Gluconeogenesis And Pentose Phosphate Pathway PDF

Gluconeogenesis and Pentose Phosphate Pathway  Gluconeogenesis, and the Pentose Phosphate Pathway Learning goals: Review glycolysis and fates of pyruvate Synthesis of glucose from simpler compounds: gluconeogenesis Oxidation of glucose in pentose phosphate pathway What a...

Gluconeogenesis and Pentose Phosphate Pathway  Gluconeogenesis, and the Pentose Phosphate Pathway Learning goals: Review glycolysis and fates of pyruvate Synthesis of glucose from simpler compounds: gluconeogenesis Oxidation of glucose in pentose phosphate pathway What are the important things to note in studying metabolic pathways? Function of the pathway Reactants, products, and overall reaction Cells involved Specific sites in the cell o Transport mechanism involved if needs to be transported to another site Energy balance (energy metabolism) o ATP, GTP, NADH, FADH Regulation Diseases involved Drug mode of action  Identify Key Steps How to identify key steps? - Rate limiting steps - Irreversible reactions - Connections with other pathways What are important to know? - Reactants and products - Enzyme involved and regulation REVIEW OF GLYCOLYSIS  Central Importance of Glucose Glucose is an excellent fuel. – yields good amount of energy upon oxidation −2840 kJ/mol glucose – can be efficiently stored in the polymeric form – Many organisms and tissues can meet their energy needs on glucose only. Glucose is a versatile biochemical precursor. – Many organisms can use glucose to generate: all the amino acids membrane lipids nucleotides in DNA and RNA cofactors needed for the metabolism  Four Major Pathways of Glucose Utilization Storage – can be stored in the polymeric form (starch, glycogen) – used for later energy needs Energy production – generates energy via oxidation of glucose – short-term energy needs Production of NADPH and pentoses – generates NADPH for use in relieving oxidative stress and synthesizing fatty acids – generates pentose phosphates for use in DNA/RNA biosynthesis Structural carbohydrate production – used for generation of alternate carbohydrates used in cell walls of bacteria, fungi, and plants Four Major Pathways of Glucose Utilization Glycolysis  Summary of Glycolysis Glucose + 2 NAD+ + 2 ADP + 2 Pi  2 Pyruvate + 2 NADH + 2 H+ + 2 ATP Used: – 1 glucose; 2 ATP; 2 NAD+ Made: – 2 pyruvate various different fates – 4 ATP used for energy-requiring processes within the cell – 2 NADH must be reoxidized to NAD+ in order for glycolysis to continue Glycolysis is heavily regulated – ensure proper use of nutrients – ensure production of ATP only when needed Summary of Glycolysis Summary of Glycolysis Regulation Hexokinase Affinity Glucose Transporters Glucose Transporters Fructose 2-6 bisphosphate Regulation Phosphofructokinase 1 Regulation by Reactants and Products Increase in reactants  activation leading to increase formation of products Increase in products  inhibition leading to decrease formation of products FATES OF PYRUVATE Fates of Pyruvate GLUCONEOGENESIS  Gluconeogenesis: Making “New” Glucose Gluconeogenesis is the synthesis of glucose from small sugars and non- carbohydrate compounds. Function: supply glucose-dependent cells; bridge periods of prolonged fasting  Precursors for Gluconeogenesis Animals can produce glucose from sugars or proteins. – sugars: pyruvate, lactate, or oxaloacetate – protein: from amino acids that can be converted to citric acid cycle intermediates (or glucogenic amino acids) Animals cannot produce glucose from fatty acids. – product of fatty acid degradation is acetyl-CoA – cannot have a net converstion of acetyl-CoA to oxaloacetate Plants, yeast, and many bacteria can do this, thus producing glucose from fatty acids.  Glycolysis versus Gluconeogenesis Glycolysis occurs mainly Gluconeogenesis occurs in the muscle and brain. mainly in the liver.  Glycolysis versus Gluconeogenesis Opposing pathways that are both thermodynamically favorable – operate in opposite direction end product of one is the starting compound of the other Glycolysis reactions are essentially irreversible in vivo and cannot be used in gluconeogenesis – must be bypassed with exergonic reactions – no ATP generated during gluconeogenesis – different enzymes in the different pathways – differentially regulated to prevent a futile cycle  Gluconeogenesis Is Expensive 2 Pyruvate + 4 ATP + 2 GTP + 2 NADH + 2 H+ + 4 H2O  Glucose + 4 ADP + 2 GDP + 6 Pi + 2 NAD+ Costs 4 ATP, 2 GTP, and 2 NADH Physiologically necessary: Brain, nervous system, and red blood cells generate ATP ONLY from glucose. Allows generation of glucose when glycogen stores are depleted: – during starvation – during vigorous exercise – can generate glucose from amino acids, but not fatty acids Gluconeogenesis Gluconeogenesis  Conversion of pyruvate to phosphoenol pyruvate Takes place in two steps pyruvate carboxylase is a biotin dependent mitochondrial enzyme that converts pyruvate to oxaloacetate in presence of ATP & CO2 This enzyme regulates gluconeogenesis & requires acetyl COA for its activity.  Oxaloacetate is synthesized in the mitochondrial matrix. It has to be transported to the cytosol. Due to membrane impermeability, oxaloacetate cannot diffuse out of the mitochondria. It is converted to malate & transported to cytosol. In the cytosol, oxaloacetate is regenerated.  The reversible conversion of oxaloacetate to malate is catalysed by MDH, present in mitochondria & cytosol In the cytosol, phosphoenolpyruvate carboxykinase converts oxaloacetate to phosphoenol pyruvate.  GTP (not ATP) is used in this reaction and the CO2 is liberated. For the conversion of pyruvate to phosphoenol pyruvate, 2ATP equivalents are utilized. Gluconeogenesis  Conversion of Fructose 1,6-bisphosphate to fructose 6-phosphate Phosphoenolpyruvate undergoes the reversal of glycolysis until Fructose 1,6-bisphosphate is produced. The enzyme Fructose 1,6-bisphosphatase converts Fructose 1,6-bisphosphate to Fructose 6-phosphate and it requires Mg2+ ions. This is also a regulatory enzyme.  Conversion of glucose 6-phosphate to glucose Glucose 6-phosphatase catalyses the conversion of glucose 6-phosphate to glucose. It is present in liver & kidney but absent in muscle, brain and adipose tissue. Liver can replenish blood sugar through gluconeogenesis, glucose 6- phosphatase is present mainly in liver. Gluconeogenesis Regulation  Gluconeogenesis Regulation *Reciprocal Regulation  Gluconeogenesis from amino acids The carbon skeleton of glucogenic amino acids (all except leucine & lysine) results in the formation of pyruvate or the intermediates of citric acid cycle. Which, ultimately, result in the synthesis of glucose.  Glucogenic Amino Acids Glucogenic amino acids = able to undergo net conversion to glucose Intermediates of the citric acid cycle can also undergo oxidation to oxaloacetate Glucogenic Amino Acids, Grouped by Site of Entry Pyruvate α-Ketoglutarate Succinyl-CoA Fumarate Oxaloacetate Alanine Arginine Isoleucine Phenylalanine Asparagine Cysteine Glutamate Methionine Tyrosine Asparate Glycine Glutamine Threonine Serine Histidine Valine Threonine Proline Tryptophan  Gluconeogenesis from glycerol Glycerol is liberated in the adipose tissue by the hydrolysis of fats (triacylglycerols). The enzyme glycerokinase (found in liver and kidney, absent in adipose tissue) activates glycerol to glycerol 3-phosphate. It is converted to DHAP by glycerol 3-phosphate dehydrogenase. DHAP is an intermediate in glycolysis.  Gluconeogenesis from propionate Oxidation of odd chain fatty acids & the breakdown of some amino acids (methionine, isoleucine) yields a three carbon propionyl COA. Propionyl COA carboxylase acts on this in the presence of ATP & biotin & converts to methyl melonyl COA  Which is then converted to succinyl CoA in the presence of B12. Succinyl CoA formed from propionyl CoA enters gluconeogenesis.  Gluconeogenesis from lactate (CORI cycle) It is a process in which glucose is converted to Lactate in the muscle and in the liver this lactate is re-converted to glucose. In an actively contracting muscle, pyruvate is reduced to lactic acid which may tend to accumulate in the muscle. To prevent lactate accumulation, body utilizes cori cycle. ANAEROBIC RESPIRATION Fates of Pyruvate  Anaerobic Glycolysis: Fermentation Generation of energy (ATP) without consuming oxygen or NAD+ No net change in oxidation state of the sugars Reduction of pyruvate to another product Regenerates NAD+ for further glycolysis under anaerobic conditions The process is used in the production of food from beer to yogurt to soy sauce.  Animals Undergo Lactic Acid Fermentation Reduction of pyruvate to lactate, reversible During strenuous exercise, lactate builds up in the muscle. – generally, less than 1 minute The acidification of muscle prevents its continuous strenuous work. The lactate can be transported to the liver and converted to glucose there. Requires a recovery time – high amount of oxygen consumption to fuel gluconeogenesis – restores muscle glycogen stores Lactic Acid Fermentation Lactic Acid Fermentation Lactic Acid Fermentation The Cori Cycle The Cori Cycle PENTOSE PHOSPHATE PATHWAY Four Major Pathways of Glucose Utilization  Two Phases of the Pentose Phosphate Pathway Simplified AKA Hexose Monophosphate Shunt Occurs in all cell types but primarily in the liver – Generally, 10% of glucose in metabolized thru the PPP – In the liver 30%  Pentose Phosphate Pathway The main products are NADPH and ribose 5-phosphate. NADPH is an electron donor. – reductive biosynthesis of fatty acids and steroids (50%) – repair of oxidative damage – Important in cytochrome P450 enzyme Ribose-5-phosphate is a biosynthetic precursor of nucleotides. – used in DNA and RNA synthesis – or synthesis of some coenzymes Also interconverts sugars – to produce trioses, hexoses and pentoses  Oxidative Phase Generates NADPH and a Pentose  Oxidative Phase Generates NADPH and a Pentose Ribulose-5-phosphate can be used to generate ribose-5-phosphate for DNA/RNA. Ribulose-5-phosphate can also be used to generate xylulose-5-phosphate for nonoxidative PPP. NADPH and Oxidative Stress  Nonoxidative Phase Regenerates G-6-P from R-5-P Used in tissues requiring more NADPH than R-5-P – such as the liver and adipose tissue  PPP Clinical Correlates: Glucose-6-Phosphate Dehydrogenase Deficiency Deficiency of G6PD – 7.5% of world population deficient – 35% prevalence in certain areas of Africa – X-linked recessive inheritance May cause: – Hemolytic Anemia Oxidative triggers: – Infections: Typhoid fever, pneumonia – Drugs: Antimalarial, sulfonamides, nitrofurantoin, analgesics – Certain food: fava beans  Summary Glycolysis, a process by which cells can extract a limited amount of energy from glucose Fermentation, a process by which cells can continue using glycolysis to extract energy in anaerobic conditions Gluconeogenesis, a process by which cells can use a variety of metabolites for the synthesis of glucose The differences between glycolysis and gluconeogenesis – how they are both made thermodynamically favorable – how they are differentially regulated to avoid a futile cycle The pentose phosphate pathway, a process by which cells can generate pentose phosphates and NADPH. The pentose phosphates can be regenerated into glucose-6-phosphate, which requires no ATP.