Why call cytochrome p450
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why is it called cytochrome P450 origin of name
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The name cytochrome P450 breaks down into three parts, each with a specific meaning:
Breaking Down the Name
1. "Cyto-" (cell)
These enzymes are bound to cell membranes — primarily the smooth endoplasmic reticulum (microsomes) of liver, intestinal, and kidney cells. "Cyto" refers to this cellular (membrane-bound) localization.
2. "-chrome" (pigment/color)
From the Greek chroma (color). Cytochromes are heme-containing proteins that appear pigmented because the heme iron absorbs visible light. The "-chrome" suffix is shared by all cytochromes (a, b, c, P450, etc.).
3. "P450" — the defining feature
This is the most specific part of the name. The "P" stands for pigment, and 450 refers to the wavelength in nanometers (450 nm).
The origin: In 1962, Omura and Sato isolated a pigment from rabbit liver microsomes with a unique spectroscopic signature. When this protein was reduced (iron in Fe²⁺ state) and then complexed with carbon monoxide (CO), it showed a striking absorption peak at exactly 450 nm in its UV-visible spectrum. This Soret band at 450 nm is characteristic of the CO-bound, ferrous (Fe²⁺) form of the enzyme.
No other heme protein produces this exact 450 nm peak under these conditions — which is why it became the defining identifier.
Summary
| Part | Meaning |
|---|---|
| Cyto- | Cell membrane-bound |
| -chrome | Colored / heme-containing pigment |
| P | Pigment |
| 450 | Absorption maximum at 450 nm (when reduced + CO-bound) |
So the full name literally means: "a membrane-bound, heme-containing pigment that absorbs light at 450 nm."
Why the CO trick?
The CO-binding trick used by Omura and Sato is a clever spectroscopic technique — CO binds tightly to the ferrous (Fe²⁺) heme iron, shifting the Soret absorption band to a unique wavelength. For most cytochromes this shift lands around 420 nm, but CYP450 enzymes shift it to the distinctive 450 nm, which is what makes them spectroscopically identifiable and what gave them their name.
— Basic Medical Biochemistry: A Clinical Approach, 6e (Lippincott)
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