Session 3 Flashcards
Describe the key features of pyruvate dehydrogenase reaction
- Pyruvate dehydrogenase is a large multi-enzyme complex
- Occurs in mitochondrial matrix (pyruvate shunted across mitochondrial membrane)
- Irreversible because of loss of Carbon dioxide - key regulatory step
- Acetyl CoA cannot be converted back to pyruvate to form glucose in gluconeogenesis
- PDH deficiency causes lactic acidosis
What happens to Pyruvate at the end of glycolysis?
- Converted to Acetyl CoA by pyruvate dehydrogenase in the mitochondrial matrix
What cofactors does pyruvate dehydrogenase require?
- FAD
- Thiamine pyrophosphate
- Lipoic acid
- All act catalytically
- Allows complicated reaction to be performed in a controlled manner
Why is pyruvate dehydrogenase very sensitive to vitamin B deficiency?
- 4 B vitamins are involved in the reaction
What inhibits pyruvate dehydrogenase?
- Acetyl-CoA (acetyl CoA from B oxidation of fatty acids is used in stage 3 catabolism instead of from glucose) (allosteric inhibition)
- NADH (allosteric
- ATP inhibition)
- Phosphorylation
What activates pyruvate dehydrogenase?
- Pyruvate
- NAD +
- ADP (allosteric)
- Insulin (promotes dephosphorylation) - causes glucose to increase in cells
What is stage 3 of glucose catabolism?
- TCA/Krebs cycle
What are the main features of the TCA cycle?
- Occurs in mitochondria
- Oxidative
- Single pathway for the catabolism of sugars, fatty acids, ketone bodies, alcohol and amino acids
- Acetate is converted to 2 CO2 molecules
- Produces some energy as ATP/GTP
- Produces precursors for other reactions
- Only works in presence of oxygen
What is produced by the TCA cycle?
- 6 NADH
- 2 FAD2H
- 2 GTP
Per glucose mol (2 turns of cycle)
How is the TCA cycle regulated?
- ATP/ADP ratio
- NADH/NAD+ ratio
- Isocitrate dehydrogenase catalysts an irreversible reaction and is allosterically inhibited by NADH and activated by ADP a
How many ATP molecules are produced by the TCA cycle per glucose molecule?
- Approximately 32
Give example of intermediates used in anabolism?
- Citrate -> fatty acids
- a-ketoglutarate -> amino acids
- Succinate -> amino acids/haem
- Malate -> amino acids
- Oxaloacetate -> amino acids/glucose
What and where do other compounds enter the TCA cycle?
- Glucose, fatty acids, alcohol and some amino acids enter as Acetyl CoA
- Amino acids also enter directly at a-ketoglutarate, Succinate, Malate and Oxaloacetate
How many ATP/GTP molecules have been produced from glycolysis and the TCA cycle per glyph ode molecules?
- 2 from glycolysis
- 2 from TCA cycle
- Most of the energy is in the chemical bond energy of the e- in NADH and FAD2H
What has happened by the end of stage 3 catabolism?
- All C-C bonds broken and C atoms oxidised to CO2
- All C-H binds broken and H atom (H+ and e-) transferred to NAD+ and FAD
What are the feature of stage 4 catabolism?
- Takes place on the inner mitochondrial membrane
- Involves electron transport and ATP a synthesis
- NADH and FAD2H are re-oxidised
- O2 is required
- Produces large amounts of ATP
What processes happen in stage 4 catabolism?
- Electron transport: electrons from NADH and FAD2H transferred through a range of carrier molecules to oxygen
- Oxidative phosphorylation: free energy used to drive ATP synthesis
Couples the energy from dissipation of the proton motive force to the synthesis of ATP from ADP
How many ATP a molecules are produced with the oxidation of NSDH and FAD2H?
- 2 mol NADH: 5 mols ATP
- 2 mol FAD2H: 3 mols ATP
How is oxidative phosphorylation regulated?
- High ATP/low ADP: lack of substrate (inhibition)
What are uncouplers?
- Increase the permeability of the inner mitochondrial membrane to protons
- Dissipate the protons gradient, reducing the protein motive force
- No drive for ATP synthesis
- Eg dinitrophenol, dinitrocresol
What are inhibitors?
- Block electron transport
- Prevents O2 accepting electrons
- Eg cyanide, carbon monoxide
What are uncoupling proteins?
- Proteins that uncouple electron transport chain from ATP production to produce heat
- UCP1-5
Where are uncoupling proteins located?
- Inner mitochondrial membrane and allow a leak of protons back across
- UCP 1 (thermogenin) is in brown adipose tissue - important in non-shivering thermogenesis
How non-shivering thermogenesis occur in brown adipose tissue?
- In response to cold, noradrenaline is released from the sympathetic nervous system
- Stimulates lipolysis releasing fatty acids for B-oxidation
- Forms NADH and FAD2H, driving electron transport and proton motive force
- Noradrenaline also activates UCP1, allowing protons to re-enter matrix, dissipating proton motive force as heat
What are other functions of the UCP proteins?
- UCP2 is widely distributed in the body, could be linked to diabetes, obesity, metabolic syndrome and heart failure
- UCP3 is found in skeletal muscle, brown adipose tissue, and is involved in modifying fatty acid metabolism and protecting against reacting species damage
What are the key features of lipids?
- Structurally diverse
- Hydrophobic - mostly insoluble in water
- Contain C, H and O (phospholipids contain P and N)
- More reduced that carbohydrates - release more energy when oxidised, requires more Oxygen for oxidation
What are the classes of lipids?
- Fatty acid derivatives
- Hydroxy-methyl-glutaric acid derivatives (c6 compound)
- Vitamins
What lipids are in fatty acid derivative class?
- Fatty acids: fuel molecule
- Triacylglyerides: fuel storage and insulation
- Phospholipids: components of membranes and plasma lipoproteins
- Eicosanoids: local mediators
What lipids are in the hydroxy-methyl-glutaric acid derivatives
- Ketone bodies: water soluble fuel molecules
- Cholesterol: membranes and steroid hormone synthesis
What lipids are in the vitamin class?
- A, D, E and K
What are the main features of triacylglyerides?
- Hydrophobic
- Can be stored in anhydrous form
- Stored in adipose tissue
- Utilised in prolonged exercise, starvation, pregnancy
- Storage/mobilisation under hormonal control
Where does stage 1 of triacylglyceride occur?
- GI tract
- Extracellular
What happens during stage 1 of triacylglyceride catabolism?
- Hydrolysed by pancreatic lipases in small intestine
- Forms fatty acids and glycerol
What happens to fatty acids from triacylglyerides?
- Converted back to triacylglyerides in GI tract
- Packaged into lipoprotein particle: chylomicrons
- Released into circulation via lymphatics
- Carried to adipose tissue where it is stored as triacylglyeride
- Released as fatty acids when needed
- Carried to tissues as albumin-fatty acid complex
What happens in adipose cells when extracellular glucose is low?
- Fatty acids are released (on albumin) as an alternative fuel
How is triacylglyeride storage controlled?
- Hormonal control
What hormones activate triacylglyeride storage?
- Insulin
What hormones inhibit triacylglyeride storage?
- Glucagon
- Adrenaline
- Cortisol
- Growth hormone
- Thyroxine
What happens to glycerol after it is hydrolysed from dietary triacylglyerides?
- Enters bloodstream and is transported to the liver
- Is converted to glycerol phosphate
Describe the conversion of glycerol to glycerol phosphate
- Glycerol -> glycerol phosphate
- ATP -> ADP
- Glycerol kinase
What happens to the glycerol phosphate after conversion from glycerol?
- Used in triacylglyeride synthesis
- Converted to dihydroxyacetone phosphate (NAD+ -> NADH) and enters glycolysis
What are features of fatty acids?
- Saturated/unsaturated
- Amphipathic
- Some essential fatty acids eg linolenic acid
- Reduced and hydrophobic (good for energy storage)
Where does stage 2 of lipid metabolism occur (fatty acids)?
- Mitochondria
What happens in fatty acid catabolism?
- Fatty acids are activated outside the mitochondrion (by linking to CoA)
- Transported across inner mitochondrial membrane by the carnitine shuttle
- Cycles through sequence of oxidative relations, removing a C2 at each cycle
What happens during fatty acid activation?
- Occurs outside of mitochondria
- Linked to CoA by fatty acyl CoA synthase
- Requires ATP a
When do triacylglyerides in adipose tissue undergo lipolysis?
- Stress eg aerobic exercise, starvation, lactation
How is lipolysis of triacylglyerides in adipose tissue controlled?
- Enzyme hormone-sensitive lipase
What activates lipolysis?
- Glucagon
- Adrenaline
- Cortisol
- Growth hormone
- Thyroxine
What inhibits lipolysis?
- Insulin
Why do activated fatty acids need the carnitine shuttle to cross the mitochondrial membrane?
- Are hydrophobic
What are the features of the carnitine shuttle?
- Regulates rate of fatty acid oxidation as controls entry into mitochondria
- Inhibited by malonyl CoA (prevents newly synthesised fatty acids in cytoplasm from immediately being transported into mitochondria and oxidised)
- Defects: poor exercise tolerance and high levels of triacylglyerides in muscle cells
What does the B oxidation of fatty acids require?
- Mitochondrial NAD+ and FAD
- Oxygen presence
What are the features of b oxidation?
- Oxidises
- Removes 2C (acetate) each cycle
- No direct synthesis of ATP
- All intermediates are linked to CoA and C atoms of fatty acid are converted to Acetyl CoA
- More energy derived from fatty acid oxidation than glucose oxidation
Where doesn’t b oxidation occur?
- Brain
- Red blood cells
- White blood cells
What happens to Acetyl CoA (catabolism and anabolism)?
- CO2
- Fatty acids -> triacylglyerides/phospholipids
- Hydroxymethyl glutaric acid (HMG) -> ketone bodies/cholesterol (-> steroid hormones)
What ketone bodies are there?
- Acetone
- Acetoacetate
- B-hydroxybutyrate
What are the different levels of ketones in the blood plasma?
- Normal: 10 mM
Where are ketone bodies synthesised?
- Acetoacetate and B-hyroxybutyrate: liver
- Acetone: spontaneous decarboxylation of Acetoacetate
What are the properties of ketone bodies?
- Water-soluble (allows high plasma concentrations and excretion in urine (ketouria)
- Acetoacetate and B-hydroxybutyrate can cause ketoacidosis in high concentrations in plasma
- Acetone is volition and can be excreted by lungs (can be smelt on breath of untreated type 1 diabetics)
How are ketone bodies synthesised?
- Acetyl~CoA -> Hydroxymethyl glutaryl CoA (HMG~CoA) (synthase)
- HMG~CoA -> mevalonate (HMG~CoA reductase) -> cholesterol
- HMG~CoA -> Acetoacetate (Lyase) -> acetone/B-hydroxybutyrate
How is ketone body synthesis regulated?
- Fed state ie high insulin:glucagon ratio: Lyase is inhibited/reductase is activated; Cholesterol synthesis activated
- Starvation/diabetes ie low insulin: glucagon ratio: Lyase is activated/reductase inhibited; ketone body synthesis
What does ketone body synthesis require?
- Excessive lipolysis of in adipose tissue to supply substrate of fatty acids
- Low plasma insulin:glucagon ratio usually due to a fall in insulin
What uses ketone bodies?
- All tissues containing mitochondria including CNS
What happens to ketone bodies?
- Converted to Acetyl CoA
- Oxidised in stage 3 of catabolism
