Q8(c). Active sites and functions of cytochrome c and cytochrome P-450

Cytochrome c

1. Coordination sphere: Fe in a haem c (protoporphyrin IX bound to the protein through two thioether links to cysteines). Four porphyrin N in the plane; axial ligands His-18 (imidazole N) and Met-80 (thioether S): six-coordinate, low-spin in both oxidation states.
2. Both axial positions are occupied by the protein, so O₂ cannot bind: the iron is built for electron transfer, not for ligand binding.
3. Function: a small, water-soluble one-electron carrier in the mitochondrial respiratory chain: Fe^(III)(cyt c) + e⁻ ⇌ Fe^(II)(cyt c), E^(°′)=+0.26 V. It takes electrons from complex III (cytochrome bc₁) and passes them to complex IV (cytochrome c oxidase), which reduces O₂ to water.
4. Fast electron transfer is possible because the low-spin Fe changes very little in size or geometry between Fe(III) and Fe(II) (small reorganisation energy). Released into the cytoplasm, cytochrome c also signals cell death (apoptosis).

Cytochrome P-450

1. Coordination sphere: haem b (protoporphyrin IX) iron with four porphyrin N; the proximal axial ligand is a cysteine thiolate (Cys-S⁻); the distal sixth site holds water in the resting Fe(III) state and is the site where O₂ binds.
2. The name comes from the intense Soret band at 450 nm of the Fe(II)–CO complex; the strongly donating thiolate shifts it from the usual 420 nm.
3. Function: a monooxygenase (hydroxylase): it inserts one O atom of O₂ into a C–H bond and reduces the other to water: RH + O₂ + 2H⁺ + 2e⁻ → ROH + H₂O. Electrons come from NADPH through a reductase (or an Fe–S protein).
4. Catalytic cycle: substrate binds and displaces water (Fe(III) becomes high spin) → one-electron reduction to Fe(II) → O₂ binds → second electron and two H⁺ → O–O cleavage, water leaves, giving the Fe(IV)=O porphyrin radical (compound I) → it abstracts H from R–H and the OH “rebounds” to R° → ROH leaves, Fe(III) is restored.
5. The thiolate's strong electron donation (“push”) helps the O–O bond to break heterolytically and makes compound I powerful enough to hydroxylate unactivated alkanes.
6. Roles: in the liver P-450 enzymes oxidise drugs and toxins to more soluble compounds for excretion (detoxification); they also make steroid hormones from cholesterol and convert camphor (bacterial P-450_(cam)).

Application. P-450 metabolism decides how long a drug stays in the body; one drug that blocks a P-450 enzyme can raise the blood level of another (drug–drug interaction), and some pro-drugs and carcinogens are activated by P-450 oxidation.