Cytochromes P450

            The Cytochromes P-450s are enzymes found in bacteria, fungi, and throughout the plant and animal kingdoms. All of these oxidizing/reducing enzymes include a very conserved cysteine containing region near the carboxy termini (from yeast to humans) which serves as an axial S-donor ligand to the heme Fe and gives it a unique reactivity. The term P450 comes from a unique absorbance spectra (with a peak at 450 nm) characteristic of the CO-adducts of Fe hemes with S-thiolate axial ligands.
P450s are large, multidomain proteins typically with a single active heme site. Different domains, often seperate subunits, are used to bind the redox partners, store reducing equivalents and act as controls of the enzymatic activation.
The heme active site is buried in P450s, you can just barely see the yellow of the cofactor in this spacefilling view. Thus the protein controls what substrates can get into it, as well as the timing of electron reducing equivalents needed for the catalysis to proceed.
The efficiency of the substrate oxygenation depends on the structural properties of the active site of the enzyme. The binding sites for the substrates is the hydrophobic domain of the polypeptide chain in the distal pocket above the heme the group.

 
In humans, the cytochrome P-450 active site performs a number of vital functions: it is "O+" agent involved in steroid synthesis; it oxygenates arachidonic acid to start the chemical signaling cascade which initiates a wide range of biological processes; it converts fat-soluble compounds (xenobiotics) ingested from plants, drugs, or environmental contaminants into more water soluble analogues. Often these xenobiotics are oxidized to more reactive species; this is thought to be a mechanism of carcinogenisis.
How it works (you'll need to learn this cycle):
 
Substrate binding induces the loss of the FeIII-bound H2O group, and increases the redox potential of the heme iron (from -300mV to a more positive -170mV). This allows the cytochrome to be easily reduced. Thus conversion from FeIII to FeII occurs only after binding to a substrate. Once the heme is reduced, a rapid series of steps produce the oxygenated product. First, the FeII state binds dioxygen, then a second electron equivalent induces the heterolytic cleavage of dioxygen resulting in ferryl/porphyrin radical and the release of water, as shown below; there are still many questions about how this occurs.
The ferryl/radical state then initiates the O-atom transfer, and for alkanes this results in an overall insertion of an O-atom into a C-H bond. The actual pathway for this last step of the cycle is still much debated. We will go over it in detail in class. But the protein obviously needs to protect and control such a highly reactive species.
Siroheme Proteins

Other similar heme proteins, with the Fe bound to axial thiolate ligands, are know which perform other funcitons: one important class is the siroheme proteins, which have an Fe4S4 cluster bound to the heme by a bridging thiolate ligand. These proteins are found in plants and function to reduce nitrite and sulfite to ammonia and hydrogen sulfide as feedstock for needed protein synthesis. Read about our models for these enzymes in the Farmer Group webpage.