Cytochrome P450 System Dynamics Insights

1. What is the resting state of the iron atom in the heme group of a Cytochrome P450 enzyme

Ferrous Fe2

Ferric Fe3

Ferryl Fe4

Metallic Fe0

In its resting state the heme iron is in the trivalent ferric form. It must be reduced to the divalent ferrous form later in the cycle to bind molecular oxygen.

Explanation

In its resting state the heme iron is in the trivalent ferric form. It must be reduced to the divalent ferrous form later in the cycle to bind molecular oxygen.

2. The Cytochrome P450 system is located primarily in the smooth endoplasmic reticulum of hepatocytes

True

False

These enzymes are membrane bound within the microsomes of the smooth ER which provides the lipophilic environment necessary for processing non polar drug molecules.

Explanation

These enzymes are membrane bound within the microsomes of the smooth ER which provides the lipophilic environment necessary for processing non polar drug molecules.

3. Which protein is the essential electron donor that transfers electrons from NADPH to the P450 heme iron

Cytochrome b5

NADPH Cytochrome P450 Reductase

Alcohol Dehydrogenase

Hemoglobin

The reductase contains FAD and FMN cofactors which act as a bridge to transfer two electrons from NADPH one at a time to the P450 enzyme.

Explanation

The reductase contains FAD and FMN cofactors which act as a bridge to transfer two electrons from NADPH one at a time to the P450 enzyme.

4. Which of the following are the primary requirements for a complete P450 catalytic turnover

Molecular Oxygen O2

NADPH

Substrate drug molecule

Glucuronic acid

The system is a monooxygenase. It requires the substrate to bind the iron to be reduced O2 to be activated and NADPH to provide the reducing equivalents.

Explanation

The system is a monooxygenase. It requires the substrate to bind the iron to be reduced O2 to be activated and NADPH to provide the reducing equivalents.

Submit

5. What is the specific axial ligand that coordinates to the heme iron and is responsible for the 450 nm spectral shift

Histidine

Cysteine thiolate

Methionine

Lysine

The sulfur atom of a conserved cysteine residue binds to the iron. This thiolate coordination is unique to P450s and determines their redox potential and spectral properties.

Explanation

The sulfur atom of a conserved cysteine residue binds to the iron. This thiolate coordination is unique to P450s and determines their redox potential and spectral properties.

6. The binding of a substrate to the ferric P450 enzyme triggers a change in the iron spin state from low spin to high spin

True

False

Substrate binding displaces a water molecule from the iron. This spin state shift makes the reduction of the iron by the reductase much more favorable.

Explanation

Substrate binding displaces a water molecule from the iron. This spin state shift makes the reduction of the iron by the reductase much more favorable.

7. What is the highly reactive oxygen species formed after the cleavage of the O O bond in the P450 cycle

Superoxide

Perhydroxyl radical

Iron oxo species Compound I

Singlet oxygen

Compound I is an Fe4 oxo porphyrin radical cation. It is powerful enough to abstract a hydrogen atom from a stable C H bond of a drug molecule.

Explanation

Compound I is an Fe4 oxo porphyrin radical cation. It is powerful enough to abstract a hydrogen atom from a stable C H bond of a drug molecule.

8. Which outcomes can occur during the P450 cycle if the activated oxygen species is not successfully transferred to a substrate

Production of superoxide

Production of hydrogen peroxide

Production of water uncoupling

Production of ozone

These are known as uncoupling reactions. They consume NADPH and O2 without metabolizing the drug and can lead to oxidative stress within the cell.

Explanation

These are known as uncoupling reactions. They consume NADPH and O2 without metabolizing the drug and can lead to oxidative stress within the cell.

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9. In the Cooper and Brodie nomenclature what does the P in P450 stand for

Protein

Pigment

Peroxidase

Portal

It was originally described as a pink pigment in liver microsomes that absorbed light at 450 nm when reduced and treated with carbon monoxide.

Explanation

It was originally described as a pink pigment in liver microsomes that absorbed light at 450 nm when reduced and treated with carbon monoxide.

10. One molecule of water is produced as a byproduct for every molecule of substrate hydroxylated by P450

True

False

P450 is a monooxygenase. One oxygen atom goes into the substrate and the other is reduced by two protons and two electrons to form H2O.

Explanation

P450 is a monooxygenase. One oxygen atom goes into the substrate and the other is reduced by two protons and two electrons to form H2O.

11. What is the effect of an enzyme inducer like phenobarbital on P450 dynamics

It inhibits protein synthesis

It increases the expression of P450 proteins

It changes the heme iron to copper

It makes the enzyme work at 100 degrees Celsius

Inducers increase the transcription of CYP genes. This leads to a higher concentration of enzymes in the liver which speeds up the metabolism of drugs.

Explanation

Inducers increase the transcription of CYP genes. This leads to a higher concentration of enzymes in the liver which speeds up the metabolism of drugs.

12. Which factors can lead to P450 inhibition

Competitive binding at the active site

Covalent modification of the heme group

Coordination of a lone pair to the heme iron

Increasing the concentration of NADPH

Drugs like ketoconazole inhibit P450 by coordinating their nitrogen atoms directly to the heme iron preventing oxygen from binding.

Explanation

Drugs like ketoconazole inhibit P450 by coordinating their nitrogen atoms directly to the heme iron preventing oxygen from binding.

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13. Which P450 isoform is responsible for metabolizing approximately 50 percent of all clinically used drugs

CYP2D6

CYP3A4

CYP1A2

CYP2E1

CYP3A4 is the most abundant P450 in the liver and intestines. Its large active site allows it to accommodate a vast range of different drug structures.

Explanation

CYP3A4 is the most abundant P450 in the liver and intestines. Its large active site allows it to accommodate a vast range of different drug structures.

14. Suicide inhibition refers to a drug being metabolized into a reactive species that then irreversibly binds to the P450 enzyme

True

False

Also known as mechanism based inhibition the enzyme essentially destroys itself by converting a pro inhibitor into a reactive group that bonds to the protein or heme.

Explanation

Also known as mechanism based inhibition the enzyme essentially destroys itself by converting a pro inhibitor into a reactive group that bonds to the protein or heme.

15. What is the primary role of Cytochrome b5 in some P450 reactions

It replaces the P450 reductase

It can provide a second electron to the P450 cycle

It acts as a structural scaffold

It prevents the binding of oxygen

While not always required Cytochrome b5 can act as an alternative electron donor for the second reduction step in the catalytic cycle.

Explanation

While not always required Cytochrome b5 can act as an alternative electron donor for the second reduction step in the catalytic cycle.

Enzyme Power: Cytochrome P450 System Dynamics Quiz

1. What is the resting state of the iron atom in the heme group of a Cytochrome P450 enzyme

2.

What first name or nickname would you like us to use?

2. The Cytochrome P450 system is located primarily in the smooth endoplasmic reticulum of hepatocytes

3. Which protein is the essential electron donor that transfers electrons from NADPH to the P450 heme iron

4. Which of the following are the primary requirements for a complete P450 catalytic turnover

5. What is the specific axial ligand that coordinates to the heme iron and is responsible for the 450 nm spectral shift

6. The binding of a substrate to the ferric P450 enzyme triggers a change in the iron spin state from low spin to high spin

7. What is the highly reactive oxygen species formed after the cleavage of the O O bond in the P450 cycle

8. Which outcomes can occur during the P450 cycle if the activated oxygen species is not successfully transferred to a substrate

9. In the Cooper and Brodie nomenclature what does the P in P450 stand for

10. One molecule of water is produced as a byproduct for every molecule of substrate hydroxylated by P450

11. What is the effect of an enzyme inducer like phenobarbital on P450 dynamics

12. Which factors can lead to P450 inhibition

13. Which P450 isoform is responsible for metabolizing approximately 50 percent of all clinically used drugs

14. Suicide inhibition refers to a drug being metabolized into a reactive species that then irreversibly binds to the P450 enzyme

15. What is the primary role of Cytochrome b5 in some P450 reactions