TCA and ETC

Question 1

What is the Citric Acid Cycle?

Answer 1

Or krebs cycle or TCA cycle.
Takes place in matrix of mitochondria.
Only happens in aerobic conditions.
Final common pathway for oxidation of fuel molecules - amino acids, fatty acids, glucose.

Question 2

What is pyruvate Dehydrogenase?

Answer 2

Acts on pyruvate in mitochondria.
Consists of 3 enzymes, and has vitamin B1 (thiamine) as cofactor.
E1, E2, and E3.

Question 3

What does pyruvate dehydrogenase do?

Answer 3

Catalyses the irreversible oxidative decarboxylation of pyruvate to produce Acetyl CoA, CO2 and NADH.
Happens pre-Krebs

Question 4

What is the energy expenditure of pyruvate dehydrogenase?

Answer 4

It strips out the CO2 from pyruvate and generates NADH as pyruvate binds to Acetyl CoA.
This allows it to enter the TCA cycle.
So no ATP generated here, but 2 NADH per glucose is generated, and overall, 4 NADH and 2 ATP are generated so far.

Question 5

What is energy requirement matching?

Answer 5

The source of pyruvate or other intermediate into TCA comes from different sources depending on what has been consumed most.

Question 6

What is the process of the TCA cycle?

Answer 6

Control half: Acetyl CoA interacts with 4C oxaloacetate to form 6C citrate.
CO2 released and NADH generated to form 5C metabolite.
More CO2 released and NADH generated to form 4C metabolite.
Regeneration half: Energy generated through ATP.
Succinate goes to oxaloacetate and generates FADH and NADH.

Question 7

What is the control half?

Answer 7

CO2 and NADH are slowly released.

Question 8

What is the regeneration half?

Answer 8

The molecules are going into the right form to make oxaloacetate to start the cycle again.

Question 9

What are the products of the TCA cycle?

Answer 9

Per cycle: 1ATP
3 NADH
1 FADH2
Per glucose x2

Question 10

What is substrate level phosphorylation?

Answer 10

A metabolite is acted on by an enzyme to generate ATP from ADP.
e.g. in glycolysis, or 1st step in TCA

Question 11

What is oxidative phosphorylation?

Answer 11

Generates NADH and FADH2 which generates 28-32 ATP molecules.
In the ETC chain and TCA cycle.

Question 12

How do NADH and FADH generate energy?

Answer 12

NADH and FADH create electron donors.
Uses the ETC and proton pumps to generate energy.

Question 13

What is the structure of the mitochondria?

Answer 13

Matrix - inside
Inner membrane
Outer membrane
Inner membrane space

Question 14

How do electrons move in the ETC?

Answer 14

Membrane proteins are electron acceptors from NADH and FADH2, then couple this to transfer protons across the membrane through complexes into intermembrane space.
Electrons then add to oxygen and protons to form water.

Question 15

Why does the TCA cycle only happen in aerobic conditions?

Answer 15

Oxygen is the terminal acceptor, without it H2O doesn’t form.

Question 16

What happens to protons in ETC?

Answer 16

High concentration in intermembrane space (low in mitochondria).
Protons interact with protein - ATP synthase.
Takes the proton gradient and allows protons to move through the protein and into mitochondria, and generates ATP.

Question 17

What are uncoupling protein 1?

Answer 17

Channel protein expressed in the mitochondria of adipocytes in brown adipose tissues.
Allows protons to cross inner mitochondrial membrane without generating ATP.
This generates heat.

Question 18

Why must UCP1 be regulated?

Answer 18

You do not want all protons to go through it because you need to generate ATP.

Question 19

What happens when there are plenty of food sources available?

Answer 19

Key intermediates such as ATP, NADH, Acetyl CoA and citrate accumulate.
ATP and citrate inhibit PFK allosterically causing Glucose 6-phosphate to accumulate.

Question 20

What does the accumulated G-6P do?

Answer 20

It can be switched to production of glycogen and pentoses by the Pentose Phosphate Pathway, which also produces NADPH.

Question 21

How can glycolysis be stopped early on when there is plenty of sources?

Answer 21

Acetyl units can be drawn from central pathway to synthesise fatty acids, because there is enough energy.

Question 22

What is acetyl polymerisation?

Answer 22

In excess energy source:
Acetyl-CoA and energy makes 4C Malonyl-CoA and ADP.
Malonyl-Acyl Carrier Protein and Acetyl CoA and 2NADPH makes Butyryl-CoA and CO2.
The fatty acids are then stored in liver or adipose tissue.

Question 23

What is required for acetyl polymerisation?

Answer 23

Acetyl units need transporting across the mitochondrial membrane as process happens in cytosol.
Acetyl CoA carboxylase is activated by citrate when lots of energy available, and when there’s lots of insulin can convert to fatty acids.

Question 24

What is acetyl Co-A carboxylase?

Answer 24

The enzyme responsible for acetyl polymerisation /fatty acid synthesis.
It requires biotin as a co factor, and NADPH.

Question 25

What is fat metabolism?

Answer 25

Fatty acids are released from triglycerides and into the central pathway.
This is catalysed by a hormone sensitive lipase, activated by a cAMP responsive kinase and then transported.

Question 26

When does fat metabolism occur?

Answer 26

When ATP and NADH are low, and ADP and NAD+ high.
It is stimulated by Epinephrine, and inhibited by insulin.

Question 27

What is B-oxidation?

Answer 27

Fatty acids are acetyl unit (2C) polymers.
Fatty acids can be transported into the mitochondria via carnitine shuttles.
CoAs, FAD and NAD are added to yield 8 acetyl CoA units which produces 131 ATP.

Question 28

When is B-oxidation used?

Answer 28

At low to moderate intensity exercise most energy requirements in muscles can be met from B-oxidation, only a small amount from glucose oxidation.
During short term intense exercise it comes from glucose.

Question 29

How are proteins metabolised?

Answer 29

Proteins are basically acetyl units with nitrogen attached.
The nitrogen becomes ammonia, which becomes urea, which is excreted.

Question 30

Where do amino acids enter the TCA cycle?

Answer 30

Can either become pyruvate, acetyl CoA, or at Succinate and Fumerate and other parts of the cycle.

Question 31

What is the limitation of amino acid metabolism?

Answer 31

Transamination:
Amino acids are converted to a-keto acid in the TCA cycle.
This uses a-ketoglutarate, which is converted to glutamate.
But a-ketoglutarate levels need maintaining so the TCA cycle can continue.

Question 32

How is transamination resolved?

Answer 32

Oxidative deamination: a-ketoglutarate is regenerated by releasing NH3 from amino acids.
NH3 reacts with CO2 to form urea, so the a-ketoglutarate can continue to TCA cycle.

Question 33

Where does deamination occur?

Answer 33

Mitochondrial matrix.

Question 34

When does deamination occur?

Answer 34

Activated by ADP and GDP (low energy)
Inhibited by ATP, NADH, GTP (high energy)

Question 35

How is urea removed?

Answer 35

The urea cycle in the liver.
CO2 and ammonia, uses energy and amino acid aspartate forms urea which can be excreted.
Also forms fumerate, which can go back to TCA, but uses ATP, so amino acids are a last resort.