1 Ó Department of Biochemistry 2013 (E.T.) Basic concept and design of metabolism The glycolytic pathway 2 Metabolism Metabolism – processes at which living organism utilizes and produces energy. Living organisms require a continual input of free energy for three major purposes: – the performance of mechanical work in cellular movements, – the active transport of molecules and ions across membranes, – the synthesis of macromolecules and other biomolecules from simple precursors. Roles of Metabolism • to provide energy (catabolic processes) • to synthesize molecules (anabolic processes) • both types of processes are tightly connected 3 The metabolic interplay of living organisms in our biosphere Two large groups of living organisms according to the chemical form of carbon they require from the environment. Autotrophic cells ("self-feeding" cells) – green leaf cells of plants and photosynthetic bacteria – utilize CO2 from the atmosphere as the sole source of carbon for construction of all their carbon-containing biomolecules. They absorb energy of the sunlight. The synthesis of organic compounds is essentially the reduction (hydrogenation) of CO2 by means of hydrogen atoms, produced by the photolysis of water (generated dioxygen O2 is released). Heterotrophic cells – cells of higher animals and most microorganisms – must obtain carbon in the form of relatively complex organic molecules (nutrients such as glucose) formed by other cells. They obtain their energy from the oxidative (mostly aerobic) degradation of organic nutrients made by autotrophs and return CO2 to the atmosphere. Carbon and oxygen are constantly cycled between the animal and plant worlds, solar energy ultimately providing the driving force for this massive process. 4 j0149627 MPj04116610000[1] MCj04338230000[1] MCj04321230000[1] CO2 Small molecules macromolecules Proteins, lipids , saccharides energy O2 The metabolic interplay in human Green plants Absorb energy of the sunlight Autotrophs CO2 Heterotrophs 5 Energy in chemical reactions Gibbs free energy (D G) The maximal amount of useful energy that can be gained in the reaction (at constant temperature and pressure) aA+bB ® cC+dD The ΔG of a reaction depends on the nature of the reactants (expressed by the ΔGº term) and on their concentrations (expressed by the second term). DGº´ = – RT ln K D G0´ pH = 7,0, T=25 oC) 6 •They permanently take up nutrients with the high enthalpy and low entropy •Nutrients are converted to waste products with low enthalpy and high entropy •Energy extracted from nutrients is used to power biosynthetic processes and keep highly organised cellular structure •A part of energy is converted to heat •Living organism can never be at equilibrium • Steady state - open systems in which there is a constant influx of reactants and removal of products •Reactions are arranged in series, product of one reaction is a substrate of the following reaction • • Living organisms as open systems 7 Biochemical processes exergonic endergonic Endergonic reactions can proceed only in coupling with exergonic reactions Transfer of energy from one proces to another process in enabled by „high-energy“ compounds Mostly ATP is used. Phosphoryl group PO32- is transferred from one to another compound in process of coupling 8 -PO32- is transferred by the enzyme kinase from ATP to glucose. Principles of coupling DGo´ = +13,8 kJ/mol DGo´ = -30,5 kJ/mol Example 1: Formation of glucose-6-phosphate glucose + Pi ® glucose-6-P + H2O DGo´ = - 16,7 kJ/mol glucose + ATP ® glucose -6-P + ADP ATP + H2O ® ADP + Pi 9 9 DG 1 > 0 pyruvate + HCO3- oxalacetate Example 2: Carboxylation of pyruvate HCO3- + ATP ® ADP + -OCO-O-PO32- phosphocarbonate -OCO-O-PO32- + biotin ® -OOC-biotin + Pi -OOC-biotin + pyruvate ® oxalacetate biotin + ATP + HCO3- → carboxybiotin + ADP + Pi carboxybiotin + pyruvate → biotin + oxalacetate DG 2 < 0 DG < 0 Partial reactions: ATP ADP + Pi biotin DG < 0 10 10 ATP + HCO3- ® ADP + -O -P-O-C-O- O- O O phosphocarbonate N N H O S C O enzym C O O- Carboxylate anion is activated by binding Pi and by means of biotin attached to enzyme is transferred to pyruvate Carboxylation of biotin - formulas Remember: •Biotin is necessary for carboxylation reactions •Carboxylation is always connected with ATP cleavage 11 The term „high-energy compound“ (also „macroergic compound“ or „energy rich compounds“ ) The most important is ATP O O H O H 2 O O C H O P O P O O O ~ P O O O ~ + H+ H N N N N N H 2 ATP + P O O O H O 2 O O O H O H N N N N N H 2 2 ADP P O O O C H O P O O O ~ 12 ATP provides energy in two reactions: ATP + H2O ® ADP + Pi D G0´ = -30,5 kJ/mol ATP + H2O ® AMP + PPi D G0´ = -32,0 kJ/mol Reactions are catalyzed by enzymes Similarly GTP, UTP a CTP can provide energy 13 Compounds that by hydrolytic cleavage provide energy that is comparable or higher than DG0´of ATP hydrolysis Most often derivatives of phosphoric acid containing phosphate bonded by: Øanhydride Ø amide Øenolester bond The other high-energy compounds (esters of phosphoric acid are not macroergic compounds) 14 Most important macroergic phosphate compounds These compounds are formed in metabolic processes. Their reaction with ADP can provide ATP = substrate phosphorylation Compound D G0 (kJ/mol) typ compound phosphoenolpyruvate -62 enolester carbamoyl phosphate -52 mixed anhydride 1,3-bisphosphoglycerate -50 mixed anhydride phosphocreatine -43 amide 15 Energy- rich compounds may be also thioesters (e.g. acyl group bonded to coenzym A) D G0 = -31,0 kJ/mol 16 How are formed energy-rich compounds during metabolism ? „combustion of nutrients“ • nutrients in food (lipids and saccharides, partially proteins) contain carbon atoms with low oxidation number • they are continuously degraded (oxidized) to various intermediates, that in decarboxylation reactions release CO2 • electrons and H atoms are transferred to redox cofactors (NADH, FADH2 ) and transported to terminal respiratory chain •energy released by their reoxidation is utilized for synthesis of ATP (oxidative phosphorylation) • several high energy compounds are formed directly during the metabolism of nutrients – they provide ATP in a reaction with ADP (phosphorylation of ADP on substrate level) 17 Formation of ATP in the cell ØOxidative phosphorylation Accounts for more than 90% of ATP generated in animals = the synthesis of ATP from ADP and Pi ADP + Pi ® ATP catalysed by ATP-synthase Reaction is driven by the electrochemical potential of proton gradient across the inner mitochondrial membrane. This gradient is generated by the terminal respiratory chain, in which hydrogen atoms, as NADH + H+ and FADH2 produced by the oxidation of carbon fuels, are oxidized to water. The oxidation of hydrogen by O2 is coupled to ATP synthesis. ØPhosphorylation of ADP on substrate level Transfer of -PO32- from energy rich compound to ADP 18 phosphoenolpyruvate pyruvate ADP ATP pyruvate kinase succinyl coenzyme A succinate + CoA Examples of substrate-level phosphorylations In glycolysis 1,3-bisphosphoglycerate 3-phosphoglycerate ADP ATP 3-phosphoglycerate kinase In the citrate cycle GDP + Pi GTP thiokinase ADP + H+ ATP creatine kinase phosphocreatine creatine In skeletal muscle phosphocreatine serves as a reservoir of high-potential phosphoryl groups that can be readily transferred to ATP: 19 ATP in cells •Life expectancy of an ATP molecule is about 2 min. •It must be permanently synthesized •Momentary content of ATP in a human body is about 100 g, but 60-70 kg is produced daily •Adenylate kinase maintains the equilibrium between ATP, ADP a AMP • ATP + AMP 2 ADP • • • • • • 20 [ATP]/[ADP] ratio (in most cells 5-200) Energy charge of the cell: The energy charge of most cells ranges from 0.80 to 0.95 Energy status of a cell 21 Control of metabolism Metabolism is regulated by controlling § catalytic activity of enzymes allosteric and cooperative effects, reversible covalent modification, substrate concentration § the amount of enzymes synthesis of adaptable enzymes § the accessibility of substrates compartmentalization segregates biosynthetic and degradative pathways, the flux of substrates depends on controlled transfer from one compartment of a cell to another § the energy status of the cell of which the energy charge or the phosphorylation potential are used as indexes § communication between cells hormones, neurotransmitters, and other extracellular molecular signals often regulate the reversible modification of key enzymes 22 Metabolism of sacharides 1 Celular metabolism of glucose 23 Transport of Glucose into the cells Glucose transporters Transmembrane proteins facilitating a transport of glucose 2 main types: GLUT (1-14)* and SGLT** * glucose transporter ** sodium-coupled glucose transporter Molecules of glucose are strongly polar, they cannot diffuse freely across the hydrophobic lipid bilayer 24 GLUT 1-GLUT 14, family of transporters with common structural features (isoforms) but a tissue specific pattern of expresion: ~ 500 AA, 12 transmembrane helices Mechanism of transport: facilitated diffusion (follows concentration gradient, do not require energy) 25 Differences between the GLUT transporters • affinity to glucose • different way of regulation • tissue specific occurence 26 Glucose transporters Typ characteristics GLUT 1 Basal glucose uptake (ercs, muscle cells at resting conditions, brain vessels ..) GLUT 2 Liver, b cells of pancreas , kidney GLUT 3 Neurons, placental cells GLUT 4 Muscle, adipocytes – dependent on insulin - GLUT 5 Transport of fructose, small intestine - GLUT 7 Intracelular transport liver 27 Transport of glucose by GLUT Two conformational states of transporters Extracellular space cytosol glucose 28 Extracellular space cytosol 29 Extracellural space 30 GLUT 1 deficiency Inherited deficiency of glucose transporter Transport across the blood brain barrier is reduced The glucose concentration in cerebrospinal fluid is decreased. Moreover GLUT1 is essential also for transport of glucose into neurons and glial cells As glucose is the principal source of fuel to the brain, GLUT1 deficiency causes impaired provision of energy ® epileptic encephalopathy Observation: decreased level of glucose in liquor Treatment: The only known treatment to date is a very restrictive diet called the ketogenic diet 31 GLUT 4 carriers are regulated by insulin (muscle, adipocytes) insulin receptor "Sleeping“ GLUT4 in the membranes of endosomes insulin > 32 insulin receptor insulin vesicles move to the membrane Binding of insulin to its receptor > 33 insulin receptor Vesicles fuse with the plasma membrane Transport of glucose into the cell > 34 The presence of insulin leads to a rapid increase in the number of GLUT4 transporters in the plasma membrane. Hence, insulin promotes the uptake of glucose by muscle and adipose tissue. GLUT 4 receptors– conclusion: 35 Transport of glucose into the epithelial cells of the small intestine and renal tubules cells SGLT - transporters • Mechanism: cotransport with Na+ secondary active transport •glucose and Na+ bind to two specific sites of the carrier • they are transported into the cell at the same time (without energy requirement) • Na+ is consequently transported outside the cell by ATPase (active transport - consumption of ATP) • glucose is consequently transported outside the cell by GLUT2 36 Cotransport of glucose with Na+ Lumen of the small intestine enterocyte 37 Transporter changes conformation after the binding glucose and Na+ lumen enterocyte 38 Na+ and glucose enter the cell (symport) lumen enterocyte 39 Na+/K+-ATPase is located on the capillary side of the cell and pumps sodium outside the cell (active transport) enterocyte Extracelular space 40 glucose is exported to the bloodstream via uniport system GLUT-2 (pasive transport) enterocyte glucose extracellular space 41 41 SGLT1 deficiency Hereditary disturbance in the transport of glucose and galactose Rare disorder (autosomal recessive patern). This failure of active transport prevents the glucose and galactose from being absorbed. Symptoms become apparent in the first weeks of a baby's life. Severe diarrhea leading to life-threatening dehydration, destabilization of the acidity of the blood and tissues (acidosis), stomach cramps. Why diarrhea? The water that normally would have been transported across the brush border with the sugar instead remains in the intestinal tract to be expelled with the stool, resulting in dehydration of the body's tissues and severe diarrhea. 42 Glucose-6-phosphate is formed immediately after the glucose enters a cell: glucose + ATP glucose-6-P + ADP Glucose metabolism in cells Enzymes hexokinase or glucokinase 43 Glucose phosphorylation phosphorylation glucose glucose ATP ADP glucose-6-P Hexokinase, (glucokinase) The reverse reaction is catalyzed by glucose-6-P phosphatase This occurs only in liver (and in less extent in kidney). 44 • Phosphorylation reaction traps glucose in the cell. • Glc-6-P cannot diffuse through the membrane, because of its negative charges. • Formation of Glc-6-P maintains the glucose concentration gradient and accelerates the entry of glucose into the cell. • Only liver (kidney) can convert Glc-6-P back to glucose and release it to blood • The addition of the phosphoryl group begins to destabilize glucose, thus facilitating its further metabolism Consequence: 45 GLUCOKINASE X HEXOKINASE Characteristics Hexokinase Glucokinase Occurence Specifity Inhibition Afinity to Glc Inducibility KM (mmol/l) In many tisues broad (hexoses) Glc-6-P high no 0,1 liver, pancreas glucose Is not inhibited low by insulin 10 46 Glucose concentration, mmol/l hexokinase glucokinase Glucose concentration and rate of phosphorylation reaction KM - hexokinase KM - glucokinase Vmax of glucokinase Vmax of hexokinase 5 10 Concentration of fasting blood glucose 47 • Hexokinase is inhibited by glucose 6-phosphate, the reaction product. High concentration of this molecule signals that the cell no longer requires glucose for energy, for storage in the form of glycogen, or as a source of biosynthetic precursors, and the glucose will be left in the blood. •glucokinase functions only when the intracellular concentration of glucose in hepatocyte is elevated (after a carbohydrate rich meal) • hexokinase in liver functions at lower concentrations of glucose 48 Role of glucokinase in pancreas Glucokinase in b- cells of pancreas functions as a blood glucose sensor When blood glucose level is high (after the saccharide rich meal), glucose enters the b-pancreatic cells (by GLUT2) and is phosphorylated by glucokinase Increase of energy status in the cell enhances the release of insulin 49 Conversions of Glc-6P in the cells and their significance Pathway Význam Glycolysis Synthesis of glycogene Pentose phosphate path. Synthesis of derivatives Energy gain, synthesis of fatty acids from acetyl-CoA Formation of glucose stores Source of pentoses, source of NADPH Synthesis of glycoproteins, proteoglycans 50 Glycolysis •Significance: energy gain, formation of intermediates for other processes, includes also the metabolism of galactose and fructose • Occurs in most of tissues • Location: cytoplasma • Reversible, enzyme catalyzed reactions • Three reactions are irreversible Aerobic glycolysis At adequate supply of oxygen, pyruvate is converted to acetylCoA Anaerobic glycolysis When oxygen is lacking, pyruvate is converted to lactate 51 Three stages of glycolysis: Trapping the glucose in the cell and destabilization by phosphorylation. Cleavage into two three-carbon units. Oxidative stage in which new molecules of ATP are formed by substrate-level phosphorylation of ADP. 52 Reactions of glycolysis 1. Formation of Glc-6-P ATP ADP ATP hexokinase, glucokinase Non-reversible Formation of glucose-6-P from glycogen (no consumption of ATP) 53 OH Pi phosphorylase phosphoglucomutase glukosa-1-P glycogen glucose-6-P 54 2. Izomerization of glucose-6-P Fructose-6-P phosphoglucoisomerase reversible 55 3. Formation fructose-1,6-bisphosphate phosphofructokinase The rate of this reaction determines the rate of the whole glycolysis ATP ATP ADP Non-reversible 56 Properties of enzymes that are rate limiting step of a metabolic pathway • The molar activity (turnover number, kcat) of the particular enzyme is smaller than those of other enzymes taking part in the metabolic pathway. • The reaction rate does not usually depend on substrate concentration [S] because it reaches the maximal value Vmax. • The reaction is practically irreversible. The process can be reversed only by the catalytic action of a separate enzyme. 57 Regulation of phosphofructokinase • allosteric inhibition by ATP and citrate • allosteric activation by AMP, ADP and by fructose-2,6-bisphosphate in liver* * The formation of fructose-2,6-bisP is controled by hormones 58 Regulatory effect of fructose-2,6-biP on glycolysis and gluconeogenesis in liver 2,6-biP is allosteric efector of phosphofructokinase fructose 1,6-bisphosphatase (glycolysis) (gluconeogenesis) activation inhibition Formation of fructose-2,6-biP Stimulation by fructose-6P ¯ inhibition by glucagone 59 aldolase reversible reversible 4.Formation of triose-phosphates reversible Glyceraldehyde-3-P Dihydroxyaceton-P Triosaphosphate isomerase 60 Pi Anhydride bond glyceraldehyde-3-P dehydrogenase reversible !! NAD+ NADH +H+ glyceraldehyde-3-P Ester bond 5. Oxidation and phosphorylation of glyceraldehyde-3-P Note, that reaction enters Pi , not ATP !!!! 1,3-bisphospoglycerate 61 Compare: glyceraldehyde-3-phosphate 3-phosphoglycerate C H 2 C H H O C H O P O O O O - - O P O O O C H 2 C H H O C O O - - - 62 62 thioester bond Anhydride bond 1,3-bisphosphoglycerate - + HS E S H NAD+ NADH +H+ + Enzyme with s SH group reversible reversible E Oxidation and phosphorylation of glyceraldehyde-3-P Phosphorolytic cleavage NAD+ is consumed in this reaction, NADH is formed 63 Phosphoglycerate kinase 3-phosphoglycerate 2x ATP 1,3-bisphosphoglycerate ADP ATP reversible 6.Formation of 3-phosphoglycerate and ATP Formation of ATP by substrate-level phosphorylation: 1,3 BPG is high-energy compound (mixed anhydride), Energy released during PO32- transfer is utilized for ATP synthesis 64 mutase 2,3-bisphosphoglycerate phosphatase 3-phosphoglycerate + Pi 1,3-bisphosphoglycerate reversible Formation of 2,3-bisphosphoglycerate, side reaction in erytrocytes Binding to Hb, significance for release of O2 This reaction does not provide ATP !!! 65 phosphoglycerate mutase 2-phosphoglycerate 3-phosphoglycerate reversible 7. Formation of 2-phosphoglycerate 66 + H2O enolase (inhibition by F-) enolase reversible 8. Formation of phosphoenolpyruvate 2-phosphoglycerate phosphoenolpyruvate Whem blood samples are taken for mesurement of glucose, it is collected in tubes containing fluoride 67 ATP 2x ATP ADP ATP Pyruvate kinase Non reversible !!! 9. Formation of pyruvate phosphoenolpyruvate Pyruvate kinase Activation by fructose-1,6-bisP Inactivation by glucagon (substrate-level phosphorylation) 68 Which reactions of glycolysis are non-reversible? 69 Conversions of pyruvate lactate acetyl CoA alanine glutamate 2-oxoglutarate ADP,Pi ATP,CO2 oxalacetate ethanol (in microorganisms) NAD+ NADH, CO2 NADH+ H+ NAD+ 70 Formation of lactate - anaerobic glycolysis LD Significance of this reaction: Regeneration of NAD+ consumed in formation 2,3-bisP-glycerate when NAD+ is lacking, the glycolysis cannot continue NADH cannot be reoxidized in respiratory chain 71 Lactate dehydrogenase (LD) Enzyme catalyzes the reaction in both directions Isoenzymes LD1 - LD5 Subunit H (heart) Subunit M (muscle) LD1 LD2 LD3 LD4 LD5 72 • short-term inexercising mucle » 14 % • in erythrocytes (lacking mitochondria) » 25 % • skin » 25 % • brain » 14 % • mucose cells of small intestine » 8 % • kidney medulla, testes, leukocytes, lens Formation of lactate In average 1,3 mol/day (a man, 70 kg) Concentration of lactate in blood: » 1 mmol/l Changes at intensive muscle work (up to 30 mmol/l) 73 Cori cycle – tranport of lactate from tissues into the liver, utilization for gluconeogenesis LIVER glucose MUSCLE glucose pyruvate lactate BLOOD lactate pyruvate LD LD 74 Energetic yield of glycolysis 1. Direct gain of ATP by substrate-level phosphorylation 2 ATP per one glucose Consumption of ATP Gain of ATP Þ - + This yield is the same for both, anaerobic and aerobic glycolysis. This is the only yield in anaerobic glycolysis 75 - Reoxidation of NADH from reaction 5 (glyceraldehyd-P®1,3-bisP-glycerate) : Transfer by „shuttles“ into the respiratory chain – a yield 2x 2-3 ATP 2. Further yield of ATP at aerobic glycolysis: - Conversion of pyruvate to acetylCoA (2 NADH) 2x3 ATP - Conversion of acetylCoA in citric acid cycle 2x12 ATP 76 Total energy yield of aerobic glycolysis Reaction ATP yield glucose ®2 pyruvate (substrate level phosporylation) 2 NADH ® 2NAD+ 2 4-6* Aerobic glycolysis till pyruvate: * Depending on shuttle type (see lecture Respiratory chain) Further conversions of pyruvate: Reakce ATP yield 2 pyruvate ® 2 acetylCoA + 2 NADH 2 acetyl CoA ® 2 CO2 + 6 NADH + 2 FADH2 6* 2x 12 Total maximal energy yield 36-38 ATP * (2x NADH to the resp. chain) 77 Energy yield of anaerobic glycolysis Anaerobic glycolysis till pyruvate: Reaction ATP yield glucose ®2 x pyruvate (substrate-level phosphorylation) 2 NADH ® 2NAD+ 2 0 Formation and consumption of NADH at anaerobic glycolysis Reaction Yield/loss NADH 2 glyceraldehyde-3-P ® 2 1,3-bisP-glycerát 2 pyruvate ® 2 lactate In sum +2 -2 0 78 •The energy yield of anaerobic glycolysis is only 2 ATP from substrate level phosphorylation • it is only small portion of the total energy conserved in molecule of glucose • it has high significance at situations when • supply of oxygen is limited • tissue do not dispose of mitochondrias (ercs, leukocytes, ..) • it is necessary to spare lactate for gluconeogenesis 79 79 Oxidative decarboxylation of pyruvate • pyruvate dehydrogenase complex CH3COCOOH + CoA-SH + NAD+ CH3COSCoA + CO2 + NADH + H+ acetylCoA mitochondrial matrix • conversion of pyruvate to acetylCoA Cofactors necessary for this reaction: : thiamindiphosphate, lipoamide, CoA, FAD, NAD+ Mechanismus is in more details discussed in the lecture Citric acid cycle