Amino acid metabolism

1. Amino acid metabolism

2. proteins • Only foodstuff that can form structures (tissues and enzymes) • Made up of amino acids • Protein synthesis, enzyme formation • Can serve as fuel during long-term work • 0.8 g/kg recommended for adults; probably too low for athletes

3. Protein structure: Amino acids • Essential vs non-essential • Essential: NOT made by body • Non-essential: made by the body

4. Protein structure Amino acid • Carboxyl and amino termini come together to from protein structures (peptides)

5. Proteins in the diet • Digested in stomach and small intestine • Hydrocholoric acid (stomach) • Trypsin, chymotrypsin, carboxypeptidase (from pancreas) • Polypeptidases and dipeptidases in intestinal cells finish digestion

6. The amino acid pool • Free amino acids in the liver, skeletal muscle, plasma, interstitial fluid and intracellular water • All interconnected in that metabolism in one affects the others • Continuous excretion of nitrogenous end-products • Necessitates constant input of new amino acids • So, CONSTANT Protein turnover

7. Nitrogen balance • Nitrogen is a component of AAs • Thus, used as a marker of protein metabolism • Protein intake necessary to balance nitrogen turnover (input vs excretion) • 0.8-1.0 g/kg is sufficient for most • 1.2-1.6 g/kg is the highest recommendation for athletes

8. Removal of nitrogen • Before amino acids can be used as fuel, nitrogen group must be removed • Two ways • Deamination • Transamination • Glutamate is a key player in both

9. Removal of Nitrogen • Deamination • Occurs in liver 1) Requires NAD+ as oxidizing agent 2) Produces ammonium ion 3) α-ketoglutarate can be used in Kreb’s cycle • Called anaplerotic (to fill) addition to Kreb’s cycle 2 1 3

10. Removal of Nitrogen • Transamination • Much more common • Transfers amine group from amino acid to keto acid • SGPT and SGOT transaminases in liver AA Keto acid AA Keto acid

11. Excretion of nitrogenous wastes 2 1 • Ammonia (small amt) • Most is excreted as urea • Urea cycle 1) Formation of carbamoyl phosphate from ammonia and Co2 2) Addition of aspartate 3) Production of fumarate (Kreb’s cycle intermediate) 4) Produces Urea 4 3

12. Gluconeogenic amino acids 1 • Some amino acids used for gluconeogenesis 1)Pyruvate to OOA 2)OOA to PEP 3)PEP begins “reverse glycolysis” or gluconeogenesis • So, amino acids that give rise to pyruvate and oxaloacetate • Can form phosphoenolpyruvate • Can be converted to glucose 2

13. Anaplerotic and cataplerotic reactions • Anaplerotic (adding to) • Cataplerotic (emptying) • These Rx add to or deplete the Kreb’s cycle • Glutamate-glutamine • Key intraorgan nitrogen transport vehicle, fuel source for GI tract and immune system and gluconeogenic precursor

14. Branched chain amino acids • Leucine, Isoleucine and valine (LIV) • Catabolized mostly in skeletal muscle • Leucine: • Forms acetyl-CoA, acetoacetate and glutamate • Leucine is thus called ketogenic Transamination

15. AA metabolism • AA can be used in the following ways • Structural (proteins) • Anaplerotic additions to Kreb’s cycle • This keeps the Kreb’s cycle working • Oxidized directly • Branched chain AA • Other contributions to energetics • Ketogenic • Produce ketone bodies when broken down • Glucogenic • Contributes to gluconeogenesis

16. Glucose-alanine cycle • Used during fasting • Alanine can come from glycolysis or AA metabolism • Glycolysis • Kreb’s cycle backs up during starvation • Pyruvate transaminated to alanine • Alanine converted to glucose in liver

17. Glucose-alanine II 1 • Other amino acids can also form alanine (glucogenic AA, anything that gives rise to pyruvate or OOA) • So when Pyruvate builds up, converted to Alanine (1) • Alanine shuttled to liver • Converted to glucose

18. Effects of endurance training on AA metab • Greater rates of AA metabolism in trained subjects • Greater oxidation in human subjects during exercise

19. AA metabolism • Note that leucine oxidation increases during exercise • This increases is linear with respect to exercise intensity • Particularly true in fasted state

20. AA metabolism 1 • Note that alanine appearance increases during exercise (1) and this can come from AA leucine (2) • Also, glucose infusion reduces AA oxidation (3) 2 3

21. AA metabolism • However, exercise training does not appear to increase AA metabolism in human subjects • If anything, it is reduced

22. Ammonia scavenging during high intensity exercise • During high intensity exercise, AMP is formed • Adenylate kinase Rx • ADP + ADP ATP + AMP • AMP then inhibits AK Rx if it builds up • AMP deaminated to IMP • Muscle releases ammonia (NH4+) during contraction • Contains nitrogen • Purine nucleotide cycle

23. Ammonia scavenging 4 2 • Formation of glutamine (1) helps to transport ammonia in blood • Ammonia is toxic • Transamination • Glutamine goes to kidney (2) • Urea (3) and glucose formed (4) 3 1

24. Neuro-endocrine control of blood glucose

25. Hormones • Chemical messengers • Produced and stored in a gland • Secreted into the blood • General and specific effects • Two basic types • Steroid • Produced from cholesterol by adrenal cortex and gonads • Polypeptides • Amino acids

26. Hormones • Powerful effects • Precisely regulated • Feedback control (negative feedback) • Mechanisms of action • Affect cell permeability (insulin) • Activate an enzyme (epinephrine) • Protein synthesis (GH)

27. Blood glucose homeostasis • When fed • Liver glycogenolysis • When fasted • Gluconeogenesis • SNS helps in this • Epi stimulates liver glycogenolysis and gluconeogenesis • Hormones • Released into blood • Epinephrine and nor-epinephrine

28. Hepatic glucose production during exercise • Maintenance of blood glucose levels is paramount • Fuel source • Anaplerotic additions to Kreb’s • Allows fat metabolism • Needed by brain and CNS • Hormones that help maintain blood glucose • glucoregulatory

29. Glucose homeostasis • How difficult is this? • Normal adult • Blood volume = 5L • Blood glucose = 100 mg/dl (1 g/L) • 5g or 20 kcals (4kcal/g) worth of energy • Only enough to support 1 min of maximal activity! • This means • We must get plenty of CHO prior to and even during activity • Liver supplements this

30. Glucose homeostasis • Glucose production increased in 2 ways • Increased absorption from gut and liver output • Liver glycogenlosis • Liver gluconeogenesis

31. Glucose homeostasis Why increased? • Note how addition of arm exercise increases catecholamine levels • glucoregulatory hormone (raises blood glucose) • Insulin falls • Decreases blood glucose • Thus hormonal changes help maintain blood glucose levels

32. Catecholamines and blood glucose • Epinephrine and nor-epinephrine • Epi binds to β-receptor • Activates adenylate cyclase • Muscle contraction increases intracellular Ca2+ and Pi • Stimulates glycogenolysis • Muscle and liver • Supports liver glucose production • Also increases lipolytic rate

33. Cyclic AMP • Made from ATP (1) • Intracellular messenger • Activates many processes in metabolism • Example • Glycogenolysis • Epinephrine binds to receptor (2) • Adenyl-cyclase creates cAMP (3) • cAMP activates phosphorylase 1 EPI 2 3

34. Insulin and glucagon • Insulin • β cells of the islets of langerhans of pancreas • Glucagon • α cells • Along with epinephrine and nor-epinephrine, main hormones of glucose homeostasis

35. Insulin response to exercise • Falls in response to exercise • Epinephrine suppresses insulin secretion • Thus • Glucose production is increased

36. Why insulin? • Insulin • Helps facilitate glucose transport across sarcolemma during rest • Uses glucose transporters (GLUT) • GLUT-4 • Insulin mobilizes transporters from intracellular pool • Transporters move to sarcolemma

37. Glucose transport: exercise insulin • Muscular contraction • “insulin-like” effect • GLUT-4 can translocate due to insulin or Ca2+ • So, muscular contractions • Cause release of Ca2+ • This causes translocation of Glut-4 receptors • Important as epinephrine (released during exercise) inhibits insulin

38. Neuro-endocrine control of hepatic glucose production • Gluconeogenesis • Liver and kidneys • 3 different enzymes than glycolysis • Pyruvate carboxylase • PEP carboxylase • Fructose 1,6 biphosphatase • Glucose 6-phosphatase • Liver only • So, muscle resynthesizes glycogen, liver and kidneys, glucose Pyruvate kinase

39. Gluconeogenesis • Those 4 enzymes are either nonexistent or in small supply in skeletal muscle • Found in large amts in liver and kidneys • Pyruvate kinase (last step of glycolysis): • virtually irreversible in skeletal muscle • In liver, can be inhibited by cAMP and phosphorylation (Ca2+-dependent protein kinase) • Reduces glycogenloysis and promotes gluconeogenesis

40. Gluconeogenesis E • Pyruvate coverted to oxaloacetate (A) • High acetyl-CoA, low ADP • Oxaloacetate converted to Phosphoenolpyruvate (B) • Low ADP • Phosphoenolpyruvate converted to Fructose 1, 6 bisphosphate (C) • F 1,6 bisphosphate converted to F6P (D) • High citrate, low AMP • Converted to glucose (E) D C B A

41. Hepatic glucose production The following hormones increase gluconeogenesis • Inhibit pyruvate kinase • Glucagon • Epinephrine • Nor-epinephrine • Insulin • Inhibits gluconeogenesis

42. Can muscle make glucose? • Glycolytic muscle can produce glycogen from lactate • Glyconeogenesis • Likely occurs early in recovery • Muscle lacks G6 phosphatase • So can’t release glucose from cell • However, it is possible that debranching enzyme can release glucose from glycogen • May help explain very rapid inc in blood glucose (fig 9-13)