electron donor to dye cauons. Degradation of the NCS ligand to the CN ligand by a intramolecular electron-transfer reaction, which reduces consequently the Ru(III) state to the Ru(II) state, occurs within 0.1-1 sec [153], whereas the rate for the reduction of Ru(in) to Ru(II) by the direct electron transfer from I ions into the dye cations is on the order of nanoseconds [30]. This indicates that one molecule of N3 dye can contribute to the photon-to-current conversion process with a turnover number of at least 107—10s without any degradation [153]. Taking this into consideration, N3 dye is considered to be sufficiently stable in the redox electrolyte under irradiation. 

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