Heme oxygenase-2 (HO-2), which degrades heme into biliverdin, free iron, and CO, is responsible for cerebral vasodilatation. HO-2 is composed of a structured region including the heme-binding site and a C-terminal disordered region. The C-terminal region has two characteristic cystein residues, which the isozyme HO-1 lacks. It is shown that the C-terminal region including the cystein residues is engaged in the regulation of the enzyme activity. However, the role of the C-terminal region in the activity is elusive, and is not easy to explain by the crystal structure alone, since it is disordered. We hypothesized that the C-terminal region transiently interacts with the structured region to regulate the structural dynamics responsible for the activity. To test this, we have examined the relationship between the activity and structural dynamics of HO-2 using two constructs with and without the C-terminal region, referred to as long HO-2 and short HO-2, respectively.
The heme-degradation activity assay showed that long HO-2 had 1.5 times higher activity than short HO-2. To explain the difference in the activity, we performed the PRE, relaxation dispersion, and CLEANEX-PM experiments. As expected, the PRE experiment indicated that the C-terminal region weakly interacts with the structured region. Interestingly, the relaxation dispersion and CLEANEX-PM experiments showed that many residues in the structured region fluctuate, and some of them are unfolded transiently. It should be noted that the residues affected by PRE from the C-terminal region are also shown to fluctuate by the relaxation dispersion and CLEANEX-PM experiments. These results suggest that the change of the fluctuating mode by the interaction with the C-terminal disordered region regulates the HO-2 activity.