Lecture 5: Redox reactions

Question 1

Which of oxidation/reduction is gaining/losing electrons?

Answer 1

OIL RIG
Oxidation is Losing electrons
Reduction is Gaining electrons

Question 2

What is the Reduction/Redox Potential and what is it measured in?

Answer 2

The reduction/redox potential is a measure of the tendency of a chemical species to acquire electrons and thus be reduced. It is measured in Volts (V), millivolts (mV) or E (1 E = 1 mV).

Question 3

What does a more positive reduction potential indicate?

Answer 3

The more positive the reduction potential, the greater the species affinity for electrons and the greater the tendency to be reduced.

Question 4

What is the standard reduction potential?

Answer 4

The standard reduction potential, E0, is measured under standard conditions and is defined relative to a standard hydrogen electron reference electrode, which is arbitrarily given a redox potential of 0.00 volts.

Question 5

What is the significance of the difference between the redox potentials of two substances?

Answer 5

The greater the difference between the redox potential of two substances (delta E), the greater the ‘power’ with which electrons will flow spontaneously from the less positive to the more positive substance.

Question 6

Which of our equations contains the redox potential?

Answer 6

Delta G = -nF delta E

Question 7

When delta E is positive, is delta G positive or negative?

Answer 7

Negative.
Spontaneous:
+ve E
-ve G

Question 8

Give the redox active centre(s) which accept 2 electrons at once.

Answer 8

NAD+

Question 9

Give the redox active centre(s) which accept 2 electrons, one at a time, forming an intermediate semiquinone.

Answer 9

FAD, FMN, Coenzyme Q

Question 10

Give the redox active centre(s) which which only accept 1 electron.

Answer 10

FeS, Copper, Cytochromes

Question 11

What does FMN stand for?

Answer 11

Flavin mononucleotide

Question 12

Describe the structure of Coenzyme Q.

Answer 12

Coenzyme Q has a hydrophobic aromatic ring and also has a long hydrophobic chain made of repeating isoprene units. Often has 10 isoprene units, so often is abbreviated to Q10.

Question 13

Where is Coenzyme Q found in the cell and how does its structure suit this?

Answer 13

Coenzyme Q is found in the centre of the inner mitochondrial membrane. It is largely hydrophobic with an aromatic ring and repeated-isoprene chain. It is not water soluble, but is instead lipid soluble, so it is energetically favourable for it to be situated inside the membrane. It is free within the membrane and can move around/along the membrane in the ETC.

Question 14

Give the various names for Coenzyme Q.

Answer 14

Coenzyme Q, ubiquinone, quinone, CoQ, Q10, ubidecarenone.

Question 15

Give Coenzyme Q as a quinone, semiquinone and quinol.

Answer 15

quinone: CoQ
semiquinone: CoQH
quinol: CoQH2

Question 16

Give some detail about Iron-Sulphur centres.

Answer 16

FeS centres are prosthetic loops with between 1 and 4 iron atoms and can be in many different arrangements, with the iron complexed to sulphur. Enzymes in the ETC may contain several FeS centres.

FeS centres accept one electron, regardless of the number of iron atoms.

Question 17

Give some detail about cytochromes.

Answer 17

Cytochromes are proteins with heme (haem) prosthetic groups. They absorb light at characteristic wavelengths. They can be type a, b or c. The absorbance changes upon oxidation/reduction of the heme iron.
Some cytochromes are part of a large integral membrane complex, but cytochrome c is a small, water-soluble protein with a single heme group found in the inter-membrane space. The departure of cyt c from the inter-membrane space acts as a signal for apoptosis.

Question 18

What is a heme?

Answer 18

A prosthetic group of cytochromes containing an iron atom in a porphyrin ring systme. The Fe is bonded to 4 N atoms of the porphyrin ring.

Hemes in the 3 classes of cytochrome (a, b, c) differ slightly in substituents on the porphyrin ring system. A common feature is 2 propionate side chains.

In cytochromes, it is only the Fe (redox active centre) to be oxidised/reduced (Fe2+/Fe3+).