Metabolism – questions and answers

1. Metabolism – questions and answers Phase 2 MBChB Gastroenterology system block Gerhard H. W. May

2. Which one is FALSE about lipids? • Triglycerides are the main storage form of lipids in the human body • Some lipids contain phosphate groups, carbohydrates, or proteins • Fatty acids can be used for gluconeogenesis • Lipids are mostly hydrocarbon and water-insoluble • Lipid absorption is required for the absorption of some vitamins Countdown

3. Which one is FALSE about fat catabolism? • The first step of fat catabolism is lipolysis • Fatty acids first have to be activated to acetyl-CoA • Activation occurs in the cytoplasm • The pathway for breakdown of fatty acids is termed beta oxidation • Beta oxidation occurs in the mitochondrial matrix Countdown

4. Lipid metabolism - overview dietary fat stored fat triglyceride fatty acyl-carnitine fatty acyl-carnitine lipolysis fatty acyl-CoA fatty acids glycerol activation fatty acyl-CoA b oxidation fatty acid synthesis malonyl-CoA acetyl-CoA citrate cycle acetyl-CoA citrate citrate glycolysis glycolysis dietary carbohydrates Mitochondrion Cytoplasm Membrane

5. Which one is FALSE about ketone bodies? • They are formed in the liver from acetyl-CoA • They diffuse into the blood stream • They are toxic to peripheral tissues which prefer glucose • They accumulate as a by-product of beta oxidation under conditions of extreme starvation • They can lead to ketoacidosis Countdown

6. Ketone bodies • Formed in liver mitochondria • from acetyl-CoA from b oxidation • Diffuse into the blood stream and to peripheral tissues • Important molecules of energy metabolism for heart muscle and renal cortex • converted back to acetyl-CoA, which enters TCA cycle

7. How often does beta oxidation happen for the breakdown of a C14 fatty acid? • Four times • Five times • Six times • Seven times • Eight times Countdown

8. What are the products of the breakdown of a C14 fatty acid? • 8 acetyl-CoA, 7 NADH + H+, 7 FADH2 • 7 acetyl-CoA, 6 NADH + H+, 6 FADH2 • 7 acetyl-CoA, 6 NADPH + H+, 6 FADH2 • 7 acetyl-CoA, 6 NADH + H+, 6 FADH2, 6 GTP Countdown

9. Acyl-carnitine b oxidation • Cycle of reactions • in mitochondrial matrix • Four steps in each cycle • Products of each cycle: • 1 acetyl-CoA • 1 FADH2 • 1 NADH + H+ • 1 fatty acyl-CoA, shortened by 2 carbon atoms TCA cycle

10. Which one is FALSE about amino acid catabolism? • The amino group of amino acids is removed to form urea • Removal of the amino group only occurs in liver cells • Carbon skeletons of all amino acids can enter the TCA cycle • Glucogenic amino acids can be used to form glucose • Ketogenic amino acids can be completely oxidised in the TCA cycle Countdown

11. Which one is FALSE about the removal of nitrogen? • Amino groups from amino acids are directly transferred to carbamoylphosphate • One nitrogen of urea is derived from aspartate • One nitrogen of urea is derived from free ammonium • Free ammonium is toxic Countdown

12. The urea cycle • Urea is synthesised in a complex series of reactions • in the liver • urea/ornithine cycle • one nitrogen from free ammonium, the other from aspartic acid • carbon from CO2

13. Which one is FALSE about fatty acid synthesis? • It is a reductive process • It occurs in the cytoplasm • Citrate transports acetyl groups from mitochondria to the cytoplasm • It requires NADPH • Fatty acid synthase consists of 7 distinct peptides Countdown

14. Fatty acid synthesis vs. degradation

15. The donor molecule of carbon atoms to a growing fatty acid is... • Acetyl-CoA • Malonyl-CoA • Citrate • Acyl-CoA Countdown

16. Acetyl-CoA carboxylase - regulation • Insulin signals the fed state: • stimulates storage of fuels and synthesis of proteins • Glucagon signals starved state, epinephrine signals requirement for energy: • mobilise glycogen stores • Citrate stimulates allosterically: • citrate levels are high when acetyl-CoA and ATP are abundant • Antagonised by palmitoyl CoA: • abundant when fatty acids are in excess

17. What is NOT the meaning of ‘citrate stimulates allosterically’? • Citrate binds to a site separate from the catalytic cleft of acetyl-CoA carboxylase • Citrate helps insulin to stimulate the catalytic activity of acetyl-CoA carboxylase • When there is lots of citrate it activates acetyl-CoA carboxylase • Binding of citrate to acetyl-CoA carboxylase activates the catalytic activity of the enzyme • Individual acetyl-CoA carboxylase enzyme molecules can be affected by a number of small molecule effectors, citrate is one of them Countdown

18. Which one is FALSE about glycogen and its synthesis? • Liver glycogen can serve as a source for blood glucose • Muscle glycogen can only be used for energy generation in the muscle cell • Glucose has to be bound to UDP before it can be transferred onto glycogen • Glycogen synthase can not introduce branches • Glycogen synthase starts new glycogen molecules by binding one glucose molecule to a glycogenin protein 10 Countdown

19. What is glycogenin? Glycogen Glycogenin protein • Glycogenin is a protein and sits at the centre of a glycogen polymer • It has catalytic activity and can covalently bind up to four glucose molecules to itself, essentially forming the starting point of the glycogen polymer • This is important because glycogen synthase can only add glucose residues to an existing glycogen chain

20. Which one is FALSE about glycogen breakdown? • Glucose is removed from glycogen as glucose-6-P • Liver cells can create free glucose from glycogen • Glucagon inhibits glycogen synthesis and stimulates glycogen breakdown • Insulin inhibits glycogen breakdown and stimulates glycogen synthesis 10 Countdown

21. glucose-6-phosphatase In liver only Glycogen breakdown summary glycogen Rate limiting step glycogen phosphorylase glucose-1-phosphate phosphoglucomutase glucose-6-phosphate glucose-6-phosphatase glucose glycolysis Blood via GLUT2 transporter

22. Which one is FALSE about gluconeogenesis? • It is a pathway for the synthesis of new glucose from non-carbohydrate precursors • Lactate, glycerol, and glucogenic amino acids are precursors for gluconeogenesis • It is glycolysis run in reverse • It occurs mainly in the liver • It is a very energy-consuming process • Glucagon stimulates it, insulin inhibits it 10 Countdown

23. Gluconeogenesis • Requires four unique liver enzymes • Proceeds via the synthesis of oxaloacetate in mitochondria • the TCA cycle intermediate which accepts acetyl groups • important for accepting acetyl groups from fat breakdown • Energy consuming

24. Amino acids are oxidised in the TCA cycle

25. Amino acids as precursors for gluconeogenesis

26. Which one is FALSE about metabolism? • Catabolic pathways break down reduced substrates for energy generation • Anabolic pathways synthesise oxidised products from small, reduced precursors • Anabolic pathways require the input of energy in the form of ATP • Anabolic pathways require the input of reducing force in the form of NADPH • Catabolic pathways generate reduced cofactors which can drive oxidative phosphorylation 10

27. ATP as energy currency

28. Metabolism - redox reactions ADP + Pi ATP Reduced biosynthetic products ANABOLISM Oxidised precursors NADP+ NADPH + H+ Reduced fuel CATABOLISM Oxidised products NAD+ (FAD) NADH + H+ (FADH2) OXIDATIVE PHOSPHORYLATION H2O O2 ATP ADP + Pi

29. Which one is FALSE about glycolysis? • It generates 2 ATP molecules per glucose molecule broken down • It forms the first stage of carbohydrate catabolism • It occurs in the mitochondrial matrix • It generates two NADH + H+ per glucose molecule broken down 10

30. Glycolysis • Stage 1: glucose is trapped and destabilised • Stage 2: two interconvertible three-carbon molecules are formed • Stage 3: generation of ATP

31. Pyruvate... • Directly enters oxidative phosphorylation • Is converted to oxaloacetate and is used for fatty acid degradation • Is converted to acetyl-CoA and then enters the TCA cycle 10

32. Which one is FALSE about the TCA cycle? • It provides precursors for gluconeogenesis • Breakdown products from carbohydrate, fat and protein metabolism feed into it • It consumes NADH and FADH2 • Carbon skeletons of amino acids enter it at different points • It occurs in the mitochondrial matrix 10

33. The TCA cycle - overview • Eight reactions in total • A two-carbon unit (from acetyl-CoA) condenses with a four-carbon unit • The resulting six-carbon unit is decarboxylated twice • yields CO2 • Four oxidation reactions • yield NADH + H+ and FADH2 • One GTP is formed • energy • The four-carbon unit is recreated

34. Which one is FALSE about oxidative phosphorylation? • Electrons from NADH and FADH2 are transferred onto O2 • The electron transport chain is located in the inner mitochondrial membrane • Protons are pumped from the intermembrane space to the mitochondrial matrix • Protons flow back across the membrane, following their concentration gradient • Proton flow is coupled to ATP synthesis • Oxidative phosphorylation consists of two separate proton pump systems 10 Countdown

35. Essence of oxidative phosphorylation • Electrons from NADH and FADH2 are used to reduce O2 to H2O • Their energy is used to pump protons (H+) from the mitochondrial matrix to the intermembrane space • pH decreases in intermembrane space, increases in the matrix • Protons flow back across the membrane, following their concentration gradient • Energy of proton flow is used to phosphorylate ADP to ATP

36. Electron transport and H+ pump • NADH donates electrons to complex I • FADH2 donates electrons to complex II • Complexes I, III and IV pump H+ out while transporting electrons • Electrons are finally transferred onto O2

37. The P/O ratio... • Is a measure of the number of ATP molecules formed per oxygen atom reduced • For NADPH is 2.5 • Measures the ratio between NADPH and NADH in the cell 10 Countdown

38. Which of the following is FALSE about redox potentials? • The redox potential of a molecule is a measure of its likelihood to loose an electron • The more negative a molecule’s redox potential, the more likely it is to donate an electron • Electrons can be transferred from molecules with negative redox potentials to those with more positive redox potentials • The difference in redox potentials between substrate and product of a redox reaction relates to the free energy change of the reaction • NADH has a more negative redox potential than FADH2, therefore transfer of its electrons to oxygen releases less energy 10 Countdown

39. Redox potentials • The standard redox potential E’o of a (reduced) substance X is a measure for how readily X donates an electron (in comparison with H2) X- X + e- • A negative E’o means that the reduced form of X has a lower affinity for electrons than H2, a positive E’o means the opposite

40. Some standard redox potentials NADH + H+ ‘likes’ to donate electrons more than those molecules below it

41. Some standard redox potentials FADH2 has a lower redox potential

42. Some standard redox potentials Molecules of the electron transport chain have even lower redox potentials

43. Some standard redox potentials Molecular oxygen has the lowest redox potential in this table and is most likely to accept electrons

44. How are redox potentials related to ATP? • Electrons are handed from NADH (or FADH2) to molecules of lower redox potential, like a relay baton • Redox potentials: NAD+/NADH - 0.32 V ½ O2/H2O + 0.82 V • Transferring electrons from a negative to a positive redox potential releases energy ½ O2 + NADH + H+ H2O + NAD+ Go’ = - 220.1 kJ/mol • Oxidative phosphorylation converts the energy inherent in these electrons into chemical energy in the form of ATP Difference: 1.14 V For FADH2: 1.04 V