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Mechanisms for the direct electron transfer of cytochrome c induced by multi-walled carbon nanotubes.

Zhao HZ, Du Q, Li ZS, Yang QZ - Sensors (Basel) (2012)

Bottom Line: There are several possible mechanisms that explain the DET of Cyt c.In the presence of MWCNTs, the secondary structure of Cyt c changes, which exposes the active site, then, the orientation of the heme is optimized, revolving the exposed active center to the optimum spatial orientation for DET; and finally, a transition of spin states is induced, providing relatively high energy and a more open microenvironment for electron transfer.These changes at different nano-levels are closely connected and form a complex process that promotes the electron transfer of Cyt c.

View Article: PubMed Central - PubMed

Affiliation: Department of Environmental Engineering, Peking University, Beijing 100871, China. zhaohuazhang@pku.edu.cn

ABSTRACT
Multi-walled carbon nanotube (MWCNT)-modified electrodes can promote the direct electron transfer (DET) of cytochrome c (Cyt c). There are several possible mechanisms that explain the DET of Cyt c. In this study, several experimental methods, including Fourier transform infrared spectroscopy, circular dichroism, ultraviolet-visible absorption spectroscopy, and electron paramagnetic resonance spectroscopy were utilized to investigate the conformational changes of Cyt c induced by MWCNTs. The DET mechanism was demonstrated at various nano-levels: secondary structure, spatial orientation, and spin state. In the presence of MWCNTs, the secondary structure of Cyt c changes, which exposes the active site, then, the orientation of the heme is optimized, revolving the exposed active center to the optimum spatial orientation for DET; and finally, a transition of spin states is induced, providing relatively high energy and a more open microenvironment for electron transfer. These changes at different nano-levels are closely connected and form a complex process that promotes the electron transfer of Cyt c.

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EPR spectra of Cyt c and Cyt c/MWCNTs.
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f6-sensors-12-10450: EPR spectra of Cyt c and Cyt c/MWCNTs.

Mentions: As has been reported, the spin state equilibrium of the heme iron could modulate both substrate binding and the oxidation-reduction reactions of the cytochrome [19], and electron transfer is always accompanied by a change in iron spin state [20]. We speculate that the spin state influences the electron transfer activity. EPR spectra were utilized to investigate the conversion of the spin states, and the heme iron axial ligation is shown in Figure 6. As shown in Figure 6, the signals of the native Cyt c represent a mixture of high and low spin states, characterized at g = 6.19 and g = 4.36, respectively [17]. The signal at g = 1.98 is from the strong Fe-S ligation in the heme redox center. Fe-S ligation is important for the bioactivity of Cyt c, and damage to the Fe-S ligation will cause deactivation of the protein. When Cyt c interacts with MWCNTs, the Fe-S ligation signal nearly stay the same, suggesting that Fe-S ligation has not been destroyed, and the protein retains its bioactivity in this process. The low spin signal has a red shift to g = 4.45 along with a decrease in intensity while the high spin signal is observed at g = 6.26 along with an increase in intensity. This result suggests that the interaction between Cyt c and MWCNTs induces a transition of the heme iron from a low spin state to a high spin state. Cyt c has six heme iron coordination sites [45]. Low spin hemes are six-coordinated, which means that the six coordination sites of the heme iron are occupied by intrinsic ligands. In contrast, high spin heme compounds are formally five-coordinated, leaving a coordination site open for the binding of extrinsic ligands [19]. X-ray structures of model compounds and proteins have shown that the high-spin iron atom sits about 0.5 Å out of the porphyrin plane while the low-spin iron is in-plane [46,47]. Accordingly, high spin states have a relatively high energy and provide a more open microenvironment for electron transfer than low spin states. According to the observed transition to high spin states as shown in Figure 6, the active center of the heme ring is in a state that facilitates DET. In addition, the transition of the spin states is accompanied by significant nuclear reorganization [20] to optimize the orientation of the heme ring for DET, which is similar to the results from the UV-vis spectroscopy.


Mechanisms for the direct electron transfer of cytochrome c induced by multi-walled carbon nanotubes.

Zhao HZ, Du Q, Li ZS, Yang QZ - Sensors (Basel) (2012)

EPR spectra of Cyt c and Cyt c/MWCNTs.
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC3472837&req=5

f6-sensors-12-10450: EPR spectra of Cyt c and Cyt c/MWCNTs.
Mentions: As has been reported, the spin state equilibrium of the heme iron could modulate both substrate binding and the oxidation-reduction reactions of the cytochrome [19], and electron transfer is always accompanied by a change in iron spin state [20]. We speculate that the spin state influences the electron transfer activity. EPR spectra were utilized to investigate the conversion of the spin states, and the heme iron axial ligation is shown in Figure 6. As shown in Figure 6, the signals of the native Cyt c represent a mixture of high and low spin states, characterized at g = 6.19 and g = 4.36, respectively [17]. The signal at g = 1.98 is from the strong Fe-S ligation in the heme redox center. Fe-S ligation is important for the bioactivity of Cyt c, and damage to the Fe-S ligation will cause deactivation of the protein. When Cyt c interacts with MWCNTs, the Fe-S ligation signal nearly stay the same, suggesting that Fe-S ligation has not been destroyed, and the protein retains its bioactivity in this process. The low spin signal has a red shift to g = 4.45 along with a decrease in intensity while the high spin signal is observed at g = 6.26 along with an increase in intensity. This result suggests that the interaction between Cyt c and MWCNTs induces a transition of the heme iron from a low spin state to a high spin state. Cyt c has six heme iron coordination sites [45]. Low spin hemes are six-coordinated, which means that the six coordination sites of the heme iron are occupied by intrinsic ligands. In contrast, high spin heme compounds are formally five-coordinated, leaving a coordination site open for the binding of extrinsic ligands [19]. X-ray structures of model compounds and proteins have shown that the high-spin iron atom sits about 0.5 Å out of the porphyrin plane while the low-spin iron is in-plane [46,47]. Accordingly, high spin states have a relatively high energy and provide a more open microenvironment for electron transfer than low spin states. According to the observed transition to high spin states as shown in Figure 6, the active center of the heme ring is in a state that facilitates DET. In addition, the transition of the spin states is accompanied by significant nuclear reorganization [20] to optimize the orientation of the heme ring for DET, which is similar to the results from the UV-vis spectroscopy.

Bottom Line: There are several possible mechanisms that explain the DET of Cyt c.In the presence of MWCNTs, the secondary structure of Cyt c changes, which exposes the active site, then, the orientation of the heme is optimized, revolving the exposed active center to the optimum spatial orientation for DET; and finally, a transition of spin states is induced, providing relatively high energy and a more open microenvironment for electron transfer.These changes at different nano-levels are closely connected and form a complex process that promotes the electron transfer of Cyt c.

View Article: PubMed Central - PubMed

Affiliation: Department of Environmental Engineering, Peking University, Beijing 100871, China. zhaohuazhang@pku.edu.cn

ABSTRACT
Multi-walled carbon nanotube (MWCNT)-modified electrodes can promote the direct electron transfer (DET) of cytochrome c (Cyt c). There are several possible mechanisms that explain the DET of Cyt c. In this study, several experimental methods, including Fourier transform infrared spectroscopy, circular dichroism, ultraviolet-visible absorption spectroscopy, and electron paramagnetic resonance spectroscopy were utilized to investigate the conformational changes of Cyt c induced by MWCNTs. The DET mechanism was demonstrated at various nano-levels: secondary structure, spatial orientation, and spin state. In the presence of MWCNTs, the secondary structure of Cyt c changes, which exposes the active site, then, the orientation of the heme is optimized, revolving the exposed active center to the optimum spatial orientation for DET; and finally, a transition of spin states is induced, providing relatively high energy and a more open microenvironment for electron transfer. These changes at different nano-levels are closely connected and form a complex process that promotes the electron transfer of Cyt c.

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