L6- Energy and redox signalling

Question 1

What is oxidation and reduction?

Answer 1

Oxidation is the loss of electrons from an element

Reduction is the gain of electrons from an element

Question 2

What is redox potential?

Answer 2

The ability or tendency of a species to be oxidised or reduced

Question 3

What is the redox potential of Zn, H and Cu?

Answer 3

Zn= -0.76V (strong reducer, easily oxidised)
H= 0V
Cu= 0.34V (strong oxidiser, easily reduced)

more positive means it is reduced

Question 4

How is redox potential measured?

Answer 4

The voltage generated when e- flow between a redox couple and the standard redox couple : H2/H+ +2e-

Question 5

What are the standard conditions?

Answer 5

pH= 0 (H+= 1mol/L)
H2 at 1atm pressure
25 degrees celcius

Question 6

What is the equation for energy released in a redox reaction?

Answer 6

Redox potential of species reduced- Redox potential of species oxidised (red-ox)

Question 7

Write out the key redox reactions and their redox potentials

Answer 7

NADH
NADPH
FADH2
H2O

Question 8

What is the equation to work out free energy change of a reaction?

Answer 8

ΔG°= 96.5 x Mol of e- x Redox potential difference

Question 9

What are the roles of redox reactions in cells?

Answer 9

1. Key cellular functions require transfer of electrons via oxidoreductase enzymes e.g ETC, nitric oxide synthase
2. Signalling- thiol switches- cysteine and methionine residues on proteins can be oxidised and reduced. Their oxidation states impact protein function and structure.
3. Damage due to free radicals and disruption of thiol networks

Question 10

What is the ultimate source of cell ‘reducing power’?

Answer 10

NADP+ + H+ + 2e- NADPH

E0= -0.32 V

Question 11

What is the purpose of the pentose phosphate pathway?

Answer 11

Generates NADPH and pentose sugars
Anabolic (unlike glycolysis)
NADPH is an important reducing substrate for processes involved in redox regulation

Question 12

What are the steps in the pentose phosphate pathway?

Answer 12

Glucose
Hexokinase
Glucose-6-phosphate
Glucose-6-phosphate dehydrogenase
Pentose sugars 
(NADPH and CO2)

Question 13

What are the main reactive oxygen species?

Answer 13

Superoxide: O2- • Highly reactive, membrane impermeable except anion channels

Peroxynitrite: ONOO-

Peroxide: H2O2 Less reactive, more mobile and permeable

Hydroxyl: OH• So reactive it can barely exist as an independent species

Question 14

What generates ROS?

Answer 14

When molecular oxygen reacts with electrons and it is catalysed by a number of oxidoreductase enzymes such as NADPH oxidase and nitric oxide synthase.

Question 15

What is NADPH oxidase?

Answer 15

Discovered as phagocytic NOX2 in neutrophils which produce ROS to kill pathogens
NOX1-5 involved in cell signalling and oxidant damage in disease

Question 16

What is nitric oxide synthase?

Answer 16

Normally electrons from NADPH pass through a flavoprotein chain to a haem complex where L-arginine is oxidised to L-citrulline and NO

ROS form due to lack of L-arginine or BH4 or oxidative damage

Question 17

What do ROS do?

Answer 17

• Oxidant stress- protein/DNA damage, gene transcription
• Normal cell signalling- growth, proliferation, gene transcription, ion channels
• Important modulators of cell redox state
• Powerful oxidising agents (proteins to change function) e.g oxidation of cysteine residues form disulphide bonds

Question 18

How is the cell protected from ROS?

Answer 18

Glutathione (antioxidant)
Superoxide dismutase (1&2)
Catalase (H2O2)
Thioredoxins and peroxiredoxins
Cytoprotective proteins e.g haemoxygenase

Question 19

What diseases do ROS cause?

Answer 19

Cardiovascular disease
Inflammation
Atherosclerosis
Diabetes

Question 20

How do ROS cause atherosclerosis?

Answer 20

Oxidised LDLs promote plaque formation and induce ROS production

Lipid accumulation, vascular damage, inflammation leads to upregulation of NOX isoforms, activation of xanthine oxidase and uncoupling of NO synthase leading to increased ROS

ROS and oxidised LDLs trigger cytochrome c release from mitochondria causing apoptosis of cells and ER stress response elements.