Anabolic and Alternative Pathways: A Review of Gluconeogenesis and the Pentose Phosphate Pathway

End Chapter 18 Lecture Title: (Gluconeogenesis) Lecture Date: 31/10/2021 Questions • • • • Where it take place ? cytosol & mitochondria the liver 90% and Which organs use gluconeogenesis ? Inin kidneys 10% The amount of glucose when it Why it is important? Raise decreases due to lack of carbohydrates What are the sources of glucose in glycerol and blood other than glycogen ? lactate, amino acids Gluconeogenesis • The synthesis of glucose from none carbohydrate sources like lactate, glycerol and amino acids. – gluconeogenesis is not the exact reversal of glycolysis; that is, pyruvate to glucose does not occur by reversing the steps of glucose to pyruvate. – It is impossible to reverse any kinase reaction under physiological conditions. – – gluconeogenesis occur in the cytosol & mitochondria gluconeogenesis takes place in the liver 90% and in kidneys 10% Sheet Note#1 Lactate : come from Pyruvate then go to liver and give us glucose. Glycerol :come from triglyceride breakdown Amino acid : 18 glycogenic and 2 ketogenic (Lysine and leucine) glycolysis irreversible reactions: hexokinase, phosphofructokinase, and pyruvate kinase . gluconeogenesis important severe muscular exercise and starvation Gluconeogenesis – There are three irreversible steps in glycolysis --- phosphoenolpyruvate to pyruvate + ATP (pyruvate kinase) --- fructose-6-phosphate to fructose-1,6Bisphosphate(phosphofructokinase) --- glucose to glucose-6-phosphate(hexokinase) – The net result of gluconeogenesis is reversal of these three steps, but by different reactions and using different enzymes (bypassing) Sheet note #2: In production of glucose from lactic we consume 6 ATP or GTP +2 ATP Gluconeogenesis • – – Step 1: carboxylation of pyruvate (1st bypass) requires biotin ( vitamin B complex(coenzyme)) pyruvate carboxylase is subject to allosteric control; it is activated by acetyl-CoA O CH 3 CCOO Pyruvate 3 Carbons + CO 2 + ATP biotin pyruvate carboxylase O CH 2 + A DP + CCOO 2 H+ COOOxaloacetate Pi + 4 Carbons Sheet note #3: Oxaloacetate can’t leave the mitochondria, so it’s left in the form of malate, Biotin • Biotin is a carrier of CO2 (carboxylation) HN ONH H H 2 N- 1. H e nz y m e 2 . CO 2 COO - S Biotin O AT P O C - + O N NH H H S C O N H- e nz y me Gluconeogenesis – Decarboxylation of oxaloacetate is coupled with phosphorylation by GTP to give PEP(Phosphoenolpyruvate) O OPO3 2 - CH2 CCOO- + P GT CO 2 Oxaloacetate CH2 = CCOOCO2 Phosphoenolpyruvate + GDP + – The net reaction of carboxylation/decarboxylation is P yruva te + A TP + Sheet note #4: multiplying by 2 , so 4 ATPs are required GT P P hosphoe nolpyruva te H+ + A DP + GD P + Pi + – Net reaction is close to equilibrium: G0’ = 2.1 kJ•mol-1 2 Gluconeogenesis • Second different reaction (2nd bypass) in gluconeogenesis CH2 O P O 3 2 O CH2 O P O 3 2 HO H + H2 H O O H HO H -D-Fructose-1,6-bisphosphate fructose 1,6-bisphosphatase Mg 2 + CH2 O P O 3 2 CH2 O H O HO H H O H HO H -D-Fructose-6-phosphate + Pi – G° = -16.7•kJ mol-1 – Fructose-1,6-bisphosphatase is an allosteric enzyme, inhibited by AMP and F2,6P and activated by ATP Gluconeogenesis • Third different reaction (3rd bypass) in gluconeogenesis CH 2 OPO 3 2 HO HO O +H OH O OH -D-Glucose-6-phosphate glucose-6phosphatase 2 G°’ = -13.8 kJ•mol1 HO HO CH 2 OH O + OH P OH -D-Glucose i The Cori Cycle • The Cori cycle – Under vigorous anaerobic exercise, glycolysis in muscle tissue converts glucose to pyruvate; NAD+ is regenerated by reduction of pyruvate to lactate – Lactate from muscle is transported to the liver where it is reoxidized to pyruvate and converted to glucose – Thus, the liver shares the stress of vigorous exercise Sheet note #5 In the state of vigorous exercise The muscle work on glycolysis and covert lactate to glucose by Gluconeogenesis The Cori Cycle Sheet note #6: we found NADH in low concentration in muscles because it is consumed there to produce ATP (glycolysis). Also, NADH produced in the liver, so we find it in high concentration there (glycogenesis) Control of carbohydrate metabolism How ? Control of carbohydrate metabolism • – – – – Allosteric: fructose-2,6-bisphosphate (F2,6P) high concentration of F2,6P stimulates glycolysis; a low concentration stimulates gluconeogenesis. concentration of F2,6P in a cell depends on the balance between its synthesis (catalyzed by phosphofructokinase-2) and its breakdown (catalyzed by fructose bisphosphatase-2) AMP inhibits FBPase and stimulates PFK each enzyme is controlled by phosphorylation/ dephosphorylation Fructose-2,6-phosphate differ Fructose-1,6-bisphosphate Sheet Note#7 Fructose-2,6-bisphosphate like switch one time on and another time off . FBPase: Fructose bisphosphatase This role in metabolism :there is no anabolism and catabolism at the same time Fructose-2,6-bisphosphate Fructose-2,6-bisphosphate is an allosteric activator of phosphofructokinase (a glycolytic enzyme) and an allosteric inhibitor of fructose bisphosphate phosphatase (an enzyme in the pathway of gluconeogenesis). p. 520 Reciprocal Regulation of Gluconeogenesis and Glycolysis in the Liver Check this video : https://www.youtube.com /watch?v=ardjd4h2Seo Sheet note #8: Enzymes that inhibit the Glycolysis activate in the Gluconeogenesis Control of carbohydrate metabolism Allosteri c Covalent modificatio n Substrat e cycles Geneti c Effectors (substrates, products, or coenzymes) of a pathway inhibit or activate an enzyme Inhibition or activation of an enzyme depends on formation or breaking of a covalent bond, often by phosphorylation or dephosphorylation Two opposing reactions (such as formation or breakdown of a substance) are catalyzed by different enzymes, which are activated or inhibited separately The amount of enzyme present is increased by protein synthesis Control of carbohydrate metabolism • Substrate cycling – opposing reactions can be catalyzed by different enzymes and each opposing enzyme or set of enzymes can be regulated independently Fructose-6-phosphate + Fructose 1,6bisphosphate phosphofructokinase AT P Fructose 1,6bisphosphate fructose-1,6+ H2 O bisphosphatase G0' (kJ•mol -1) + ADP -25.9 + P Fructose-6phosphate i -8. 6 Major Control Points in Carbohydrate Metabolism • – Three steps in glycolysis are major control points in glucose metabolism Hexo`kinase Inhibited by high levels of glucose 6-phosphate Phosphofructokinase, – – – • – – – When glycolysis is inhibited through glucose 6-phosphate builds up, shutting down hexokinase Pyruvate kinase (PK) is an allosteric enzyme Inhibited by ATP and alanine Activated by fructose-1,6-bisphosphate PK has 3 different isoenzymes M predominates in muscle, L in liver, and A in other tissues Native PK is a tetramer Liver isoenzymes are subject to covalent modification Control of Pyruvate Kinase Feed forward activation Feed backward inhibition • • Summar y A number of control mechanisms operate in carbohydrate metabolism. These include allosteric effectors, covalent modification, substrate cycles, and genetic control. In the mechanism of substrate cycling, the synthesis and the breakdown of a given compound are catalyzed by two different enzymes. Pentose Phosphate Pathway Questions Where it take place ? cytosol Which organs use PPP ? RBCs Why it is important ? Pentose Phosphate Pathway (PPP) – As the name implies, five-carbon sugars, including ribose, are produced from glucose – The oxidizing agent is NADP+; it is reduced to NADPH, which is a reducing agent in biosyntheses e.g. lipid – PPP is composed from two reactions: 1. Oxidative reactions: begins with two oxidation steps (using NADP+) to give ribulose-5-phosphate 2. Non-oxidative reactions: a series of carbon-shuffling steps during which three-, four-, five-, six-, and sevencarbon monosaccharide phosphates are produced – ATP production is not an important concern Sheet note #9(1): Don't forget that NADPH is a proton donor and NADH is an electron carrier Cheak this : https://www.youtube.com/wa tch?v=nkTDi2WIm4w Sheet note #9(2): Oxidative reaction is irreversible Non-oxidative reaction is reversible Sheet note #9(3): NADP+ important in lipid and cholesterol metabolism & Hermon synthesis PPP oxidative reactions H HO H CHO OH H OH NAD + NADP P OH H H COOH OH NADP + HO H HNADPH OH H OH CH 2 OPO 32 6-Phosphogluconate - CH 2 OPO 32 Glucose-6-phosphate COOOH C O H OH H OH H CH 2 OPO 32 - CH 2 OH C O + CO 2 H OH H OH CH 2 OPO 32 Ribulose-5-phosphate Sheet note #10: The responsible Enzyme of this reaction is glucose 6 phosphate dehydrogenase enzyme Cheak this : https://www.youtube.com /watch?v=nMWmmJLEI9I Non-oxidative reactions(shoveling of carbon) RN A CH2 OH C O H OH H OH CH2 OPO 32 Ribulose-5phosphate CHO OH OH OH CH2 OPO 32 Ribose-5-phosphate H H H CH2 OH C O HO H H OH CH2 OPO 32 Xylulose-5-phosphate - CH2 OH C O HO H H OH H OH H OH CH2 OPO 32 Sedoheptulose7-phosphate CHO OH CH2 OPO 32 Glyceraldehyde3-phosphate H Cheak this : https://www.youtube.com /watch?v=dWwXvo2vpBc CH 2 OH C O HO H OH H H OH H OH CH 2 OPO 32 Sedoheptulose-7-phosphate CHO H OH CH 2 OPO 32 Glyceraldehyde-3phosphate CHO H OH H OH CH 2 OPO 32 Erythrose-4-phosphate CH 2 OH C O HO H OH H H OH CH 2 OPO 32 Fructose-6phosphate CH 2 OH O C HO H H OH CH 2 OPO Xylulose-5- CHO OH CH 2 OPO 32 Glyceraldehyde3-phosphate H 3 2- phosphat e CHO OH H H OH CH 2 OH C HO OH OH H H OH To glycolysis CH2 OPO 32 CH2 OPO 32 - - Erythrose- 4phosphate Fructose-6phosphate Pentose Phosphate Pathway Sheet note #11 (1): Glyceraldehyde-3- phosphate and Fructose-6- phosphate are intermediate in glycolysis Sheet note #11 (2): Transaldolase: the process of transferring aldehyde group Transketolase : the process of transferring Ketone group Pentose Phosphate Pathway – The carbon-shuffling reactions are catalyzed by --- transketolase for the transfer of two-carbon units requires thiamine pyrophosphate as a coenzyme --- transaldolase for the transfer of three-carbon units • – – – Control of the pentose phosphate pathway glucose-6-phosphate (G6P) can be channeled into either glycolysis or the pentose phosphate pathway if ATP needed, G6P is channeled into glycolysis if NADPH or ribose-5-phosphate are needed, G6P is channeled into the pentose phosphate pathway Sheet note #12: Thiamine pyrophosphate is vit B1 Relationship between PPP and Glycolysis G-6-PD • • • Sheet note #13: deficiency of G-6-PD lead to hemolytic anemia More than 400 variants of G-6-PD have been characterized, which show less activity than normal. G-6-PD is the most common human enzyme deficiency in the world. It affect an estimated 400 million people. Hemolysis, abdominal pain, dizziness, headache, dyspnea, palpitation, neonatal jaundice. Sheet note #14: H2O2 is free radicals Glutathione is a substa made from three amin acids glycine, cysteine and glutamic acid Glutathione acts as an antioxidant Precipitating Factors • • • • Infection & other ac. Illness(diabetic ketoacidosis) Drugs: Antimalarials, Antipyretics or Antibiotics Fava beans Neonatal jaundice : due to decrease hepatic catabolism or increase production of bilirubin. Pentose Phosphate Pathway • Summary of oxidative reactions • Glucose-6-phosphate + 2 NADP Ribulose-5-phosphate + CO2 + 2 NADPH • Summary of non-oxidative reactions Reactant Enzyme Products Transketolase C 5 + C5 C 7 + C3 7 C + Transaldolase C 3 C 5 Net: + C4 3 C 5 Transketolase C 6 + 6 + C3 C 4 C 2C 6 + C3