(A) The catalytic peroxidase cycle for HRP, MPO and EPO. For MPO and EPO, compound I is reduced directly to the resting state via conversion of halide to hypohalite (red path). (B, C, D) Molecular modeling of carboxylated SWCNTs binding to the active sites of (B) HRP, (C) MPO and (D) EPO. Panel B reproduced from [21] Copyright American Chemical Society 2009, Panel C reproduced from [26] Copyright Macmillan Publishers, Ltd. 2010, and Panel D reproduced from [29] Copyright Wiley-VCH 2013.