Mitochondrial synthesis of ATP: Kreb's cycle and oxidative phosphorylation.

Question 1

What is the concentration of ATP in most cells?

Answer 1

6mM

Question 2

How much ATP does the average human body contain?

Answer 2

75g

Question 3

What is the average turnover of ATP per day?

Answer 3

75kg

Question 4

What proportion of the energy in food is converted to ATP and what proportion of that is converted to useful work?

Answer 4

50% and 50%

Question 5

Where is most of the body’s ATP synthesised?

Answer 5

Mitochondria

Question 6

What occurs in the mitochondrial matrix space?

Answer 6

Contains wide range of enzymes. Krebs cycle, fatty acid oxidation, urea cycle (in liver). High concentrations of substrates cofactors and ions. Contains mitobhondrial DNA, RNA and ribosomes.

Question 7

What is present in the mitochondrial intermembrane space?

Answer 7

Has metabolite and ion concentrations similar to cytosol. Contains cytochrome c.

Question 8

What is the link reaction?

Answer 8

Pyruvate -> acetyl CoA

Pyruvate transported across the inner membrane from cytosol to matrix and reaction catalysed by pyruvate dehydrogenase.

Question 9

Which vitamin is essential for the function of pyruvate dehydrogenase?

Answer 9

Thaimine (vit B1)

Question 10

What conditions are caused by lack of thiamine?

Answer 10

Beriberi - neuro/cardio symptoms

Wernicke-Korsakoff syndrome - in alcohol abuse.

Question 11

Why is pyruvate dehydrogenase a key decision point in metabolism?

Answer 11

Acetyl CoA cannot be converted back to glucose in humans. Conversion of pyruvate to acetyl CoA commits the C atoms of glucose to energy production or lipid synthesis.

Question 12

What inhibits pyruvate dehydrogenase?

Answer 12

High energy levels

Question 13

What are the two stages of the Kreb’s cycle?

Answer 13

1. Synthesis of 6-C compound (citrate) which then looses 2C as 2xCO₂ to become a 4-C compound (succinyl CoA).
2. Oxidation of 4-C compound to regenerate oxaloacetate and initiate another round of the cycle.

Question 14

What are the main outputs of the Kreb’s cycle?

Answer 14

Reduced coenzymes NADH and FADH₂ as electron carriers.

CO₂ - waste product.

1 GTP (=1 ATP)

Question 15

What are the control points in the citric acid cycle?

Answer 15

Pyruvate -> acetyl CoA - inhibited by ATP, acetyl CoA and NADH

Acetyl CoA -> citrate - inhibited by ATP, NADH, succinyl CoA and citrate

Isocitrate -> alpha-ketoglutarate - inhibited by ATP, NADH. Stimulated by ADP and NAD+

Alpha-ketoglutarate -> succinyl-CoA - inhibited by ATP, NADH, and succinyl-CoA.

Question 16

What are key enzymes in the Kreb’s cycle?

Answer 16

Citrate synthase, isocitrate dehydrogenase and alpha-ketoglutarate dehydrogenase.

Question 17

Why do patients with type I diabetes synthesise ketone bodies?

Answer 17

Cannot use glucose effectively in the absence of insulin.

Gylcolysis inhibited and pyruvate low.

Gluconeogenesis is not inhibited - oxaloacetate and malate removed to form glucose.

In the absence of insulin fatty acids are used to supply acetyl CoA. Acetyl CoA levels high.

Lack of oxaloacetate prevents acetyl CoA from entering Kreb’s cycle so ketones synthesised instead.

Question 18

What is the last ATP synthesis step?

Answer 18

Oxidative phosphorylation.

Question 19

What are the two tightly coupled processed that make up oxidative phosphorylation?

Answer 19

Electron transport and ATP synthesis.

Question 20

What is electron transport?

Answer 20

The energy (reduction potential) of the electrons in NADH/FADH2 is used to create a proton gradient across the inner mitochondrial membrane - OXIDATION.

Question 21

How does ATP sysnthesis occur after oxidative phosphorylation?

Answer 21

Energy from the proton gradient is used to phosphorylate ADP to synthesise ATP - PHOSPHORYLATION.

Question 22

What are the membrane-bound components of the electron transport chain?

Answer 22

Complexes I-IV

Question 23

How does the electron transport chain work?

Answer 23

Electrons are transferred from one electron carrier to another.

Complex I - NADH -> NAD+

Complex II - FADH2 -FAD

Yields 2 electrons, both pass through complex 3, ubiquinone and cytochrome c

At complex IV they are donated to oxygen reducing it to water.

Energy is released from each step.

Question 24

Which step in the electron transport chain is blocked by cyanide?

Answer 24

O₂ + 2H -> H₂O at complex IV

Question 25

How is the energy from the electron transport chain used?

Answer 25

To power the proton gradient. Each pair of electrons = 10 electrons.

Question 26

How is ATP synthesised?

Answer 26

Protons flow through the F0F1 ATPase enzyme down the concentration gradient created by the electron transport chain. This provides the energy required to synthesise ATP.

Question 27

How is ADP transported into the mitochondrian while ATP is transported out?

Answer 27

Via an antiporter in the inner mitochondrial membrane.

Question 28

How is phosphate transported into the mitochondria?

Answer 28

Movement of one proton out.

Question 29

How many protons must move across the mitochondrial inner membrane for the synthesis of one ATP?

Answer 29

4

Question 30

What substrate cannot cross the inner membrane?

Answer 30

NADH

Question 31

What are uncouplers?

Answer 31

Weak acids which are soluble in the membrane. When they penetrate the inner mitochondrial membrane they diffuse freely. At inner membrane space they associate with protons, at matrix surface they release protons. This dissipates the proton gradient but electron transport can continue without ATP synthesis.

Question 32

What is the purpose of uncoupling?

Answer 32

When electron transport is uncoupled from ATP synthesis the energy is released as heat.

Question 33

How does brown adipose tissue work?

Answer 33

Contains lots of mitochondria which contain thermogenin (uncoupling protein 1). When core body temperature drops sympathetic nervous system releases noradrenaline, leads to increased concentration of free fatty acids in cytosol, which activate thermogenin.

Question 34

Is brown adipose tissue more prevelant in infants or adults?

Answer 34

Infants.