TCA Cycle + Oxidative Phosphorylation

Question 1

Give an overview of the tricarboxylic acid cycle

Answer 1

• Mitochondrial
• Single Pathway
• Acetyl (CH3CO-) converted into 2CO2
• Oxidative (NAD+ and FAD required)
• Some energy produced as ATP/GTP
• Also produces precursors for biosynthesis
• 2 cycles for every glucose entering glycolysis
• Strategy- to produce molecules that readily lose CO2
• Central pathway in catabolism of sugars, fatty acids, ketone bodies, amino acids, alcohol

Question 2

How is the tricarboxylic acid regulated?

Answer 2

• Regulated by energy availability e.g. ATP/ADP ratio and also NADPH/NAD+ ratio
• Certain enzymes necessary e.g Isocitrate dehydrogenase and alpha- ketoglutarate dehydrogenase

Question 3

Describe the process of mitochondrial electron transport

Answer 3

• In the mitochondrial matrix FAD2H and NADPH release electrons to move through the PTC’s. 3 out of 4 PTC’s pump protons out of the matrix into inter membrane space. Electrochemical gradient produced, Proton Motive Force generated.
• Oxygen is final electron acceptor at PTC4

Protons can only move back into the matrix by ATPase. This is favourable when there is an electrochemical gradient achieved.

Question 4

What is uncoupling?

Answer 4

• Protons being pumped into inter membrane space and re-entering without passing through ATP synthase
• Energy lost as heat
• Proton leak accounts for 25% of BMR

Question 5

What are the 3 uncoupling proteins?

Answer 5

UCP1 : found in brown adipose tissue. contains thermogenin

UCP2: widely distributed in body, linked to metabolic syndrome

UCP3: found in brown adipose tissue, skeletal muscle and the heart. Protects against ROS damage

Question 6

Describe the action of noradrenaline, with respect to coupling

Answer 6

• neurotransmitter of sympathetic nervous system
• stimulates lipolysis to provide fatty acids (fuel) for oxidation in brown adipose tissue
• activates UCP1
• increased NADH and FADH2 levels (increased pmf so energy lost as heat)

Question 7

List the key products of the Krebs Cycle

Answer 7

• 6 NADH’s are generated
• 2 FADH2 is generated
• 2 ATP are generated
• 4 CO2’s are released

Biosynthetic intermediates are:

• haem
• fatty acids
• glucose
• amino acids

Question 8

Why do electrons in NADH use more PTC’s than electrons in FADH2?

Answer 8

Electrons in NADH have more energy than in FAD2H. So, NADH uses 3 PTC’s, FADH2 uses only 2.

Question 9

How is oxidative phosphorylation regulated by ATP:ADP ratio?

Answer 9

High ATP:ADP ratio means there is no substrate for ATP synthase. Inward flow of H+ stops. Concentration of H+ in the intermitochondrial space increases and prevents further H+ pumping- stopping electron transport.

Question 10

Explain the effect of uncouplers on oxidative phosphorylation

Answer 10

-Uncouplers increase the permeability of the mitochondrial inner membrane to protons, dissipating the proton gradient, thereby reducing the proton motive force. No drive for ATP synthesis and no oxidative phosphorylation. However no inhibition of electron transport.

Question 11

Explain the effect of inhibitors and genetic defects on oxidative phosphorylation

Answer 11

• Inhibitors block electron transport e.g cyanide prevents acceptance of electrons by oxygen. Can be lethal
• Genetic defects in proteins encoded by mtDNA (some subunits of the PTCs and ATP synthase). Decrease in electron transport and ATP synthesis.

Question 12

What is pyruvate dehydrogenase sensitive to?

Answer 12

• PDH is a 5 enzyme complex. The different enzyme activities require various cofactors (FAD, thiamine pyrophosphate, lipoic acid). B-vitamins provide these factors, so reaction is sensitive to Vitamin B deficiency.
• Reaction is irreversible, so is a key regulator step