L8: CYP450 enzymes

Question 1

What is the main function of cytochrome P450 enzymes?

Answer 1

Phase I metabolism — they detoxify or activate drugs by oxidation

Question 2

Where are CYP450 enzymes mainly found in the cell?

Answer 2

Endoplasmic reticulum (ER) and mitochondria

Question 3

What gives CYP450 enzymes their name?

Answer 3

They form a pigment absorbing at 450 nm when reduced and exposed to CO

Question 4

What is the prosthetic group in CYP450 enzymes?

Answer 4

A single haem molecule with Fe³⁺ coordinated to four nitrogens and one cysteine residue

Question 5

Outline the CYP450 enzyme reaction mechanism.

Answer 5

RH + NADPH + O₂ → ROH + NADP⁺ + H₂O

Question 6

What are the four parts of CYP450 nomenclature?

Answer 6

Family (CYP1)
Subfamily (CYP1A)
Isoform (CYP1A2)
Allele (CYP1A2*3)

Question 7

What percentage homology separates CYP450 families?

Answer 7

Less than 40%.

Question 8

What is the percentage homology within CYP450 subfamilies?

Answer 8

40–70%.

Question 9

What is the role of NADPH-CYP450 reductase?

Answer 9

Transfers electrons from NADPH to CYP450 using FAD and FMN

Question 10

What anchors CYP450 enzymes into the ER membrane?

Answer 10

Phosphatidylcholine

Question 11

How is mitochondrial CYP450 electron transfer different?

Answer 11

It uses adrenodoxin instead of FMN/FAD

Question 12

What are common reactions catalyzed by CYP450?

Answer 12

N-dealkylation, O-dealkylation, aliphatic & aromatic hydroxylation, deamination, oxidation

Question 13

What are some inducers of CYP450 enzymes?

Answer 13

Polycyclic aromatic hydrocarbons (PAH), barbiturates

Question 14

What is a positive effect of CYP450 metabolism?

Answer 14

Inactivation of drugs → easier excretion

Question 15

What is a negative effect of CYP450 metabolism?

Answer 15

Activation of procarcinogens like benzo[a]pyrene and toxic metabolites from drugs like paracetamol

Question 16

Which CYP450s are involved in xenobiotic metabolism?

Answer 16

Families 1, 2, and 3

Question 17

Which CYP450s are involved in vitamin/fatty acid oxidation?

Answer 17

CYP4 and CYP26 (retinoic acid)

Question 18

Which CYP450 catalyzes steroid biosynthesis?

Answer 18

CYP27 (e.g., for oestrogen biosynthesis from cholesterol)

Question 19

Where are CYP450s most abundant?

Answer 19

Liver (also found in kidney, lung, intestine, adrenals, brain)

Question 20

Describe how the CYP450 catalytic cycle works step-by-step.

Answer 20

1. Drug (RH) binds to CYP450
2. NADPH donates 2 electrons and 2 protons → transferred via FAD and FMN to CYP450
3. Fe³⁺ in haem is reduced to Fe²⁺
4. Fe²⁺ binds O₂ → forms Fe²⁺–O₂ complex
5. Additional electrons/protons split O₂ → one O forms H₂O, the other adds -OH to the drug (ROH)
6. Final product: ROH + NADP⁺ + H₂O

Question 21

What is the function of FMN and FAD in the CYP450 reaction?

Answer 21

FMN and FAD are flavins that temporarily hold and shuttle electrons from NADPH to CYP450, enabling the reduction of the haem iron (Fe³⁺ → Fe²⁺) and activation of oxygen

Question 22

Why is Fe³⁺ important in CYP450 enzymes and not Fe²⁺ initially?

Answer 22

Fe³⁺ (oxidised) is more stable and prevents uncontrolled ROS formation. Fe²⁺ is too reactive and could generate toxic species if not tightly regulated

Question 23

Explain the role of phosphatidylcholine in CYP450 activity.

Answer 23

Phosphatidylcholine is a phospholipid from the ER membrane that helps correctly orient and anchor CYP450 enzymes within the membrane for optimal function

Question 24

Compare the sites of xenobiotic vs steroid CYP450 action.

Answer 24

Xenobiotic metabolism (CYP1–3): occurs mainly in the ER (microsomal CYPs)

Steroid biosynthesis (e.g. CYP27): occurs in mitochondria

Question 25

Explain how CYP450 enzymes activate prodrugs vs carcinogens.

Answer 25

Prodrugs: CYP450 converts inactive drugs to active therapeutic forms (e.g. cyclophosphamide) Carcinogens: CYP450 can unintentionally activate compounds like benzo[a]pyrene into DNA-binding carcinogens

Question 26

Why do different species have different substrate specificities in CYP450?

Answer 26

Although the haem and O₂-binding sites are conserved, the substrate-binding regions vary significantly between species, leading to different substrate recognition profiles

Question 27

What distinguishes microsomal vs mitochondrial CYP450s?

Answer 27

Microsomal: Found in the ER, use NADPH → FAD → FMN → CYP450 electron transfer Mitochondrial: Found in mitochondria, use NADPH → adrenodoxin reductase → adrenodoxin → CYP450

Question 28

Which of the following is true regarding CYP450 monooxygenase activity? A) Uses NADPH to directly bind substrate B) Requires flavins (FAD & FMN) to transfer electrons C) Only found in mitochondria D) Requires Mg²⁺ as a cofactor

Answer 28

B

Question 29

What feature allows identification of cytochrome P450 enzymes spectrophotometrically? A) Maximum absorbance at 450 nm when oxidised B) Maximum absorbance at 450 nm when reduced and bound to CO C) Minimum absorbance at 350 nm in reduced form D) Fluorescent emission at 420 nm

Answer 29

B

Question 30

Which of the following enzymes is most responsible for steroid biosynthesis in mitochondria? A) CYP3A4 B) CYP26 C) CYP27 D) CYP1A2

Answer 30

C

Question 31

he activation of cyclophosphamide into its therapeutic form is mediated by: A) Sulfotransferase B) CYP3A4 C) CYP2D6 D) CYP450 monooxygenase

Answer 31

D

Question 32

Which of the following correctly pairs CYP450 isoform and substrate type? A) CYP1 – steroid biosynthesis B) CYP2 – xenobiotic metabolism C) CYP3 – fatty acid metabolism D) CYP27 – detoxification

Answer 32

B

Question 33

During the CYP450 cycle, which component is first reduced by NADPH? A) CYP450 directly B) Adrenodoxin C) FAD in reductase D) Oxygen

Answer 33

C

Question 34

Which of the following is not a Phase I CYP450-mediated reaction? A) N-dealkylation B) Aromatic hydroxylation C) Glucuronidation D) O-dealkylation

Answer 34

C

Question 35

Which of the following best explains why CYP450 enzymes can handle a broad range of substrates? A) They use ATP-dependent catalysis B) The haem group is non-specific C) Substrate binding regions vary significantly D) The iron-sulphur cluster adapts conformation

Answer 35

C