Energy Production: Carbohydrate 4

Question 1

What is pyruvate dehydrogenase? Why is it important?

Answer 1

It’s an enzyme that converts pyruvate into acetyl CoA.

Question 2

What is essential for the conversion from pyruvate to acetyl CoA?

Answer 2

Pyruvate + CoA + NAD+ -> Acetyl CoA + CO2 + NADH + H+

Question 3

Where does the reaction of pyruvate to acetyl CoA take place?

Answer 3

Mitochondrial matrix

Question 4

Why is this an important step?

Answer 4

Because it is irreversible

Question 5

What is a dietary factor that is important for this reaction?

Answer 5

B-vitamins are used as cofactors. So a Vitamin B1 deficiency means that there is a build up of pyruvate.

Question 6

What are some factors that stimulate pyruvate dehydrogenase?

Answer 6

Pyruvate
CoASH
NAD+
ADP and AMP
Insulin

Question 7

What are some factors that inhibit pyruvate dehydrogenase?

Answer 7

Acetyl CoA
NADH
ATP
Citrate
Glucagon?

Question 8

What are the committing steps in TCA cycle?

Answer 8

Isocitrate (C6) going to C5. Here NAD+ goes to NADH and CO2 is also released.
Also C5 to C4 which is the next step. Here CoA is inserted but NAD+ goes to NADH and CO2 is released.

Question 9

What makes isocitrate to C5 and C5 to C4 be irreversible steps?

Answer 9

The release of CO2.

Question 10

What are the products of the TCA that we are interested in that are produced by a glucose molecule?

Answer 10

6 NADH
2 FADH2
2 GTP

Question 11

But only half of those products are produced in the TCA at one time. How can this be the case?

Answer 11

Because in glycolysis glucose goes to two pyruvates. This mean that 2 acetyl CoA are formed and the TCA cycle is done twice.

Question 12

Where does regulation occur in the TCA? What enzymes are affected?

Answer 12

In the committing steps aka Isocitrate to C5 and C5 to C4. Isocitrate dehydrogenase and alpha-ketoglutarate dehydrogenase respectively are affected.

Question 13

What inhibits or stimulates isocitrate dehydrogenase?

Answer 13

Inhibits:
ATP and NADH
Stimulates:
NAD+, ADP and AMP

Question 14

What inhibits or stimulates alpha-ketoglutarate dehydrogenase?

Answer 14

Stimulates:
NAD+ and maybe CoA
Inhibits:
NADH
ATP
Succinyl-CoA

Question 15

TCA is not only used to make a lot of reducing powers for energy. What else is it used for?

Answer 15

TCA supplies biosynthesis.
Citrate -> Fatty acids
alpha-ketoglutarate -> amino acids
Succinate -> amino acids and Haem
Malate -> amino acids
Oxaloacetate -> amino acids and glucose

Question 16

Give a brief summary of TCA

Answer 16

It’s a catabolic pathway for sugars, fatty acids, ketone bodies, amino acids and alcohol that happens in the mitochondria.
It releases CO2 by breaking carbon bonds.
Produces 6 NADH, 2 FADH2 and 2 GTP.
It also produces precursors for biosynthesis so not only catabolism but also anabolism.
It does not function in absence of O2.

Question 17

What is stage 4 in catabolism?

Answer 17

This is the electron transports chain and oxidative phosphorylation.

Question 18

Where and what happens in electron transport chain and oxidative phosphorylation?

Answer 18

It happens in the mitochondrial matrix, inner mitochondrial membrane and inter membrane space.
Here NADH and FADH2 are re-oxidised.
It requires O2 to work.
Produces a large amount of ATP.

Question 19

Briefly explain the electron transport chain.

Answer 19

NADH + H+ donates 2H+ and 2 electrons.
The donation of 2H+ happens three times. Only 2 electrons are used per cycle. The electrons go through 3 proton translocating complexes which makes the 6H+ to be transported from the mitochondrial matrix to the inter membrane space. The low energy 2 electrons then react with 2H+ and oxygen to form H2O.

Question 20

Why would protons be pumped out into the intermembrane space?

Answer 20

Because it sets up a concentration gradient of H+ called the proton motive force. This makes H+ to be transported into the matrix again via ATP synthase which makes ADP and inorganic phosphate to make ATP. This is the basis of ATP synthesis and called oxidative phosphorylation.

Question 21

How can oxidative phosphorylation be regulated?

Answer 21

By an increased concentration in ATP. This stops ADP and inorganic phosphate to convert into ATP. This means that H+ stops being transported into matrix again and there is a build up of H+ in the intermembrane space. This causes the electron transport chain to stop pumping out H+ into the intermembrane space as well.

Question 22

What gives most moles of ATP, NADH or FADH2?

Answer 22

NADH.

Question 23

How many moles of ATP do 2 moles of NADH give?

Answer 23

2 moles give 5 moles of ATP. Ideally it would give 6.

Question 24

How many moles of ATP do 2 moles of FADH2 give?

Answer 24

2 moles give 3 moles of ATP. Ideally it would give 4.

Question 25

What inhibits (not regulates) oxidative phosphorylation?

Answer 25

Cyanide (CN-) prevents acceptance of electrons by O2. Also carbon monoxide. This is because Cyanide binds with higher affinity.
Uncouplers as well.
Oxidative phosphorylation diseases

Question 26

What are uncouplers? Give examples. How does it work and why is it important in oxidative phosphorylation?

Answer 26

Uncouplers are fatty acids, dinitrophenol and dinitrocresol.

They increase the permeability of the inner mitochondrial membrane for H+.

Question 27

Why do we not want an increased permeability of the inner mitochondrial membrane for H+?

Answer 27

Because that means that H+ can cross the inner mitochondrial membrane from the intermembrane space to the matrix without the use of ATP synthase. This means that there will be less ATP synthesis.

Question 28

Where are uncouplers used as a favourable pathway?

Answer 28

In brown adipose tissue it is used to generate heat instead of ATP.

Question 29

Briefly explain the pathway in brown adipose tissue.

Answer 29

In the response to cold noradrenaline activates Lipase which releases fatty acids from triacylglycerol.
Fatty acid oxidation occurs which produces NADH and FADH2 to be fed into electron transport chain.
Fatty acids activate UCP1 which is an uncoupler.
UCP1 increases the permeability and transports H+ back into mitochondria once again. This means less that ATP synthesis does not happen but instead energy is released as heat.