Oxidative Stress and Free Radicals

Question 1

Define oxidative stress

Answer 1

Essentially an imbalance between the production of free radicals and the ability of the body to counteract or detoxify their harmful effects through neutralization by antioxidants

Question 2

What is a free radical?

Answer 2

An uncharged molecule (typically highly reactive and short-lived) having an unpaired valency electron.

Question 3

What is a ROS and provide some examples

Answer 3

Reactive oxygen species (ROS) are chemically reactive chemical species containing oxygen. Examples include peroxides, superoxide, hydroxyl radical, and singlet oxygen.

Question 4

What is a reducing agent?

Answer 4

A substance that tends to bring about reduction by being oxidized and losing electrons

Question 5

How does normal oxygen exist in our environment?

Answer 5

As a biradical/triplet molecule, having 2 unpaired electrons

• makes it less reactive than singlet molecule of oxygen
• thermodynamically favourable but not kinetically favourable

Question 6

What happens to biradical oxygen if heated or irradiated?

Answer 6

The free electrons pair to form a singlet molecule which is highly reactive
- striking a match induces this change and thus causes a reaction that produces fire

Question 7

What structures can bind with biradical oxygen?

Answer 7

Transition metals e.g. iron and copper as used in the electron transport system

Question 8

Name the reactive oxygen species found within the body as a function of cellular activity, describe how each point gets to the next through the addition of electrons

Answer 8

Biradical oxygen (·O2·) + · = superoxide (O2·-)
Superoxide (O2·-) + · = Peroxide O2(2-) (not a free radical, is a ROS)
Peroxide + 2H+ = Hydrogen peroxide (H2O2)
Peroxide + · = Hydroxyl radical (HO·)

Question 9

Why are hydroxyl radicals the most dangerous?

Answer 9

By far the most reactive ROS and react so quickly with other molecules that enzymes do not have time to react with them. Almost immediate reaction upon formation of HO·.

Question 10

What is formed when H2O2 reacts with a transition metal such as Fe(2+) or Cu+?

Answer 10

H2O2 + Fe(2+)/Cu+ = OH(-) + OH

Question 11

Name the important ROS

Answer 11

Superoxide, Peroxides and hydroxyl radical

Question 12

What does ROS damage?

Answer 12

Nucleic acids, Proteins and lipids

Question 13

How do ROS affect nucleic acids?

Answer 13

• binding with the base can cause mutations e.g. cancer

- binding with the sugar can cause breaks in strands, which can lead to mistakes occurring during repair

Question 14

How do ROS affect proteins?

Answer 14

Most common reaction occurs as ROS react with proteins to form disulphide bridges which change structure of the protein and damage function
ROS produce bridges between cysteines that normally don’t produce a disulphide bridge

Question 15

Which lipids are most affected by ROS and how do they get affected?

Answer 15

Poly-unsaturated lipids

• ROS react with the H-C-H group linking chains of poly-unsaturated lipids which results in this lipid containing a free radical
• this causes a series of chain reactions as each free radical lipid binds with another stable lipid
• can be damaging particularly in the cell membrane as OH groups within the lipids allow for H2O entry into the hydrophobic centre of the membrane thus resulting in membrane damage

Question 16

What are the sources of ROS?

Answer 16

External sources;

• Ionising radiation (e.g. cosmic radiation)
  e. g. ionising radiaiton + H-O-H = H· + ·OH
• UV radiation
• general pollution

Internal sources

• Macrophages e.g. oxidative bursts
• Mitochondrial electron transport chain e.g. complex I and complex III

Question 17

What internal defences do cells have against ROS?

Answer 17

Enzymes and small antioxidant defences

Question 18

Describe the process by which enzymes defend against ROS

Answer 18

Superoxide (O2-·) 
Enzyme - Superoxide dismutase 
Enzyme results in dismutation, therefore...
Superoxide + Superoxide --> O2 + H2O2
H2O2 can still form hydroxyl radical
Enzymes - Gluthione (GSH) peroxidase and catalase
- GSH peroxidase = H2O2 + 2GSH = 2H20
- Catalase = H2O2 + H2O2 = 2H20 +O2

Question 19

Name the enzymes involved in ROS defence

Answer 19

Superoxide dismutase - 2 molecules of superoxide react, one is reduced the other oxidised, results in O2 and H2O2 thus still needs further enzymatic activity
Gluthione peroxidase - catalyses a reaction between H2O2 and GSH to produce 2H2O and GSSG
Catalase - causes the reaction of 2 H2O2 molecules to produce 2H2O and one O2

Question 20

What happens to GSSG after the GSH peroxidase reaction?

Answer 20

GSSG + NADPH catalysed by GSH reductase = 2GSH + NADP+

Question 21

What happens to hydroxyl radicals given that they are too reactive to be enzymatically cotnrolled?

Answer 21

Small antioxidant defences

• Antioxidant + ·OH = H-O-H + Antioxidant ·
• Antioxidant · is nonreactive as a result of delocalised electrons
• generally, more conjugated bonds result in higher stability as the electron can be at any point on the compound and thus is less likely to react

Question 22

Name some lipophilic and hydrophilic antioxidants

Answer 22

Lipophilic

• Vitamin E (Tocopherol)
• Coenzyme Q (CoQ)
• Beta-carotein

Hydrophilic

• GSH
• Vitamin C (Ascorbic acid)