Dr. Mohnen s notes on GLUCONEOGENESIS

Transcription

1 Dr. Mohnen s notes on GLUCONEOGENESIS Note: Even though we did not get through all of these slides during lecture, I advise you to look them all through because they will be helpful to you as you learn the material. Note : Be sure to concentrate on Dr. DerVartanian s notes. Note : Be sure to also look at page of Dr. DerVartanian s notes and page 04 in the text for a discussion of the Warburg Effect. Study well! Gluconeogenesis: Glucose Biosynthesis (or feeding the brain ). The brain is only % of body weight, BUT Consumes 5% of total glucose and 5% of Oxygen. Glucose and glycogen depleted in 4-6 hrs during starvation without gluconeogenesis. **Gluconeogenesis Net Reaction:** + 4 ATP + GTP + NADH + H H O Glucose + 4 ADP + GDP+ NAD Pi Hexokinase Glucose-6-phosphatase Liver synthesizes glucose to maintain blood sugar. Why spend energy to convert pyruvate to glucose? USE IT OR LOSE IT! Glucose costs ATP equivalents Glucose is worth 0- total ATPs during aerobic metabolism. There are only four new steps to learn. Everything else is Glycolysis running backwards! WHERE DOES ALL OF THE come from?

2 Gluconeogenesis: The Cori and Alanine (Cahill) Cycles (Liver). Also RBCs! The Cori Cycle converts Lactate produced in muscle during anaerobic respiration to glucose in the liver to control blood sugar. RBCs are also a major source of lactate (why?). During starvation, muscle protein catabolism produces energy and pyruvate. can be used to make OAA for Glucose and FA synthesis in the Liver via the Cahill Cycle. Cori cycle Alanine (Cahill) Cycle- pyruvate produced in muscle cannot be exported. In the Cahill Cycle, the amine groups of AA s are transferred to pyruvate to produce alanine. The alanine is exported from the muscle to the liver, where deamination produces pyruvate for gluconeogenesis. Gluconeogenesis: Turning into Phosphoenolpyruvate in two steps ADP inhibits: energy is low, make ATP Acetyl-CoA activates: CAC stopped WHAT are two major sources of pyruvate used for gluconeogenesis in the liver? Lactate (Cori Cycle) and Alanine (Cahill) carboxylase Phosphoenolpyruvate carboxykinase OAA is an important metabolite in the CAC. Occurs in mitochondria Occurs in cytosol Anaerobic respiration produces lactate. What is another major sources of lactate?

3 Carboxylase uses biotin* to add CO to pyruvate. (Named for the reverse reaction) *Biotin = vitamin B 7 What is the special bond in carboxyphosphate?.. Carboxyphosphate is an α-ketoacid: the β carbon is acidic and forms a nucleophillic carbanion + PC Costs Energy: ATP hydrolysis PC Phosphoenol-pyruvate carboxykinase uses the added carboxylate as a leaving group* to produce phosphoenolpyruvate. Phosphoenolpyruvate carboxykinase Decarboxylation ALWAYS pay attention to OAA Costs Energy: GTP hydrolysis = ATP hydrolysis *OAA is an α-ketoacid, and is reactive due to the β-carbocyclic acid, and can undergo spontaneous decarboxylation

4 Gluconeogenesis: Fructose-,6-bisphosphatase (F-,6- BPase) Citrate activates: CAC is stopped, so make glucose for glycogen or release to blood AMP inhibits: Need to burn glucose to make ATP F-,6-BP inhibits: Glucose is plentiful. Replenish ATP and CAC Gluconeogenesis: Glucose-6-phosphatase (Liver). In most cells, Glc-6-P is end product--- used for glycogen, starch, etc. Glucose-6-phosphatase is found in liver, bound to ER. Liver exports Glc to blood stream for other tissues IMPORTANT for glucose Homeostasis and the Glucagon response Hexokinase Glucose-6-phosphatas 4

5 Hormonal control of glycolysis (Insulin) and gluconeogenesis (Glucagon) Insulin: Indirectly* Activates PP *Increase in glucose increases fructose- 6-phosphate, which allosterically activates Protein Phosphatase (PP)! Glut 4 Glc G6P F6P Protein Phosphatase (PP) Glucagon: Activates PKA Glucagon Protein Kinase A (PKA) Insulin: primary signal for glycogen synthesis Insulin (high blood sugar) -uptake of GLc (Glut4)!!! -Activates Glycogen Synthesis Increase in glucose increases fructose-6-phosphate, which allosterically activates Protein Phosphatase (PP)! Glycogen Granule Glucose Glut4 P PP dephosphorylates glycogen synthase to activate glycogen synthesis. active Inactive P P Glut4 The # of glycogenin molecules limits the # of granules. The physical interaction between Glycogenin and GS limits size of granules Glycogen Granules After glycogen and ATP stores are replenished, excess Glc is converted to Fatty Acids. 5

6 INSULIN: Allosteric coupling of Glycolysis and Gluconeogenesis through F-,6-BP Hexokinase NEXT SLIDES SHOW the roles of insulin (PP) and glucagon (PKA) in controlling F-,6-BP levels High concentrations of F-6-P signals that glucose is plentiful. Glucose-6-phosphatase The F-6-P allosterically ACTIVATES the enzyme PP! PP activates the enzyme PFK which produces the allosteric effector F-,6-BP AND inactivates FBPase which degrades F-,6-BP F-,6-BP: Fructose-,6-bisphosphatase (FBPase/PFK- (phosphofructokinase) PKA Insulin Indirectly Stimulates By telling cells to take up glucose! PP Know how Insulin and glucagon control F-,6-BP Levels 6

7 Kinase: Hormonal Control (We already saw this at the end of the lecture on regulating glycolysis) INHIBIT: When blood [Glc] drops, glucagon signals protein kinase A (PKA) to phosphorylate pyruvate kinase ACTIVATE: When Blood sugar is high, Insulin signals phosphoprotein phosphatase (PP) to dephosphorylate pyruvate kinase. PP PKA Insulin ) Glut4 increases Glc Note global roles of PP & F-,6-BP Glucose Hexokinase ) Glycogen Synthase Dephosphorylated by PP G-6-P PP activated by high F6P Activates glycogen synthesis PFK 5) F-,6-BP Activates PFK F-6-P F-,6-P G-6-Pase (only in Liver) F-,6-BPase Allosteric effectors control glycogen synthesis vs glycolysis F-,6-BP ) PFK makes F-,6-BP high F6P activates PP which dephosphorylates PFK, activates PFK 4) F-,6-BP inhibits F-,6-BPase 6) PK is dephosphorylated high F6P activates PP which activates PK Kinase Phosphoenoylpyruva te Phosphoenolpyruvate carboxykinase Oxaloacetate carboxylase 7

8 Glucagon Note global role of PKA & F-,6-BP 6) Hexokinase Hexokinase inhibited by G-6-P BLOCK Glc uptake! 5) Glycogen Phosphorylase activated by PKA Glucose G-6-P F-6-P 7) Glc exported from Liver Other tissues just block glycolysis G-6-Pase (only in Liver) F-,6-BP ) FBPase activated by PKA Converts F-,6-BP to F-6-P PFK ) PFK Slowed by loss of F-,6-BP F-,6-P F-,6-BPase 4) F-,6-BPase activated by loss of F-,6-BP ) PK is inactivated by PKA Kinase Phosphoenoylpyruva te Phosphoenolpyruvate carboxykinase Oxaloacetat e carboxylase Insulin: Glucose is plentiful GLucagon: Glucose is low Indirectly Activates Protein Phosphatase (PP) IN ALL CELLS BUT BRAIN & RBCs ) TAKE UP Glc (Glut4)!!! ) TURN on glycolysis (ATP) ) Activates Glycogen Synthesis 4) inhibits gluconeogenesis 5) Blocks secretion of Glucagon Activates Protein Kinase A (PKA) IN ALL CELLS BUT BRAIN & RBCs ) STOP taking up Glc!!!!! ) Break down Glycogen ) Burn fatty acids 4) LIVER: TURN ON GLUCONEOGENESIS and export glucose 8