Free Radicals, Antioxidants, & Ascorbate (Module 3 Lecture 2)

Question 1

What is a molecule with an oxygen radical called?

Answer 1

ROS (reactive oxygen species)

Question 2

Oxygen’s structure favors additions of elections how?

Answer 2

One at a time, forming oxygen free radicals

Question 3

List 5 ROSs.

Answer 3

1. Superoxide (O2*)
2. Hydroxyl radical (OH*)
3. Nitric oxide (NO*)
4. Hydrogen peroxide (H2O2)
5. Hypochlorous acid (HOCl)

Question 4

Describe the pathway from oxygen to water that includes oxygen free radicals.

Answer 4

oxygen + e- –> superoxide + e- + 2H+ –> hydrogen peroxide + e- + H+ –> water + hydroxyl radical + e- –> water

Question 5

What is the most potent ROS?

Answer 5

hydroxyl radical OH*

Question 6

What are two nonezymatic pathways to forming a hydroxyl radical?

Answer 6

1. hydrogen peroxide + ferrous iron –> Fe3+ + OH- + OH* (Fenton rxn)
2. hydrogen peroxide + superoxide + H+ –> O2 + H2O + OH* (Haber-Weiss rxn)

Question 7

How do enzymes that use oxygen protect the body from ROS?

Answer 7

By keeping the ROS intermediates tightly bound at the active sites and only releasing ROSs in the form of H2O2.

Question 8

Name two types of enzymes that use O2

Answer 8

1. Oxygenase

2. Oxidase

Question 9

What do oxygenases do?

Answer 9

Incorporate oxygen directly into the substrate.

Question 10

What do oxidases do?

Answer 10

Transfer electrons to O2 to reduce it to H2O or H2O2

Question 11

Where is cytochrome c oxidase found?

Answer 11

In the mitochondrial cristae as Respiratory Complex IV (last step of e- transport chain)

Question 12

Name two types of oxygenases

Answer 12

1. Monooxygenases

2. Dioxygenases

Question 13

What is the difference between a monooxygenase and a dioxygenase?

Answer 13

Monooxygenases use one of the O2 atoms for the substrate and the other to oxidize a cofactor, whereas dioxygenase incorporates both oxygens into the substrate, usually making a highly reactive peroxide as an immediate product.

Question 14

Give one detailed example of a dioxygenase.

Answer 14

5-lipoxygenase, which makes 5-hydroperoxiarachidonic acid (5-HPETE) from arahidonic acid (20:4, omega 6)
Note: 5-HPETE is a precursor for active lipids like leukotrienes LTB4 and LTC4 used in inflammatory responses.

Question 15

Give one detailed example of a monooxygenase.

Answer 15

Cytochrome P450 superfamily of monooxygenases work to hydroxylate:
1. Steroids (by modifying cholesterol rings)
2. Fatty acids
3. Xenobiotics (drugs and foreign chemicals)
4. Carcinogens
Note: 450 in name denotes the wavelength nm at which the prosthetic group’s heme absorbs light on all enzymes in this superfamily.

Question 16

What is another name for monooxidases and why?

Answer 16

Mixed Function oxidases because they not only work on the substrate but also transfer O atoms to FADH2, NADH, NADPH, or other cofactors.

Question 17

Explain the two different types of products formed by oxidation from a monooxidase.

Answer 17

1. RH + O –> ROH (alcohol formation)

2. Water formation by removing two hydrogen atoms from substrate, forming a double bond, and adding the O.