Some Questions on Structure-Function Relationships

Spectral similarities between P-450 and chloroperoxidase originally led to suggestions that both enzymes had thiolate ligation [20, 22, 42]. However, the two systems displayed clear differences in their catalytic activities. Furthermore, at the time when EXAFS studies of chloroperoxidase were initiated, it was not clear whether the enzyme had a free (non-disulfide linked) cysteine available to coordinate to the heme iron [100]. Also, the unusually low pH optimum of the chloroperoxidase halogenation reaction, pH 3.0 for peroxidative formation of a carbon-halogen bond [42], raised questions concerning possible protonation of the axial heme ligand(s). 

The precise structural identities of the high-valent oxo-iron states of heme systems are also of interest. Recent work by Mayer [101] indicates that the Fe =0 bond of an oxo iron (IV) complex is 0.1 A longer than expected in comparison with other metal oxo complexes. The oxygen-binding respiratory proteins hemoglobin and myoglobin will each form a relatively stable derivative 

---