P450s

Question 1

What to tasks to p450’s usually help with?

Answer 1

Drug and lipid metabolism

Question 2

What is CYP51 important for?

Answer 2

• Important for cholesterol synthesis

- Metabolism of antifungal (triazole) drugs

Question 3

What is the general structure of a p450?

Answer 3

• Prismatically Shaped
• Heme center
• NEGATIVELY CHARGED cysteine interacting with heme iron
• Distinctive 10 amino acid loop at residue 400 (involves cysteine)

Question 4

T or F: p450s must undergo conformational changes in order for there active site to be exposed

Answer 4

True

Question 5

Where can p450s be found?

Answer 5

• ER membrane mostly

- Inner Mitochondrial membrane

Question 6

Describe the topology of p450s in the ER membrane.

Answer 6

Transmembrane domain (contains N-terminus) attached to a CYTOSOLIC domain which contains the heme and cysteine.

Question 7

Will p450s stay fixed to the ER membrane even if their transmembrane portion is removed?

Answer 7

Yes

Question 8

What are the 2 phases of p450 drug metabolism?

Answer 8

O2 is added to a lipophilic substrate in 2 phases

Phase I: non water soluble (lipophilic) drug is modified to have a hydroxyl

Phase II: a sugar or glutathione is added to make the drug more water soluble

Question 9

How do p450s accomplish the cleavage of a stable O=O bond?

Answer 9

• They used NADPH

- They require another protein to aid in electron extraction of NADPH

Question 10

Describe the steps of p450 metabolism in the ER.

Answer 10

• Another MEMBRANE bound protein containing FAD and FMN comes along to pass electrons sequentially (NADPH –> FAD –> FMN), then:

1. 2 electrons are extracted from NADPH
2. electron 1 is passed along to reduce the heme iron
3. OXYGEN now binds
4. electron 2 comes in and allows for oxygen cleavage
   a. 1 Oxygen gets added to substrate (drug)
   b. 1 Oxygen gets turned into H2O

Question 11

Describe the steps of p450 metabolism in the Mitochondria.

Answer 11

• TWO CYTOSOLIC proteins are needed:
  1. FERRODOXIN REDUCTASE [containing FAD (for 2 electron extraction, w/ one electron passing)]
  2. FEREDOXIN [ Fe/S containing protein]

1. 2 electrons are extracted from NADPH
2. electron 1 is passed along to reduce the heme iron
3. OXYGEN now binds
4. electron 2 comes in and allows for oxygen cleavage
   a. 1 Oxygen gets added to substrate (drug)
   b. 1 Oxygen gets turned into H2O

Question 12

What does it mean that p450s can be induced?

Answer 12

• They can be turned on to detoxify new substances encountered in the environment
• Lots of active p450 means you’re probably encountering lots of harmful things causing p450 induction

Question 13

What p450 that is prone to polymorphism can be detected via a chip?

Answer 13

CYP2D6

Question 14

What p450 is most abundant in liver?

Answer 14

CYP3A4

Question 15

What is the result of CYP2D6 polymorphism?

Answer 15

• Entire gene is deleted

- You can’t metabolize codeine

Question 16

What causes most drug interactions?

Answer 16

2 different Drugs being metabolized by the same p450s often causes interaction

Question 17

Suppose you take drug A and drug B and both are metabolized by CYP2D6; however B has a lower Km. Which drug will you overdose on?

Answer 17

• You will overdose on drug A because it doesn’t get metabolized and just enters straight into the bloostream

Question 18

Why would you not want to (or want to) drink grapefruit juice before taking oxicontin?

Answer 18

• Grapefruit juice blocks p450s like CYP2D6 meaning the drug is not as heavily metabolized and more gets taken up into the blood.