The genes encoding human manganese superoxide dismutase (MnSOD) and Vitreoscilla hemoglobin (VHb) were fused inframe to generate a bifunctional enzyme that possessed MnSOD and peroxidase-like activities. At neutral pH, the coupling of the SOD and peroxidase reactions revealed that the bifunctional enzyme exhibited a 2.5 times shorter transient period and a 1.67 times higher reaction rate at steady-state conditions. Furthermore, the catalytic rate of the bifunctional enzyme was not affected as much by the external H2O2 scavenger catalase. This indicates that the bifunctional protein possesses a greater antioxidant capability, which is possibly due to the close proximity between the active site of MnSOD and the heme moiety of VHb. Our findings not only provide insight into the synergistic functions of SOD and peroxidase but also could potentially be used to develop novel therapeutic agents with more efficient O-2 carrying capability. (C) 2010, The Society for Biotechnology, Japan. All rights reserved.