Oxygen activation and redox partner binding in cytochromes P450

The structural underpinnings of O₂ activation in P450s have been limited by the inability to trap unstable oxy complexes in the crystalline state for structure determination. To date there are only two known oxy P450 structures. Even so, much is known about O₂ activation in P450cam and the role that redox partner binding plays in this process. Of particular importance are changes in the I helix associated with O₂ binding that enable "catalytic" waters to enter the active site and establish an H-bonding network essential for cleavage of the O--O bond. The changes in the I helix are similar to differences in the substrate-bound (closed) and substrate-free (open) conformations. With this information in hand, we have solved the structure of the substrate-free form of P450cin as well as the nitric oxide complex as a mimic of the oxy complex. This information provides some insight into the similarities and differences between P450cin and P450cam in O₂ activation.