Lecture 8 - Cytochromes P450 1 Flashcards
What is the most important family in phase 1 metabolism ?
cytochrome P450 ENZYMES
WHERE are cytochrome p450 enzymes found ?
ER and mitochondria (Eukaryotes)
what do p450s contain and what do they form?
- contain sinle haem molecule as prosthetic group
- forms complex that shows maximum absorption at 450nm when reduced and CO added
describe the history prior to 1960
Known that liver endoplasmic reticulum was the main site of xenobiotic
metabolism.
Requirement for NADPH and O2.
Possibly one or two enzymes
involved
describe the history 1960 to 1980
- Cytochrome P450 identified spectroscopically ie if reduced and CO present,
see peak at 450 nm - Some evidence for multiple forms of the enzyme
- Reconstitution experiments
describe the history 1980 onwards
- Purification of proteins and cloning of genes
- At least 50 different human P450s now known
describe the history 2000 onwards
X-ray structures of human P450s
what does the nomenclature refer to cytochrome P450?
Use root CYP followed by family, subfamily and form number e.g.
CYP2D6
less than __% homolgy bwtween different families
(10 fams)
40
e.g. CYP1 CYP2 CYP3
In sub family how much homology ?
between 40-70%
e.g of familues that have large number of subfamies
family 2 (2A to 2W)
how much homology between different isoforms?
70-95%
what percent homnology between allelic varuents ?
more than 95%
give 3 functions of cytochromes p450s
- xenobiotic metabolism
- steroid , fatty acid and vitemin oxidation
- steroid biosynthesis
where does xenobiotic metabolism and steroid fatty acid anf vitemin oxidation occur?
- endoplasmic reticulum
Cytochrome P450 enzymes in ______ help make steroids from cholesterol. e.g _____
mitochondria
Aromatase
how many sub families do CYP1,2,3 have ?
1= 2 Xenobiotic metabolism
2= >10 Xenobiotic and steroid
metabolism
3= 1 Xenobiotic and steroid
metabolism
monomeric proteins of molecular weight cytochom p450s ?
40,000 - 50, 000
give cytochrome p450 structure
- Contains haem (iron Fe³⁺) as a key part.
- Iron binds 4 nitrogens + cysteine + water (when no substrate).
- Haem & oxygen binding regions = conserved; substrate binding = varies.
- Crystal structures of most human P450s are now available.
explain the conserved areas in p450 ?
Haem-binding region (important for enzyme function)
Oxygen-binding site (helps with metabolism)
Stay similar across different P450 enzymes, while substrate-binding areas vary.
3 components needed for cytochrome p450 reaction
- cytochrome p450 enzyme
- NADPH - cytochrome p450 reductase
- phosphatidylcholine
explain properties of NADPH-cytochrome p450 reductase
- transfer electrons to P450 enzymes in the ER
- flavoprotein with FAD and FMN prosthetic groups
- binds P450 via electrostatic interactions
- mitochondrial p450s use adrenodoxin instead
explain cytochrome P450 induction
- p450 enzymes increase with cirtain xenobiotics
- more mRNA & protien = faster drug metabolism
- different inducers affect specific P450 types
example- PAHs , barbiturates
functions of CYTOCHROME P450S
- inactivate many drugs - ^ rate of exctretion
- actibvate carcinogens
- produce toxic molecules from certain harmless drugs e.g paracetamol
- activate prodrugs e.g. cyclophosphamide
describe tissue distribution of cytochrome p450
highest levels in liver
then lower in kidney , lung , intestine , adrenals and brain
- some forms detected mainly in extrahepatic tissue e.g. CYP1A1 , some steroid biosynthetic
give 4 examples of reactions catalysed by P450s
N-Dealkyation
O-dealkylatin
Aliphatic hydroxylation
Aromatic hydroxylation
