Enhanced enzyme activity through electron transfer between single-walled carbon nanotubes and horseradish peroxidase

Abstract

Better understanding of electron transfer (ET) taking place at the nano-bio interface can guide design of more effective functional materials used in fuel cells, biosensors, and medical devices. Single-walled carbon nanotube (SWCNT) coupled with biological enzymes serves as a model system for studying the ET mechanism, as demonstrated in the present study. SWCNT enhanced the activity of horseradish peroxidase (HRP) in the solution-based redox reaction by binding to HRP at a site proximate to the enzyme's activity center and participating in the ET process. ET to and from SWCNT was clearly observable using near-infrared spectroscopy. The capability of SWCNT in receiving electrons and the direct attachment of HRP to the surface of SWCNT strongly affected the enzyme activity due to the direct involvement of SWCNT in ET.

Figure 1

MS spectra for the analysis of (a) free HRP digest; (b) the supernatant after the SWCNTs-COOH adsorbed and crosslinked to HRP, washed with 0.1% Tween 20, subject to trypsin digestion, and finally removed from the mixture; (c) the supernatant after the SWCNTs-COOH went through the same treatment as (b) except for crosslinking.

Figure 2

Intrinsic fluorescence spectra of 100 μg/mL (a) HRP or (b) apo-HRP at the present of 0, 5, 10, 20, 30, 40 μg/mL SWCNT-COOH in 25 mM phosphate buffer (pH 7.4). (c) Stern-Volmer plots for dependence of fluorescence quenching and SWCNT-COOH concentration.

Figure 3

Crystal structure of HRP (PDB: 1H5H) with the heme center shown in cyan and the two peptides close to the SWCNTs-COOH surface during adsorption displayed in yellow and purple.

Figure 4

Absorbance change of 25 μg/mL SWCNT-COOH at 1,050 nm with the addition of 0.6 μg/mL HRP followed by 20 μM H₂DCF.

Figure 5

Percent change of 1.2 μg/mL HRP activity by 10 μg/mL SWCNTs-COOH and absorbance of 25 μg/mL SWCNTs-COOH at 1,050 nm under different pHs. Ten μM H2DCF was used as substrate for HRP.

Figure 6

Docking of SWCNT close to the enzyme activity center enables the participation of SWCNT in ET between the reducing substrate and the enzyme, which results in the enhanced enzyme activity.