Session 3

Question 0

Explain how the TCA cycle is regulated

Answer 0

ATP/ADP ratio
NADH/NAD+ ratio
Isocitrate dehydrogenase is allosterically inhibited by high energy NADH and activated by low energy ADP

Question 1

Describe the roles of the tricarboxylic acid cycle (TCA) in metabolism

Answer 1

Central pathway in the catabolism of sugars, fatty acids, ketone bodies, alcohol, amino acids
Occurs in mitochondria
Requires NAD+, FAD, oxaloacetate (oxidative) H+ and e- transferred
Breaks C-C bond in acetate, oxidises C –> CO2
Doesn’t function in absence of O2
Anabolic (bio synthetic) functions - interconversion:
Synthesis of non-essential amino acids
Synthesis of haem, glucose
Synthesis of fatty acids

Question 2

Describe the key features of oxidative phosphorylation

Answer 2

Occurs at inner mitochondrial membrane
Free energy released used to drive ATP synthesis
O2 required
Involve electron transport and ATP synthesis

Question 3

Explain the process of electron transport and ATP synthesis and how they are coupled

Answer 3

Electron transport:
Four highly specialised protein complexes (three are also proton translocating complexes)
Electrons are transferred sequentially - release free energy
PTCs use the free energy to move protons from inside to outside of inner mitochondrial membrane
Membrane is impermeable to protons so conc. increases (proton motive force - electrochemical potential)
NADH uses all three, FAD2H uses two
Occurs in presence of O2 - terminal electron acceptor

ATP synthesis:
Protons only re-enter mitochondrial matrix via ATP synthase complex
Greater p.m.f. = more ATP synthesised
Energy not conserved appears as heat (maintains body temp.)

Coupling:
When [ATP] high, ATP synthase stops (lack of substrate)
Protons not transported back into mitochondria
[H+] outside increases so more can’t be pumped in
Electron transport stops

Question 4

Describe how, when and why uncoupling of these processes occurs in some tissues

Answer 4

Uncouplers:
Dinitrophenols increase permeability of inner mitochondrial membrane to protons
Potential energy of p.m.f. dissipated as heat
ATP synthesis doesn’t occur, heat generated
Uncoupling proteins (UCP1-5) located on inner mitochondrial membrane
UCP1 (thermogenin) expressed in brown adipose tissue - non shivering thermogenesis so mammals survive in cold environments
Cold –> noradrenaline –> lipolysis releases fatty acids to provide fuel
UCP2 widely distributed –> diabetes, obesity, metabolic syndrome, heart failure
UCP3 found in skeletal muscle, brown adipose tissue, heart –> modifying fatty acid metabolism, protecting against ROS damage

Inhibitors:
E.g. anaerobic conditions, CO, poisons (cyanide)
NADH and FAD2H are t oxidised by ET
No energy to drive proton pumping
P.m.f. not created
No ATP synthesis therefore no heat generated
Irreversible cell damage

Question 5

Compare the processes of oxidative phosphorylation and substrate level phosphorylation

Answer 5

Oxidative phosphorylation:
Requires membrane-associated complexes (inner mitochondrial membrane)
Energy coupling occurs indirectly through generation and utilisation of p.m.f
Cannot occur in the absence of O2
Major process for ATP synthesis in cells requiring large amounts of energy

Substrate level phosphorylation:
Requires soluble enzymes (cytoplasmic and mitochondrial matrix)
Energy coupling occurs directly through formation of high energy hydrolysis bond (P-group transfer)
Can occur to a limited extent in the absence of O2 (glycolysis)
Minor process for ATP synthesis in cells requiring large amounts of energy

Question 6

Describe the various classes of lipids

Answer 6

1. Fatty acid derivatives
   Fatty acids - fuels
   Triacylglycerols - fuel, storage, insulation
   Phospholipids - membranes, plasma lipoproteins
   Eicosanoids - local mediators

2. HMGs
Ketone bodies - water soluble fuel
Cholesterol - membranes, steroid hormone synthesis
Cholesterol esters - cholesterol storage
Bile acids and salts - lipid digestion
3. Vitamins
A, D, E, K

Question 7

Describe how dietary triacylglycerols are processed to produce energy

Answer 7

TAGs are hydrolysed by pancreatic lipase in small intestine to release glycerol and fatty acids (requires bile salts and colipase)
Glycerol is metabolised in liver by glycerol kinase
Fatty acids are an energy storage,
Carried bound non-covalently to albumin called non-esterified fatty acids (NEFA) or free fatty acids (FFA) in blood to liver
Oxidised, converted to glucose or used in synthesis of TAGs
Fatty acids used by liver, skeletal muscle, heart
Not used by RBCs (no mitochondria) or CNS (can’t pass blood-brain barrier)

Question 8

Explain how, when and why ketone bodies are formed

Answer 8

1. Acetoacetate
2. Acetone
3. B-hydroxybutyrate
   Increased conc. in starvation (physiological ketosis) and untreated type 1 diabetes (pathological ketosis)
   Water soluble
   Allows high plasma conc. and excretion in urine (ketonuria)
   1+3 may cause acidosis
   2 can be smelt on the breath of untreated type 1 diabetes
   Synthesis:
   Acetyl CoA –> HMG-CoA (synthase)
   –> cholesterol (reductase)
   –> ketone bodies (lyase)
   Lyase and reductase are reciprocally controlled by insulin/glucagon ratio:
   Decreased insulin/glucagon ratio = increased lyase (ketone bodies) = decreased reductase

Question 9

Describe the central role of acetyl CoA in metabolism

Answer 9

Acetyl CoA is oxidised via stage 3 of catabolism (Kreb’s cycle)