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Gluconeogenesis- Continued 11/12/2014 Gluconeogenesis- continued 11/12/2014 Cori Cycle Exchange of energetics between 2 different systems Looks at a tissue for one reason or another takes pyruvate (end product of glycolysis) and makes lactate o Ex.: red blood cells (no mitochondria, can’t metabolize pyruvate), muscle cells under oxygen limiting conditions and lack of blood flow lactate build up o Need glucose more than the liver in each case o Lactate converts to pyruvate to OAA to PEP back to glucose in liver and glucose goes back to tissues in need Exchange of high energy molecule for low energy molecule between the 2 systems (same with the alanine cycle) Fructose overriding the signal in glucokinase regulation Fructose goes to fructose 1-phosphate o Catalyzed by fructokinase Fructose 1-phosphate then interferes with fructose 6-phosphate signal (which is a repressor of glucokinase) Acts as effector, but not sure at which site Regulation of gluconeogenesis Regulation of gluconeogenesis and glycolysis are interrelated (pretty much the same thing) 3 different chemical steps regulated that are unique to gluconeogenesis: o 1) Conversion of pyruvate to phosphoenolpyruvate by pyruvate carboxylase and phosphoenolpyruvate carboxykinase through oxaloacetate . acetyl CoA is a positive effector of this conversion . acetyl CoA is a negative effector of pyruvate dehydrogenase in pyruvate metabolism (not this pathway) regulates conversion of pyruvate to acetyl CoA or OAA determines fate of pyruvate o 2) Conversion of fructose 1,6-bisphosphate to fructose 6- phosphate by fructose 1,6-bisphosphatase . Effector control (reciprocal and simultaneous control over glycolysis and gluconeogenesis): ratios of AMP and ATP AMP is a negative effector of fructose 1,6- bisphosphatase and positive effector of glycolysis ATP is a positive effector of fructose 1,6- bisphosphate conversion to fructose 6- phosphate by fructose 1,6-bisphosphatase citrate determines how much material is available for fatty acid biosynthesis citrate is a positive effector for the conversion of fructose 1,6-bisphosphate to fructose 6-phosphate by fructose 1,6- bisphosphatase fructose 2,6-bisphosphate is a negative effector for the conversion of fructose 1,6-bisphosphate to fructose 6-phosphate by fructose 1,6- bisphosphatase phosphorylation/dephosphorylation of bifunctional enzyme controls the fructose 2,6-bisphosphate effector o Bifunctional enzyme signaled by glucagon to be phosphorylated by protein kinase a, which shifts metabolism towards gluconeogenesis . When the bifunctional enzyme is phosphorylated, it keeps the fructose 2,6-bisphosphate effector from repressing gluconeogenesis o Bifunctional enzyme signaled by insulin to be dephosphorylated by phosphoprotein phosphatase, which shifts metabolism away from gluconeogenesis . When the bifunctional enzyme is dephosphorylated, it allows the fructose 2,6-bisphosphate effector to repress gluconeogenesis o 3) Conversion of glucose 6-phosphate to glucose by glucose 6-phosphatase . fructose 6-phosphate is a positive effector for the conversion of glucose 6-phosphate to glucose This is just basic substrate availability control Hormone control of gluconeogenesis 3 metabolic components that are regulated universally by the same systems: o 1) Lipids . Lipid metabolism connection to gluconeogenesis: provides energy for liver to invest energy into oxidized molecules like lactate and reform glucose Connection between lipids and carbohydrates o 2) Carbohydrates (like glucose) . glycolysis vs. gluconeogenesis . shift in oxidative catabolic pathway (glycolysis) vs. reductive anabolic pathway (gluconeogenesis) controlled through same system o 3) Glycogen glucagon & insulin o insulin signals to the liver that there’s high glucose levels . 1) Lipids decreases fatty acid beta-oxidation increases lipid synthesis . 2) Carbohydrates (like glucose) increases glycolysis decreases gluconeogenesis . 3) Glycogen makes liver store more glycogen & stop burning glycogen . insulin activates phosphoprotein phosphatase and inactivates protein kinase a causes dephosphorylation of the bifunctional enzyme promotes glycogenesis (storage of carbohydrates as glycogen) Stops glycogenolysis (liberation of glucose from glycogen) Removes cAMP, so that the bifunctional enzyme cannot be derepressed (cAMP levels are decreased by the enzyme cAMP phosphodiesterase) Activates the phosphoprotein phosphatase o glucagon tells liver there’s low [glucose] . 1) Carbohydrates (like glucose) increases gluconeogenesis stops glycolysis . 2) Lipids increases oxidation of fatty acids stops synthesis of fatty acids . 3) Glycogen stops glycogenesis (storing carbohydrates as glycogen) promotes liberation of glucose from glycogen to supply to blood (glycogenolysis) . Glucagon inactivates phosphoprotein phosphatase and activates protein kinase a Causes phosphorylation of the bifunctional enzyme Tissue differences: liver vs. muscle o liver metabolizes glycogen to send glucose out to the blood for tissues deficient in carbohydrate energy o muscle metabolizes glycogen to send glucose through glycolysis for carbohydrate energy to supply the muscle itself Transcriptional regulation of gluconeogenesis Covalent modification requires the protein to already be there o This is a fast response, because we don’t have to go through the central dogma (transcription of the DNA, translation of the RNA, etc.) Prolonged starvation and prolonged feast conditions are regulated differently (different from covalent modification and allosteric regulation) o Through transcriptional programming that controls the amounts of proteins rather than inactivation/activation of those proteins o Slower response, but prolonged duration of modification Signals aren’t on and off (glucagon/insulin), but ratios are shifted to shift metabolic responses o Receptors of cells perceive which signal as more dominant and then respond to it Transcriptional level regulation response to Glucagon o Glucagon hits the 7 membrane protein pass receptor, the g- protein coupled receptor . This activates the g protein, which activates adenylate cyclase This Makes cAMP cAMP is a derepressor of protein kinase a o Increases protein kinase a, which sends a signal called cAMP response element binding protein (CREB) to proteins in the nucleus . CREB is activated when phosphorylated by protein kinase a . Once activated, CREB can bind to CRE (cAMP response element, which is a transcription factor) Once CREB binds to CRE, it doesn’t increase transcription on its own, but is an activator that can pull in other co-activators to increase transcription of PEPCK gene Basal components of transcription are still needed PEPCK controls conversion of OAA to PEP 4 Functions of lipids o 1) Energy . fatty acids stored in adipose tissue as long-term energy than can then be liberated by fatty acid beta-oxidation to produce acetyl-CoA that can go through the TCA cycle and create energy for tissues, such as the liver during gluconeogenesis . Nonpolar neutral lipids are mostly used for long-term energy o 2) Insulation . Adipose tissue forms a layer beneath our skin that helps us trap heat and stay warm o 3) Membrane structure . Polar lipids are mostly used in cell membrane structures . Cholesterol is an example o 4) Signaling . IP3 and DAG Inositol triphosphate signals calcium influx into the cell, which is an important positive effector of 1) phosphorylase kinase, 2) calmodulin-dependent protein kinase, and 3) protein kinase C All of these kinases function to phosphorylate/inactivate glycogen synthase, which causes a metabolic shift away from glycogenesis and towards glycogenolysis Phosphorylase kinase also phosphorylates/activates glycogen phosphorylase, which causes a shift towards glycogenolysis Diacylglycerol signals protein kinase C to phosphorylate/inactivate glycogen synthase, causing a metabolic shift away from glycogenesis and towards glycogenolysis Lipids are not easily definied, why? o No universal metabolic pathway o Simply called lipids because they’re nonpolar (chemically physically similar) o Diverse collection of molecules . Not related in structure, function, or biosynthesis to one another We are incapable of sustaining ourselves from fatty acid biosynthesis o We must consume dietary essential fatty acids . The 2 essential dietary fatty acids are: 1) linoleic 2) linolenic acid These can be used to make arachadonic acid, which can be used to make important signaling molecules Structure and Chemistry of fatty acids and acylglycerols Fatty acids are long hydrocarbon chains with a carboxyl group o Alkyl group = long hydrocarbon chain o Acyl group = long hydrocarbon/alkyl group with a carboxyl group attached . These are acyl lipids (carboxylic acid with a fatty acid as its R group) o Saturated means every carbon in the alkyl portion of the acyl lipid is connected to either another carbon by a single bond and or completely surrounded by/“saturated” with hydrogen atoms o Unsaturated means you’ve removed some hydrogen’s and replaced them with double bonds between two carbons . Double bonds between 2 carbons in a fatty acid are almost exclusively in the cis- configuration . Cis- means the hydrogen’s in the double bond are on the same side of the molecule, while trans- means they’re opposite Typical fatty acids are even chain numbers o Most have 16 and 18 carbons (such as our 5 dominant ones) Saturates and unsaturates o Saturated with hydrogens o Unsaturated have double bonds . Can have up to 6 double bonds typically . Mono- di-
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