[BCH LEC - LE 2] 08 - CHO in Everyday Life.pdf

BIOCHEMISTRY LECTURE | TRANS 8 LE Carbohydrates in Everyday Life CATHERINE L. CO-REPORTOSO, MD...

BIOCHEMISTRY LECTURE | TRANS 8 LE Carbohydrates in Everyday Life CATHERINE L. CO-REPORTOSO, MD, DPPS | Lecture Date (10/24/2023) | Version #1 02 ✔ fasted state (Concept map) OUTLINE I. Fate of Carbohydrate A. Fasting State I. FATE OF CARBOHYDRATE DIGESTION Digestion B. Reciprocal Regulation of A. Glycolysis Gluconeogenesis and B. Galactose Glycolysis C. Fructose V. Fed State vs. Fasting State II. Summary of Fate of A. Fed State Products of CHO Digestion B. Fasting State Dyspnea, III. Regulation of VI. References Glycogenolysis VII. Review Questions IV. Gluconeogenesis vs. VIII. Appendix Glycolysis Must Lecturer Book Previous Youtube ❗️ Know 💬 📖 📋 Trans 🔺 Video SUMMARY OF ABBREVIATIONS 1,3 BPG 1,3 Bisphosphoglycerate 2-PG 2-Phosphoglycerate 3-PG 3-Phosphoglycerate ADP Adenosine diphosphate AMP Adenosine monophosphate Figure 1. Digestion of Carbohydrates[Doc Reporto’s PPT] ATP Adenosine Triphosphate cAMP Cyclic adenosine monophosphate A. 1. GLUCOSE: GLYCOLYSIS DHAP Dihydroxyacetone phosphate Major pathway for glucose metabolism ETC Electron Transport Chain Breakdown of six-carbon glucose into two molecules of F6P Fructose 6-Phosphate three-carbon pyruvate F1,6BP / Fructose Fructose 1,6-bisphosphate Pathway may be aerobic or anaerobic 1,6-bisP Divided into two phases: F2,6BP / Fructose Fructose 2,6-bisphosphate → Preparatory Phase 2,6-bisP → Payoff Phase FBPase FBPase-2/PFK-2 Fructose 1,6-bisphosphatase Fructose 2,6-bisphosphatase Phosphofructokinase-2 📖 PREPARATORY PHASE Phosphorylation of glucose and its glyceraldehyde 3-phosphate conversion to G3P/GA3P Glyceraldehyde 3-phosphate Occurs during Step 1 - Step 5 Glucose 1-P (G1P) Glucose 1-phosphate 2 moles of ATP are invested in glycolysis during this phase in the Glucose 6-P (G6P) Glucose 6-phosphate following steps: G6Pase Glucose 6-phosphatase → Step 1: Phosphorylation of Glucose to Glucose-6-Phosphate G6PD Glucose 6-phosphate Deficiency → Step 3: Phosphorylation Fructose-6-Phosphate to Fructose GALT HK Galactose 1-phosphate uridyltransferase Hexokinase ❗️1,6,-Bisphosphate The two irreversible steps are the two phosphorylation steps provided with their enzymes: HFI Hereditary Fructose Intolerance → Step 1: Hexokinase (muscle), Glucokinase (liver) NADH Nicotinamide adenine dinucleotide → Step 3: Phosphofructokinase (PFK-1) PDH Pyruvate dehydrogenase Rate-limiting enzyme: Phosphofructokinase (PFK-1) Pi Inorganic Phosphate Produces two triose-phosphates: PK Pyruvate kinase → Glyceraldehyde 3-Phosphate (G3P) PEP Phosphoenolpyruvate → Dihydroxyacetone Phosphate (DHAP), which is further PFK-1 Phosphofructokinase-1 isomerized into Glyceraldehyde 3-Phosphate PEPCK Phosphoenolpyruvate carboxykinase TCA Tricarboxylic acid LEARNING OBJECTIVES ✔ Trace the fate of products of carbohydrate digestion (glucose,galactose and fructose). ✔ Calculate the net yield of ATPs formed from complete oxidation of glucose in different organs. ✔ Explain how the body maintains glucose during the fasted state. ✔ Discuss how glycogenolysis is regulated. ✔ Explain how the body synthesizes glucose during the fasting ✔ state. ✔ Discuss the reciprocal regulation of gluconeogenesis and ✔ glycolysis. ✔ Differentiate the pathways occurring in the fed state and in the LE 1 TG 25-28 | L. Matienzo, M. Matti, A.J. Mendoza, M.J. TE | E. Mejia, M.A. Mendoza, AVPAA | S. Guzman, PAGE 1 of TRANS 1 Mendoza, R. Mendoza*, M.J. Mendoza, A. Mercado, H.A. H.L. Mercado, S.M. Millares, C.Kua 18 Mercado, D. Mesina*, K.A. Morales, S.A. Morales, S.A. K. Nepomuceno Nadurata, A.F. Natino, S.B. Navarro, M. Navarro, K. Nepomuceno*, R.M. Ngo, R.A.I. Nicolas, J. Nogoy, J.C. Nueno BIOCHEMISTRY | LE 1 Carbohydrates in Everyday Life | Catherine L. Co-Reportoso, MD, DPPS STEP 2. ISOMERIZATION OF GLUCOSE 6-PHOSPHATE TO FRUCTOSE 6-PHOSPHATE Table 2. Isomerization of G6P Process Isomerization Starting Substrate Glucose 6-phosphate Enzyme Phosphoglucoisomerase Product Fructose 6-phosphate (F6P) STEP 3. PHOSPHORYLATION OF FRUCTOSE 6-PHOSPHATE TO Figure 2. Point of entry of fructose in muscle and liver[Doc Reportoso’s PPT] ❗️ ❗️ FRUCTOSE 1,6 BISPHOSPHATE Rate-limiting reaction for glycolysis 📖 PAYOFF PHASE Oxidative conversion of glyceraldehyde 3-phosphate to pyruvate, and the coupled formation of ATP and NADH 📖 Irreversible reaction PFK-1 is regulated by complex allosteric regulation → Activated by: Happens in Step 6 - Step 10 ▪ Depletion of ATP in the cell → Two molecules of G3P are present, thus reactions in the ▪ Accumulation of ADP and AMP (both are from steps will occur twice. breakdown of ATP) Produces two pyruvates → Inhibited by: Energy gain in this phase occurs through substrate-level ▪ When cell has ample supply of ATP → ❗️ phosphorylation Substrate-level phosphorylation refers to the process where ATP is directly synthesized from ADP using high-energy ▪ When the cell is well supplied by other fuels Table 3. Phosphorylation of F6P intermediates. Process Phosphorylation of F6P → Occurs in steps 7 and 10, catalyzed by their respective Starting Substrate Fructose 6-phosphate, ATP enzymes: Enzyme Phosphofructokinase-1(PFK-1) ▪ Step 7: phosphoglycerate kinase Fructose 1,6 Bisphosphate (F1,6BP) + ▪ Step 10: pyruvate kinase Product ADP → Gross yield and net yield of ATPs are different for glycolysis: ▪ Gross gain of ATP: 4 molecules ▪ Net gain of ATP: 2 molecules STEP 4. CLEAVAGE OF FRUCTOSE 1,6-BISPHOSPHATE INTO − 2 ATP molecules were utilized during the preparatory TRIOSE PHOSPHATES phase Glyceraldehyde 3-phosphate is an aldose Two fates of NADH produced in the oxidation of G3P Dihydroxyacetone phosphate is a ketose → Aerobic glycolysis: shuttled into the mitochondria → Anaerobic glycolysis: used in production of lactate by lactate Table 4. Cleavage of F1,6-BP dehydrogenase Process Cleavage of F1,6-BP Fates of NADH in various organs and tissues for metabolic fuel Starting Substrate F1,6-BP → Malate-Aspartate Shuttle Enzyme Aldolase ▪ 📖 ▪ Used in the mitochondria of the heart, liver, brain How it works: − 1) In the cytosol: NADH is oxidized, transferring Product DHAP and G3P electrons to cytosolic oxaloacetate, yielding malate STEP 5. ISOMERIZATION OF DIHYDROXYACETONE o Enzyme: cytosolic malate dehydrogenase − 2) Malate passes through the inner membrane through the malate—𝛂-ketoglutarate transporter 📖 PHOSPHATE TO GLYCERALDEHYDE 3-PHOSPHATE Isomerization of DHAP (ketone) must occur as this is the only triose phosphate which can be directly degraded in the succeeding − 3) In the matrix: malate is oxidized back to oxaloacetate, steps of glycolysis generating NADH ▪ ❗️ o Enzyme: matrix malate dehydrogenase Generates 2.5 molecules of ATP per pair of electrons → Lactate Dehydrogenase Reaction Table 5. Cleavage of F1,6-BP Process Cleavage of F1,6-BP Starting Substrate F1,6-BP ▪ 📖 ▪ Used by mature erythrocytes as they lack mitochondria How it works: − 1) NADH from glycolysis is used to convert pyruvate to Enzyme Product Aldolase DHAP and G3P lactate o Enzyme: lactate dehydrogenase STEP 6. OXIDATION AND PHOSPHORYLATION OF − 2) NAD+ regenerated from this reaction can be used for glycolysis ❗️ 📖 GLYCERALDEHYDE 3-PHOSPHATE From this step onwards, the reactions will be doubled Phosphorylation occurs as the aldehyde group of G3P is oxidized STEP 1. PHOSPHORYLATION OF GLUCOSE TO GLUCOSE to a carboxylic acid anhydride with phosphoric acid instead of a free carboxyl group ❗️Irreversible reaction 6-PHOSPHATE G6P molecule is produced by hydrolyzing ATP into ADP Removes hydride from GA3P and adds Pi to GA3P resulting in 1,3-BPG. Table 6. Cleavage of F1,6-BP Table 1. Phosphorylation of Glucose Process Oxidation and Phosphorylation of G3P Process Phosphorylation of Glucose Starting Substrate Glyceraldehyde 3- Phosphate, Pi, NAD+ Starting Substrate Glucose, ATP Glyceraldehyde-3-phosphate Enzyme Hexokinase Enzyme dehydrogenase Product Glucose 6-Phosphate (G6P) + ADP BIOCHEMISTRY Carbohydrates in Everyday Life PAGE 2 of 18 BIOCHEMISTRY | LE 1 Carbohydrates in Everyday Life | Catherine L. Co-Reportoso, MD, DPPS Product 1,3 BPG and NADH Final products 2 Pyruvate, 2 NADH, 2 H, 2 ATP, 2 H20 Location Cytosol STEP 7. SUBSTRATE-LEVEL PHOSPHORYLATION OF ADP TO Starting substrate Glucose ATP BY 1,3 BISPHOSPHOGLYCERATE Rate-limiting Phosphofructokinase-1 (PFK-1) First reaction that generates ATP enzyme Aerobic: 2 Pyruvate, 2 NADH, 2 H, 2 Table 7. Substrate-level Phosphorylation from 1,3 Products ATP, 2 H2O bisphosphoglycerate Anaerobic: Lactate Process Substrate-level phosphorylation Starting Substrate 1,3-Bisphosphoglycerate, ADP A. 2. GLUCOSE: CITRIC ACID CYCLE Enzyme Phosphoglycerate kinase Shared pathway in the metabolism of all types of fuels Product 3PG, ATP → Main pathway for ATP formation → Involves the oxidation of metabolic fuels → As electrons are stripped from acetyl-CoA through oxidation, STEP 8. ISOMERIZATION OF 3-PHOSPHOGLYCERATE large majority of the essential coenzymes required in ETC are Phosphate group “mutated” from Carbon 3 to Carbon 2. produced Generates the following: Table 8. Isomerization → 3 NADH and 1 FADH2: required for ATP synthesis in ETC Process Isomerization → 1 GTP: produced via substrate level phosphorylation Starting Substrate 3PG → 1 CO2: produced via decarboxylation Enzyme Phosphoglycerate mutase Also participates in gluconeogenesis and lipogenesis Product 2PG 4 oxidative enzymes in the TCA (aka the dehydrogenases) → Isocitrate dehydrogenase (Step 3) → a-ketoglutarate dehydrogenase (Step 4) STEP 9. DEHYDRATION OF 2-PHOSPHOGLYCERATE TO → Succinate dehydrogenase (Step 6) PHOSPHOENOLPYRUVATE → Malate dehydrogenase (Step 8) This reaction converts a compound of relatively low phosphate group transfer potential (2-PG) to one with a high phosphate group transfer potential (PEP) STEP 1. CONDENSATION OF ACETYL-CoA & → Compare the ΔG’° of 2-PG and PEP: OXALOACETATE ▪ ΔG’° for 2-PG hydrolysis = -17.6 kJ/mol Formation of a 6-carbon citrate from acetyl-CoA (2C) and ▪ ΔG’° for PEP hydrolysis = -61.9 kJ/mol oxaloacetate (4C) → In preparation for the substrate-level phosphorylation of the Irreversible step next step Highly exergonic Table 9. Dehydration of 2-phosphoglycerate to Phosphoenolpyruvate Table 13. Condensation of Acetyl-CoA & Oxaloacetate Process Dehydration Condensation of Acetyl-CoA & Process Starting Substrate 2-PG Oxaloacetate Enzyme Enolase Starting Substrate Acetyl-CoA & Oxaloacetate Product PEP, H2O Enzyme Citrate Synthase Product Citrate STEP 10. SUBSTRATE-LEVEL PHOSPHORYLATION OF ADP STEP 2. ISOMERIZATION OF CITRATE ❗️ TO ATP BY PHOSPHOENOLPYRUVATE Irreversible reaction Generation of 2nd ATP Reversible step Endergonic process PEP donates high-energy phosphate for the reaction Includes two steps: → 1st (dehydration) - citrate to cis-aconitate Table 10. Substrate-level phosphorylation from phosphoenolpyruvate → 2nd (hydration) - cis-aconitate to iso-citrate Process Substrate-level phosphorylation Starting Substrate PEP, ADP Table 14. Isomerization of Citrate Enzyme Pyruvate kinase Process Isomerization Product Pyruvate, ATP Starting Substrate Citrate Enzyme Aconitase Product Isocitrate SUMMARY OF GLYCOLYSIS Table 11. Summary of Glycolysis STEP 3. OXIDATIVE DECARBOXYLATION OF ISOCITRATE Step 1: Phosphorylation of Glucose Irreversible step Step 3: Phosphorylation of Fructose Exergonic process 3 irreversible 6-Phosphate Isocitrate dehydrogenase is the rate limiting enzyme pathways Step 10: Substrate-level Generates 1st NADH and CO2 phosphorylation of ADP to ATP by → release of Carbon from Isocitrate (6C) to form a a-ketoglutarate substrate phosphoenolpyruvate (5C) Step 7: Substrate-level phosphorylation of ADP to ATP by substrate Table 15. Oxidative decarboxylation of isocitrate 2 substrate-level 1,3-bisphosphoglycerate Process Oxidative decarboxylation phosphorylation Step 10: Substrate-level Starting Substrate Isocitrate phosphorylation of ADP to ATP by Enzyme Isocitrate Dehydrogenase substrate phosphoenolpyruvate Product a-Ketoglutarate, NADH, CO2 Substrates in the Glucose, 2 NAD, 2 Pi, 2 ADP overall reaction BIOCHEMISTRY Carbohydrates in Everyday Life PAGE 3 of 18 BIOCHEMISTRY | LE 1 Carbohydrates in Everyday Life | Catherine L. Co-Reportoso, MD, DPPS STEP 4. OXIDATIVE DECARBOXYLATION OF SUMMARY OF TCA CYCLE a-KETOGLUTARATE Irreversible step Table 21. Summary of TCA Cycle Exergonic process Step 1: Condensation of Acetyl-CoA & Generates 2nd NADH and CO2 Oxaloacetate → Release of Carbon from a-ketoglutarate (5C) to form 3 irreversible Step 3: Oxidative decarboxylation of succinyl-CoA (4C) pathways Isocitrate Step 4: Oxidative decarboxylation of Table 16. Oxidative decarboxylation of a-ketoglutarate a-ketoglutarate Process Oxidative decarboxylation 1 substrate-level Step 5: Substrate level phosphorylation Starting Substrate a-ketoglutarate phosphorylation of Succinyl-CoA Enzyme a-ketoglutarate Dehydrogenase Location Mitochondria Product Succinyl-CoA, NADH, CO2 Starting substrate Acetyl-CoA Rate-limiting Isocitrate Dehydrogenase enzyme STEP 5. SUBSTRATE LEVEL PHOSPHORYLATION OF Oxaloacetate, 10 ATP SUCCINYL-CoA → 3 NADH x 2.5 ATP = 7.5 ATP Reversible step Products → 1 FADH x 1.5 ATP = 1.5 ATP Endergonic process → 1 GTP x 1 ATP = 1 ATP Generates GTP → 2 CO2 → The free energy of thioester bond in succinyl-CoA is conserved by formation of GTP from GDP and inorganic phosphate (Pi) A. 3. GLUCOSE: PENTOSE PHOSPHATE PATHWAY Table 17. Substrate level phosphorylation of Succinyl-CoA aka Hexose Monophosphate Shunt Process Substrate level phosphorylation Bypass route of the first stage of glycolysis Starting Substrate Succinyl-CoA 2 pathways: Enzyme Succinyl-CoA Synthetase → Oxidative phase Product Succinate, GTP ▪ irreversible reaction ▪ NADP+ is reduced to NADPH → Non-oxidative phase STEP 6. OXIDATION OF SUCCINATE ▪ Reversible reaction Reversible step ▪ Formation of glycolytic intermediates: G3P and F6P Endergonic process ▪ Production of Ribose-5-Phosphate Generates FADH Reaction involves oxidation of succinate to trans-dicarboxylic acid and fumarate Table 18. Oxidation of Succinate Process Oxidation Starting Substrate Succinate Enzyme Succinate Dehydrogenase Product Fumarate, FADH2 STEP 7. HYDRATION OF FUMARATE Reversible step Endergonic process Fumarase stereospecifically adds water to the trans double bond of fumarate to form L-malate Table 19. Hydration of Fumarate Process Hydration Starting Substrate Fumarate Enzyme Fumarase Product L-Malate Figure 3. Overview of Pentose Phosphate Pathway[Mark’s] STEP 8. OXIDATION OF MALATE Reversible step Table 22. Summary of Pentose Phosphate Pathway Endergonic process Location Cytosol Generation of the third NADH Starting Substrate G6P Completes one round of the TCA cycle Rate Limiting Enzyme G6P Dehydrogenase → Oxaloacetate is free to react with acetyl-CoA, thus continuing Coenzyme Involved TPP the cycle Fructose 6-phosphate Glyceraldehyde 3-phosphate Table 20. Oxidation of Malate Products Ribose 5-phosphate Process Oxidation NADPH Starting Substrate Malate CO2 Enzyme Malate Dehydrogenase Product Oxaloacetate, NADH BIOCHEMISTRY Carbohydrates in Everyday Life PAGE 4 of 18 BIOCHEMISTRY | LE 1 Carbohydrates in Everyday Life | Catherine L. Co-Reportoso, MD, DPPS OXIDATIVE PHASE: Cofactor TPP STEP 1: Oxidation Reduction of G6P to 6-Phosphogluconolactone Product Glyceraldehyde- Sedoheptulose- Table 23. Oxidation Reduction of G6P to 6-Phosphogluconolactone 3-Phosphate 7-phosphate Process Oxidation-Reduction G6P Starting substrate STEP 6: Transfer of C atoms of Glyceraldehyde 3-phosphate NADP+ to Fructose 6-Phosphate; and Sedoheptulose 7-Phosphate Enzyme G6P dehydrogenase to Erythrose 4-Phosphate Cofactors Mg2+ 6-phosphogluconolactone Table 28. Transfer of C atoms of Glyceraldehyde 3-phosphate to Product NADPH Fructose 6-Phosphate; and Sedoheptulose 7-Phosphate to Erythrose 4-Phosphate STEP 2: Hydrolysis of 6-Phosphogluconolactone to Process Transfer of carbon atoms 6-Phosphogluconate Table 24. Hydrolysis of 6-Phosphogluconolactone to Starting Glyceraldehyde- Sedoheptulose- 6-Phosphogluconate substrate 3-Phosphate 7-phosphate Process Hydrolysis Starting substrate 6-Phosphogluconolactone Enzyme Transaldolase Enzyme Gluconolactonase Cofactors Mg2+ Cofactor TPP Product 6-phosphogluconate Product Fructose- Erythrose- 6-Phosphate 4-Phosphate STEP 3: Oxidative Decarboxylation of 6-Phosphogluconate to Ribulose 5-Phosphate STEP 7: Transfer of glycolaldehyde group of Xylulose Table 25. Oxidative Decarboxylation of 6-Phosphogluconate to 5-phosphate to Glyceraldehyde 3-phosphate; and Erythrose Ribulose 5-Phosphate 4-Phosphate to Fructose 6-Phosphate Process Oxidative decarboxylation 6-Phosphogluconate Table 29. Transfer of glycolaldehyde group of Xylulose 5-phosphate Starting substrate NADP+ to Glyceraldehyde 3-phosphate; and Erythrose 4-Phosphate to Enzyme 6-Phosphogluconate dehydrogenase Fructose 6-Phosphate Cofactors Mg2+ Process Transfer of glycolaldehyde atoms Ribulose-5-Phosphate Product NADPH Starting Xylulose- Erythrose- CO2 substrate 5-Phosphate 4-phosphate NON-OXIDATIVE PHASE: Enzyme Transketolase STEP 4: Isomerization of Ribulose 5-Phosphate to Ribose 5-Phosphate and Xylulose 5-Phosphate Cofactor TPP Table 26. Isomerization of Ribulose 5-Phosphate to Ribose Product Glyceraldehyde- Fructose- 5-Phosphate and Xylulose 5-Phosphate 3-Phosphate 6-Phosphate Process Isomerization Starting substrate Ribulose-5-Phosphate Ribulose- Ribulose- Enzyme 5-Phosphate 5-Phosphate-3- Isomerase Epimerase Ribose- Xylulose- Product 5-Phosphate 5-Phosphate STEP 5: Transfer of C atoms of Ribose 5-phosphate to Glyceraldehyde 3-phosphate; and Xylulose 5-Phosphate to Sedoheptulose 7-Phosphate Table 27. Transfer of C atoms of Ribose 5-phosphate to Glyceraldehyde 3-phosphate; and Xylulose 5-Phosphate to Sedoheptulose 7-Phosphate Process Transfer of carbon atoms Starting Ribose- Xylulose- substrate 5-Phosphate 5-Phosphate Enzyme Transketolase Figure 4. Overview of Pentose Phosphate Pathway[Springer] BIOCHEMISTRY Carbohydrates in Everyday Life PAGE 5 of 18 BIOCHEMISTRY | LE 1 Carbohydrates in Everyday Life | Catherine L. Co-Reportoso, MD, DPPS A. 4. GLUCOSE: URONIC ACID PATHWAY 1- Phosphate Alternative oxidative pathway for glucose Catalyzes the conversion of glucose to UDP-glucuronate Endergonic, reversible reaction Table 32. Isomerization Of Glucose 6-Phosphate To Glucose 1- Phosphate Process Isomerization Starting Substrate Glucose-6-Phosphate Enzyme Phosphoglucomutase Product Glucose-1-Phosphate STEP 3. Phosphorylation of Glucose-1-Phosphate and Uridine Triphosphate (UTP) to Uridine Diphosphate Glucose (UDP-Glucose) Uridine Triphosphate (UTP) combines with G1P G1P breaks down two phosphates from UTP The two phosphates are called pyrophosphates UTP becomes uridine monophosphate Figure 5. Overview of UDP-Glucose metabolism Pathway[Google] Table 33. Phosphorylation Of Glucose-1-Phosphate And Uridine Table 30. Summary of Uronic Pathway Triphosphate (UTP) To Uridine Diphosphate Glucose (UDP-Glucose) Location Cytosol Process Phosphorylation Starting substrate Glucose-6-Phosphate Starting Substrate Glucose-1-Phosphate Products Xylulose-5-Phosphate Enzyme UDP-Glucose pyrophosphorylase Glucuronic acid Product UDP Glucose A. 5. GLUCOSE : GLYCOGEN SYNTHESIS STEP 1. Phosphorylation of Glucose to Glucose 6-Phosphate STEP 4. Auto Glycosylation of UDP Glucose and Glycogen to Glycogen Primer and UDP Glucose is transported into the cell via facilitated diffusion → In the muscles: GLUT-4 Endergonic, reversible reaction → In the Liver: GLUT-2 Table 34. Auto Glycosylation Of UDP Glucose And Glycogen To Glycogen Primer And UDP Process Auto Glycosylation Of UDP Glucose and Glycogen To Glycogen Primer and UDP Starting Substrate UDP-Glucose, Glycogenin Enzyme Auto-glycosylation (Non-enzymatic reaction) Product Glycogen primer, UDP STEP 5. Glycosylation of Glycogen Primer Figure 6. Hexokinase vs. Glucokinase [Doc Reportoso’s PPT] Continue glycosylation by adding another UDP-glucose into the glycogen primer. Table 31. Phosphorylation of Glucose to Glucose-6-Phosphate Table 35. Glycosylation of Glycogen Primer Process Phosphorylation of glucose Process Glycosylation of Glycogen Prime Starting Substrate Glucose Starting Substrate Glycogen primer, UDP-glucose Enzyme Hexokinase (in the muscles) Enzyme Glycogen synthase Glucokinase (in the liver) Product 1→4 glucosyl units Product Glucose-6-Phosphate STEP 2. Isomerization of Glucose 6-Phosphate to Glucose BIOCHEMISTRY Carbohydrates in Everyday Life PAGE 6 of 18 BIOCHEMISTRY | LE 1 Carbohydrates in Everyday Life | Catherine L. Co-Reportoso, MD, DPPS STEP 6. Breakdown of ⍺-1,4-Glycosidic bonds and Formation of ⍺-1,6-Glycosidic bonds Branching of the 1→4 glucosyl units at the 8-10 glucosyl units Glycogen synthase and debranching enzyme keeps on going making more branched glycogen Table 36. Breakdown of ⍺-1,4-Glycosidic bonds and formation of ⍺-1,6-Glycosidic bonds Process Breakdown of α-1-4 Glycosidic bonds Formation of α-1-6 Glycosidic bonds Starting Substrate 1→4 glucosyl units Enzyme Branching enzyme (aka amylo-4-6-transferase) Product 1→4 glucosyl units and 1→6 glucosyl units Figure 8. Galactose Metabolism[Google] STEP 1. Phosphorylation of Galactose to Formation of UDP-glucose (continues process) Galactose-1-phosphate by Galactokinase (GALK) Table 38. Phosphorylation of Galactose to Galactose-1-phosphate by Galactokinase (GALK) Phosphorylation of Galactose to Process Galactose-1-phosphate Starting Substrate Galactose, ATP Enzyme GALK Product Galactose 1-Phosphate + ADP Figure 7. Formation of UDP-Glucose [Google} STEP 2. Conversion of Galactose-1-Phosphate to Summary Of Glycogenesis UDP-Galactose and Glucose-1-phosphate by Galactose-1-phosphate Uridyltransferase (GALT) Formation of UDP-Glucose Table 37. Summary of Glycogenesis → Accomplished by the attack of the phosphate oxygen on Galactose-1-phosphate on the α-phosphate of UDP-Glucose, Location Cytosol releasing Glucose-1-phosphate while forming UDP-Galactose → catalyzed by the enzyme GALT. [Marks’] Starting Substrate Glucose-6-Phosphate Table 39. Conversion of Galactose-1-Phosphate to UDP-Galactose Rate-limiting Glycogen synthase and Glucose-1-phosphate by Galactose-1-phosphate enzyme Uridyltransferase (GALT) Conversion of Galactose-1-Phosphate Product Glycogen Process to UDP-Galactose and Glucose-1-phosphate B. GALACTOSE Galactose-1-Phosphate Starting Substrate UDP-Glucose Glucose and Galactose are epimers at C4 Commonly found in soy, dairy products, celery, honey, kiwi, and Enzyme GALT cherry UDP-Galactose Product Inherited disorders of Fructose metabolism: Glucose-1-phosphate → Non-Classical Galactosemia ▪ Galactokinase (GALK) deficiency STEP 3. Conversion of UDP-Galactose to UDP-Glucose by ▪ Leads to increased amount of Galactose (will be reduced to UDP-Glucose Epimerase Galactitol instead, by the enzyme Aldolase Reductase) UDP-Galactose → Classical Galactosemia → Then converted to UDP-glucose by the reversible UDP-glucose ▪ Most common and severe epimerase (the configuration of the hydroxyl group on Carbon ▪ Galactose-1-phosphate Uridyltransferase (GALT) deficiency 4 is reversed in this reaction). [Marks’] ▪ Leads to increased amount of Galactose-1-phosphate, Galactose, UDP-Galactose Table 40. Conversion of UDP-Galactose to UDP-Glucose by UDP-Glucose Epimerase Conversion of UDP-Galactose to Process UDP-Glucose Starting Substrate UDP-Galactose Enzyme UDP-Glucose epimerase (Reversible) Product UDP-Glucose BIOCHEMISTRY Carbohydrates in Everyday Life PAGE 7 of 18 BIOCHEMISTRY | LE 1 Carbohydrates in Everyday Life | Catherine L. Co-Reportoso, MD, DPPS STEP 4. Conversion of Glucose-1-phosphate to Glucose–6-phosphate by Phosphoglucomutase Fates of dietary galactose[Marks’] → Similar to Fructose, therefore is parallel to Glucose Table 41. Conversion of Glucose-1-phosphate to Glucose 6-phosphate Conversion of Glucose-1-phosphate to Process Glucose 6-phosphate Starting Substrate Glucose-1-phosphate Enzyme Phosphoglucomutase Product Glucose-6-phosphate Table 42. Summary of Galactose Metabolism Location Cytosol Starting Substrate Galactose Rate Limiting Enzyme Coenzymes Figure 9. Fructose Metabolism[Google] Involved Products Glucose-6-phosphate STEP 1. Phosphorylation of Fructose to Fructose-1-phosphate by Fructokinase C. FRUCTOSE Table 44. Phosphorylation of Fructose to Fructose-1-phosphate The 2nd most common sugar in the adult diet – ingested Process Phosphorylation principally as the monosaccharide or as part of Sucrose. Found in fruits, honey, and corn syrups Starting Substrate Fructose Fructose metabolism: Enzyme Fructokinase → Mainly occurs in the liver Product Fructose-1-Phosphate → Lesser extent in small intestinal mucosa and proximal epithelium of the renal tubule STEP 2. Cleavage of Fructose-1-phosphate into DHAP and ▪ Due the presence of both Fructokinase & Aldolase B Glyceraldehyde by Aldolase-B Fructose is cleaved by the following enzymes: → Fructokinase ▪ Major kinase involved Table 45. Cleavage of Fructose-1-phosphate into DHAP and ▪ High Vmax Glyceraldehyde ▪ Found in liver Cleavage of Fructose-1-phosphate into Process → Hexokinase (I,II,III) DHAP and Glyceraldehyde ▪ High Vmax Starting Substrate Fructose-1-phosphate ▪ Found in muscle and adipose tissue Enzyme Aldolase-B ▪ Converts Fructose to Fructose-6 phosphate DHAP (glycolytic intermediate) Inherited disorders of Fructose metabolism: Product Glyceraldehyde → Essential fructosuria ▪ Fructokinase deficiency → Hereditary fructose intolerance STEP 3. Phosphorylation of Glyceraldehyde into ▪ Deficiency of Fructose-1-phosphate cleavage by Aldolase-B Glyceraldehyde 3-Phosphate by Triose Kinase Table 43. Fructose Metabolism by Conversion to Glycolytic Pathway Table 46. Phosphorylation of Glyceraldehyde into Glyceraldehyde Intermediates 3-Phosphate Process Phosphorylation Glyceraldehyde-3- Can proceed to: Starting Substrate Glyceraldehyde Phosphate Pyruvate, TCA cycle, and fatty acid Enzyme Triose Kinase synthesis Glyceraldehyde-3-phosphate (glycolytic Dihydroxyacetone Product intermediate Phosphate Can also be converted to: (DHAP) Glucose (by Gluconeogenesis) Summary of Fructose Metabolism Table 47. Summary of Fructose Metabolism Location Cytosol Starting Substrate Fructose Rate-limiting enzyme Aldolase B Product DHAP Glyceraldehyde-3-phosphate Fructose-6-phosphate (by enzyme Hexokinase) BIOCHEMISTRY Carbohydrates in Everyday Life PAGE 8 of 18 BIOCHEMISTRY | LE 1 Carbohydrates in Everyday Life | Catherine L. Co-Reportoso, MD, DPPS A. 3. GLUCOSE: PENTOSE PHOSPHATE PATHWAY Figure 10. Galactose, Fructose, and Glucose Metabolism[Google] II. SUMMARY OF FATE OF PRODUCTS OF CHO DIGESTION A. 1. GLUCOSE: GLYCOLYSIS Figure 14. Pentose Phosphate Pathway/Hexose Monophosphate Shunt[Marks’] A. 4. GLUCOSE: URONIC ACID PATHWAY Figure 15. Uronic Acid Pathway[Marks’] Mainly occurs in the liver to ensure normal blood glucose levels, but can occur in muscles as well with the use of another Figure 11. Glycolysis Pathway messenger A. 2. GLUCOSE: CITRIC ACID CYCLE A. 5. GLUCOSE : GLYCOGEN METABOLISM Figure 12. Krebs Cycle/Citric Acid Cycle/Tricarboxylic Acid Cycle[Marks’] Figure 16. Synthesis (Glycogenesis) and Degradation (Glycogenolysis) of Glycogen[Marks’] BIOCHEMISTRY Carbohydrates in Everyday Life PAGE 9 of 18 BIOCHEMISTRY | LE 1 Carbohydrates in Everyday Life | Catherine L. Co-Reportoso, MD, DPPS III. GLYCOGENOLYSIS AND ITS REGULATION In the cytosol, glycogen phosphorylase catalyzes glucose 1P GLYCOGENOLYSIS release from the ends of glycogen branches through the usage of Initial state of fasting (~6 hours) inorganic phosphate to cleave α-1,4 bonds. Limit dextrin is left Glucose absorption and consumption by all cells are suppressed, after phosphorolysis. G1P will then be converted to G6P except in the brain (Paredes-Flores, M., Mohiuddin, S., 2021). Glucagon activates this pathway by stimulating cAMP-dependent Hydrolysis of α-1,4 bonds will stop when there are four glucosyl PKA residues left in the branching point. → Released glucose molecules can join the pathway as a result of the breakdown of glycogen Table 48. Regulatory enzymes and mechanisms of Glycogenolysis Glycogen phosphorylase, glycogen debranching enzyme, and Mechanism glycogenolysis❗️ phosphoglucomutase are the main → Glycogen is broken down into G1P (due to glycogen enzymes in Enzyme Allosteric Regulation Covalent Modification phosphorylase, glucan transferase, and debranching enzyme) Phosphorylated by and some free glucose Glycogen (+) AMP glycogen ▪ caused by debranching enzyme amylo α-1,6 glucan Phosphorylase (-) G6P&ATP phosphorylase kinase; glucosidase activated → Phosphoglucomutase converts G1P to G6P Phosphorylase (+) Ca2+ Phosphorylated by → G6P is then converted into free glucose by Kinase (-) cAMP PKA; activated glucose-6-phosphatase glycogenolysis ❗️ Glycogen phosphorylase and phosphorylase kinase regulate The rate-limiting enzyme in glycogenolysis is glycogen synthase cAMP is formed when adenylyl cyclase converts ATP to cAMP; activates PKA PKA phosphorylates phosphorylase kinase → converts glycogen (phosphorolysis). On the other hand, α-1,4 glucan transferase and phosphorylase b to phosphorylase a → increases rate of amylo α-1,6 glucan glucosidase are debranching enzymes. glycogenolysis → 💬 Debranching enzymes have two activities: transfer and hydrolysis. PKA also has an effect on glycogen synthase. A phosphorylated glycogen synthase makes it inactive, therefore there is no glycogenesis. Phosphorylation occurs during fasted state since the predominant hormone is glucagon. Mainly occurs in the liver to ensure normal blood glucose levels, but can occur in muscles as well with the use of another messenger → Liver: G6P will be converted to glucose thus liver (hepatic) glycogen can contribute to blood glucose level. → Muscle: G6P will be converted to pyruvate (upon entering glycolysis) since muscles have no Glucose 6-phosphatase that can convert G6P to glucose. Figure 14. Pathway of Glycogenolysis [Google] REGULATION OF GLYCOGENOLYSIS IN THE LIVER Figure 15. Regulation of Glycogenesis & Glycogen phosphorylase and phosphorylase kinase are the key Glycogenolysis in the liver [Marks’] regulators of glycogenolysis, activated by phosphorylation. In muscle tissue, this pathway occurs when adenylyl cyclase and cAMP bind to phosphorylase kinase, which converts it to its active Allosteric Modification (in detail): form. Conversion of phosphorylase b to phosphorylase a During muscle contraction: ATP → ADP → AMP catalyzes breakdown of glycogen. BIOCHEMISTRY Carbohydrates in Everyday Life PAGE 10 of 18 BIOCHEMISTRY | LE 1 Carbohydrates in Everyday Life | Catherine L. Co-Reportoso, MD, DPPS ↑ AMP stimulates glycogenolysis by conversion of − Phosphorylation is not seen in the Calcium-calmodulin phosphorylase b (T or inactive state) to phosphorylase a (R or binding, rather it is seen in the interaction of active state) phosphorylate kinase with glycogen phosphorylase a. Ca+2 binding with the calmodulin component of phosphorylase kinase → ↑ conversion of phosphorylase kinase b to ▪ However, the phosphorylation of glycogen synthase phosphorylase kinase a → ↑ glycogenolysis (inactive) would not lead to glycogenesis. ↑ Glycogen → ↓ phosphorylase kinase b (inactive) to phosphorylase kinase a (active) → ↓ glycogenolysis G6P increases active glycogen synthase → ↓ glycogenolysis ATP inhibits glycogen synthase a → ↓ glycogenolysis ↑ Hepatic glucose → ↑ binding of free sugar to phosphorylase a → partial inhibition of phosphorylase a → ↓ glycogenolysis Figure 17. Regulation of glycogen synthesis and degradation by epinephrine and calcium [Marks’] ACTIVATION OF GLYCOGEN PHOSPHORYLASE IN THE MUSCLE DURING EXERCISE When ATP demand is high, glycogen in skeletal muscles are degraded when contraction occurs[2026 Trans] Glycogenolysis in skeletal muscles are initiated by AMP in muscle contraction, calcium in neural impulses, and epinephrine → Neural impulses from the initiation of contraction releases Figure 16. Regulation of Glycogenesis & Glycogenolysis in the liver calcium from the sarcoplasmic reticulum. The calcium released would bind to the calcium-calmodulin complex and eventually Covalent Modification (in detail): activates glycogen phosphorylase kinase Epinephrine and glucagon: Bind to GPCR which activates → Epinephrine is released when exercising adenylyl cyclase, converting ATP to cAMP. cAMP AMP does not phosphorylate glycogen phosphorylase but directly phosphorylates phosphorylase kinase b to a → ↑ converts it into its active form glycogenolysis[2026 Trans] Insulin: → ↑ phosphodiesterase activity → ↓ cAMP synthesis→ ↓ cAMP-dependent protein kinase synthesis → ↓ phosphorylation of phosphorylase b (inactive) to phosphorylase a (active) → ↓ glycogenolysis REGULATION OF GLYCOGENOLYSIS BY EPINEPHRINE AND CALCIUM Occurs in the muscle → May occur in other catecholamines other than epinephrine Epinephrine binds to α-adrenergic receptors (GPCR)[2026 Trans] → Leads to activation of GTP and phospholipase C (hydrolyzes PIP2 to IP3 and DAG), which releases calcium by endoplasmic reticulum → Calcium then binds to calmodulin to form calcium-calmodulin complex ▪ The calcium-calmodulin complex is a protein modifier which will activate phosphorylase kinase ▪ Stimulates calmodulin-dependent protein kinase which promotes inactivation of glycogen synthase ▪ Stimulates phosphorylase kinase which promotes activation → 💬 of glycogen phosphorylase This may also occur in hepatocytes because of alpha receptors, aside from beta receptors. ▪ When epinephrine bind to the alpha receptor, there would be an activation of phospholipase C located in the cell Figure 18. Activation of glycogen phosphorylase membrane. in the muscle during exercise ▪ This would cleave the PIP2 into IP3 and DAG. IV. GLUCONEOGENESIS VS GLYCOLYSIS ▪ IP3 would stimulate the release of calcium from the endoplasmic reticulum. FASTING STATE ▪ Calcium from the endoplasmic reticulum would bind to the F.S. received a text message from his girlfriend breaking up with him. calmodulin located in the phosphorylase kinase. This He decided not to eat and slept. He woke up at 10am and attended would induce the phosphorylation of glycogen his Physio class. phosphorylase a. Late Fasting → 18-72 hours after a meal BIOCHEMISTRY Carbohydrates in Everyday Life PAGE 11 of 18 BIOCHEMISTRY | LE 1 Carbohydrates in Everyday Life | Catherine L. Co-Reportoso, MD, DPPS Glucagon → Aims to maintain blood glucose and provide alternative energy sources to glucose-dependent tissues → Inhibits glycolysis mediated by cAMP-dependent protein kinase that phosphorylates and inactivates PFK2 and Pyruvate kinase. → (-) Glycolysis; (+) Gluconeogenesis GLUCONEOGENESIS Anabolic pathway in liver and kidneys that forms glucose from non-carbohydrate precursors In long fasting, the hepatic glycogen source is already depleted. It is important when carbohydrates are not available from the diet. Substrates for gluconeogenesis or glucogenic precursors include: → Lactate ▪ Product of glycolysis when the rate of glycolysis in the skeletal muscle exceeds the rate of oxidative metabolism ▪ Formed by cells that lack mitochondria such as red blood cells ▪ Synthesized through the lactic acid cycle or Cori cycle by the action of lactate dehydrogenase → Glucogenic Amino Acids ▪ Amino acids derived from the hydrolysis of tissue proteins are a major source of glucose during fasting → Propionyl-CoA ▪ Produced from the catabolism of fatty acids with an odd number of carbons Figure 19. Glycolysis and gluconeogenesis in the liver [Mark’s 5th Ed] → Glycerol ▪ Released from the hydrolysis of TAG in the adipose RECIPROCAL REGULATION OF GLUCONEOGENESIS AND ▪ Delivered in the liver and may enter gluconeogenic or the GLYCOLYSIS glycolytic pathway at dihydroxyacetone phosphate Gluconeogenesis is the reciprocal of glycolysis except for 3 Table 49. Allosteric regulations of Glycolysis and Gluconeogenesis. bypassed irreversible reactions Regulated enzymes Covalent Genetic → Irreversible Hexokinase/Glucokinase reaction Modification Regulation → Irreversible PFK-1 reaction ❗️ → Pyruvate-kinase reaction ❗️ Pyruvate to Oxaloacetate by Pyruvate carboxylase Oxaloacetate to PEP via Mitochondrial PEPCK Glycolysis Carboxylation of pyruvate to form oxaloacetate is catalyzed by Hexokinase I, II, III Glucose-6 pyruvate carboxylase in the mitochondria. (muscle) phosphate All other enzymes involved in gluconeogenesis are found in the cytosol. Hexokinase IV (liver Fructose-6 Two transport routes of OAA: glucokinase) phosphate → Decarboxylated to PEP by the mitochondrial PEPCK → Converted to malate or aspartate Phosphofructokinase AMP, ADP, Fructose ▪ Reduction to malate by malate dehydrogenase 1 (PFK-1) 2,6-bisphosphate, ATP, Citrate ▪ Converted to aspartate via transamination Phosphofructokinase ▪ Malate or aspartate is transported to the cytosol and 2 reconverted to OAA by reversal of the reactions in the mitochondria. Pyruvate Kinase Fructose ATP, Alanine, PEPCK is found both in cytosol and mitochondria. 1,6-bisphosphate Acetyl-CoA, Two moles of Glyceraldehyde 3-Phosphate long chain fatty → 1 molecule of G3P is isomerized back to DHAP acid → DHAP and G3P condense to form Fructose 1,6-bisphosphate by reversal of the aldolase reaction. Starts from 2 moles of pyruvate to form 1 mole of glucose Phosphofructokinase AMP, Fructose Citrate Energy required: Input of 6 ATP/GTP for each molecule of glucose 2 (PFK-2) 6-phosphate and no ATP produced: → Pyruvate carboxylase (-2 ATP) Gluconeogenesis → PEPCK (-2 ATP) → Phosphoglycerate kinase (-2 ATP) Glucose GLYCOLYSIS 6-phosphatase Central metabolic pathway in the cytosol of the cells that involves the breakdown of glucose and other carbohydrates to form Fructose ATP, Citrate Fructose pyruvate 1,6-Bisphosphatase 2,6-bisphospha 📖 During fasting, many of the reactions of glycolysis are reversed in te, AMP the process called gluconeogenesis → Liver produces glucose to maintain blood glucose levels. Phosphoenolpyruvate Carboxykinase ADP (PEPCK) Pyruvate Carboxylase Acetyl-CoA ADP BIOCHEMISTRY Carbohydrates in Everyday Life PAGE 12 of 18 BIOCHEMISTRY | LE 1 Carbohydrates in Everyday Life | Catherine L. Co-Reportoso, MD, DPPS Glucagon secretion is inhibited Fructose 2, Glycerol Fructose Carbohydrates, amino acids, and fats are absorbed in the 6-Bisphosphatase 3-phosphate 6-phosphate intestine. Glucose utilization by the brain is unchanged. Table 50. Covalent modifications and genetic regulation of Glycolysis Increase in glucose uptake of insulin-dependent tissues, and Gluconeogenesis. especially in the skeletal muscle. Stimulated: Regulated enzymes Activator Inhibitor → Glycolysis → Glycogenesis Glycolysis Inhibited: → Lipid oxidation Hexokinase I, II, III Triacylglycerol absorbed in the intestine are transported in (muscle) chylomicrons to peripheral tissues, which will be hydrolyzed into glycerol and free fatty acids. Stimulates amino acid uptake and protein synthesis. Hexokinase IV (liver Induced by insulin; Decrease of protein degradation in the liver, muscle, and adipose glucokinase) Repressed by glucagon 💬 tissue. Genetic defects in intestinal epithelial cells affects digestion of disaccharides after consumption, mainly disaccharidase function, Phosphofructokinas Induced by insulin; causing them to remain undigested and unabsorbed in the intestine as e Repressed by disaccharides, unable to be split into monosaccharides, and eventually 1 (PFK-1) glucagon excreted in fecal matter. FED STATE IN THE LIVER Pyruvate Kinase Phosphorylation, Induced by insulin; Glucose is taken up by the GLUT-2 transporter and is channeled thus inactive Repressed by into glycolysis and glycogenesis 1,6-bisphosphate glucagon → Excess glucose is directed into the PPP, generating NADPH + H+ which is used in biosynthetic pathways that require Phosphofructoki- Inactivated when reductions (e.g. synthesis of fatty acids and cholesterol). nase 2 (PFK-2) phosphorylated Aerobic glycolysis supplies acetyl-CoA → Acetyl-CoA Gluconeogenesis ▪ Key substrate for fatty acid synthesis Fatty acids are also esterified by glycolysis-derived glycerol which Glucose Induced by forms triacylglycerols in the process of lipogenesis 6-phosphatase glucagon, Triacylglycerols are packaged into very low density lipoprotein epinephrine, (VLDL) for transport to peripheral tissues cortisol; Repressed by FED STATE IN THE MUSCLES insulin Glycogenesis, amino acid uptake, and protein synthesis are stimulated. Fructose Phosphorylation, Induced by 1,6-Bisphosphatase thus active glucagon FED STATE IN ADIPOSE TISSUES VLDL triacylglycerols are hydrolyzed Phosphoenolpyru- Induced by Fatty acids are taken up by cells vate Carboxykinase glucagon, Triacylglycerols are resynthesized intracellularly which would (PEPCK) epinephrine, then become storage material cortisol; Repressed by FED STATE IN THE RED BLOOD CELLS insulin Erythrocytes lack mitochondria and hence are wholly reliant on (anaerobic) glycolysis and the pentose phosphate pathway (PPP) Pyruvate Induced by at all times. Carboxylase glucagon, Glucose enters red blood cells by facilitated diffusion, a process epinephrine, mediated by GLUT1 cortisol; The pyruvate formed from glucose is reduced to lactate. Repressed by insulin FED STATE IN THE BRAIN Fructose 2, 6-Bisphosphatase Activated when phosphorylated to CO2 and H2O, generating ATP 📖 Neurons generally oxidize glucose, via glycolysis and TCA cycle, FASTING STATE V. FED STATE VS FASTING STATE Post-absorptive state: Fasting for 6-12 hours FED STATE Prolonged fasting or starvation: Fasting that lasts >12 hours 📋 Also known as postprandial state or absorptive state which Liver metabolism changes from glucose utilization to glucose occurs within 4 hours after a meal production (gluconeogenesis). Geared towards energy production and storage → Key substrates for gluconeogenesis: Insulin secretion is stimulated ▪ Alanine → Directs metabolism towards synthesis and storage ▪ Lactate 💬 (anabolism). ▪ Glycerol During metabolism, Glucokinase is induced by insulin to further → Alanine and lactate are transported to the liver from muscle promote glycolysis BIOCHEMISTRY Carbohydrates in Everyday Life PAGE 13 of 18 BIOCHEMISTRY | LE 1 Carbohydrates in Everyday Life | Catherine L. Co-Reportoso, MD, DPPS → Glycerol is released during the hydrolysis of triacylglycerols B. Sucrase hydrolyzes α1,1 (a1→B2) glycosidic bond of sucrose, (lipolysis) by the hormone-sensitive lipase, which is stimulated a reducing (non-reducing) sugar, to form glucose and fructose. by glucagon. (F) → Contribution of gluconeogenesis increases with duration. Rationalization: Sucrose is not formed with a1,1 Glucagon stimulates glycogenolysis and inhibits glycolysis glycosidic bond, it is trehalose that is formed with a1,1 → DECREASE in glycogenesis glycosidic bonds; Sucrase hydrolyzes a1→B2 glycosidic 💬 → INCREASE in glycogenolysis After 15 hours fasting, glycogenolysis occurs in order to maintain blood sugar levels to normal values. bond of sucrose; Sucrose is a non-reducing sugar. C. Galactose absorption is co-transported with H+ (Na+) in the luminal side via simple passive diffusion (secondary active Glucose uptake by the muscle and adipose tissue decreases transport). (F) Hydrolysis of triacylglycerols (lipolysis) and subsequent fatty acid Rationalization: Galactose is co-transported with oxidation are stimulated. Sodium via SGLT-1 (a secondary active transport). After an overnight fast, a steady state is reached. D. Jose’s GLUT activity is more prominently disrupted in the brain → Hepatic glucose production becomes equal to peripheral (skeletal muscle, adipose tissues, and heart) than in any other glucose uptake (due to the action of Glucose-6-phosphatase organs. (F) and glucokinase). Rationalization: GLUT-4 is found in skeletal muscles, Fasting induces Glucose-6-phosphatase and suppresses adipose, and cardiac muscles. 💬 glucokinase After 16 hours fasting, blood glucose levels decrease, leading to a decrease in insulin levels and insulin-glucagon ratio (glucose levels E. In the liver, fructose’ point of entry to glycolysis is through fructose 6-P (1 mol of DHAP and Glyceraldehyde-3-phosphate) thereby creating a net ATP yield of 3 ATPs (2 ATPs). (F) 💬 affect insulin levels and not the other way around). von Gierke’s Disease causes hepatomegaly after fasting due to excess glycogen stores caused by a deficiency in G6Pase and Rationalization: In the liver, the point of entry for fructose is through 1 mol of DHAP and glyceraldehyde-3-phosphate, which runs through the hypoglycemia due to an inability to utilize glycogen for glucose glycolysis pathway and generates a net yield of 2 ATP. synthesis. The net yield of ATP in glycolysis is always 2 ATP regardless of the hexose entering the pathway. CORI CYCLE F. When comparing the glycolysis occurring in Santiago’s and Glucose, produced in the liver by gluconeogenesis, is converted Juan’s erythrocytes, Santiago’s cells do not produce any net ATP by glycolysis in muscle, red blood cells (RBC), and many other (Both produces a net yield of 2 ATPs), whereas Juan’s cells cells, to lactate. Lactate returns to the liver and is reconverted to yield a net of 2 ATPs. (F) glucose by gluconeogenesis. Rationalization: Glycolysis is not affected because we Muscle facilitates gluconeogenesis by releasing lactate, which is are talking about RBC, therefore, there is still a net yield taken to the liver and is oxidized to pyruvate, which then enters of 2 ATP on both individuals. Von Gierke’s disease gluconeogenesis. affects glycogen metabolism and gluconeogenesis, not Newly synthesized glucose is then released from the liver and is glycolysis. returned to the skeleton muscles. G. Glucokinase is not inhibited by glucose 6-P, and it can therefore GLUCOSE-ALANINE CYCLE continue to operate, whereas the accumulation of glucose 6-P Low insulin stimulates muscle proteolysis thus the release of completely inhibits hexokinases I to III. (T) amino acids, primarily alanine and glutamine. Rationalization: statement is true Alanine is taken up by the liver and converted into pyruvate. H. Fructose 1,6 bisphosphate activates PFK1 (pyruvate kinase), Glucose-Alanine cycle parallels the Cori Cycle. whereas fructose 2,6 bisphosphate inhibits Fructose 1,6 bisphosphatase. (F) Rationalization: F1,6BP allosterically activates pyruvate kinase. It is F2,6BP that allosterically activates PFK1 (and allosterically inhibits F1,6BPase) I. Increased cellular AMP (cAMP) levels and the action of protein kinase A stimulates glycogen phosphorylase making glycogenolysis active. (F) Rationalization: it should be cAMP (cyclic AMP) not cellular AMP. Metabolic event indicated is also inappropriate (glycogenolysis is not active in the fed state). Figure 20. Cori Cycle and Glucose-Alanine Cycle[Baynes, 2019] J. ATP activates (inactivates) isocitrate dehydrogenase by covalent (allosteric) modification (phosphorylation). (F) CASE STUDIES Rationalization: ATP inactivates isocitrate dehydrogenase by allosteric modification. There is no CASE 1 covalent modification in the TCA cycle. It is ADP that 4 friends attended a party at 3pm. They all ate pasta, cake, and allosterically activates isocitrate dehydrogenase. ice cream. Which of the following occurrences is/are true at 6pm? K. Insulin induces expression of hexokinase (glucokinase) Juan - normal adult therefore increasing its amount to promote glycolysis. (F) Pedro - diagnosed with genetic defects in intestinal Rationalization: Hexokinase is not induced by insulin. epithelial cells. Glucokinase is inducible by insulin. Jose - diagnosed with DM type II Santiago - diagnosed with von Gierke’s Disease CASE 2 2 friends decided to undergo intermittent fasting using the method A. Pedro exhibits (does not exhibit) glucose, galactose, and 16:8 (fasting for 16 hrs and eating during an 8-hour window). fructose in the blood. (F) Which of the following statements is/are true during the 16 fasting Rationalization: disaccharidases are not present in hours? intestinal epithelial cells due to genetic defects; Juan - normal adult disaccharides are undigested and are not absorbed, Santiago - diagnosed with von Gierke’s Disease therefore, monosaccharides are not present in blood. Jose - diagnosed with DM type II Santiago - diagnosed with von Gierke’s Disease BIOCHEMISTRY Carbohydrates in Everyday Life PAGE 14 of 18 BIOCHEMISTRY | LE 1 Carbohydrates in Everyday Life | Catherine L. Co-Reportoso, MD, DPPS A.There is decreased secretion of insulin in response to increased Rationalization: The reaction catalyzed by pyruvate (decreased) glucose in the portal blood. (F) carboxylase utilizes 2 moles of ATP because 2 moles of Rationalization: In the context of fasted state, pyruvate are converted to oxaloacetate using 2 moles of decreased glucose will lead to decreased secretion of ATP. insulin. CASE 4 B.Hepatic glycogen releases glucose in response to dephosphorylation (phosphorylation) of glycogen phosphorylase. Different case scenario: (F) Rationalization: Glucagon stimulates cAMP-mediated A. A 24-year-old man running a marathon, pyruvate produced in phosphorylation of glycogen phosphorylase. his muscle is converted to alanine (lactate). (F) C.Glucose 6P is dephosphorylated to glucose by means of Rationalization: In a highly intensive exercise, the glycogen phosphorylase (glucose-6-phosphatase). (F) muscles utilize anaerobic glycolysis, which produces Rationalization: lactic acid. Lactate dehydrogenase catalyzes reduction D.Phosphorylation of hepatic glycogen synthase prevents glucose of pyruvate to lactate. incorporation into glycogen. (T) B. G6PD deficient patient with Urinary Tract Infection was given Rationalization: phosphorylated glycogen synthase is Sulfamethoxazole-Trimethoprim antibiotic to develop hemolytic inactivated. anemia. Lab findings showed an increased (decreased) amount E.Calcium binds to muscle glycogen phosphorylase (calmodulin), of reduced glutathione. (F) activating it without phosphorylation. (F) Rationalization:: Wrong drug: Sulfamethoxazole Rationalization: Calcium binds to calmodulin, not should not be given to patients with G6PD deficiency, it glycogen phosphorylase. Binding occurs without the will lead to reduced amount of glutathione. need for phosphorylation. C. In patients with arsenic poisoning, glycolysis still produces 2 F.Santiago experienced hypoglycemia because of low glycogen moles of pyruvate but 0 net ATP yield. The reaction catalyzed by (glucose) stores due to deficiency of glycogen synthase glyceraldehyde 3P dehydrogenase (phosphoglycerate kinase) is (glucose-6-phosphatase). (F) bypassed, explaining the 0 ATP yield. (F) Rationalization: The body is not able to synthesize Rationalization: both glyceraldehyde-3-phosphate glucose due to lack of G6Pase. dehydrogenase and phosphoglycerate kinase is G.Pyruvate (DHAP/Glycerol-3-Phosphate) is the precursor of bypassed but the bypass of phosphoglycerate kinase is glycerol backbone of TAG explaining Santiago's the enzyme that explains the 0 ATP net yield. hypertriglyceridemia. (F) D. Hemolytic anemia may result from the deficiency of pyruvate Rationalization: DHAP or Glycerol-3-Phosphate is the kinase, glucose 6P dehydrogenase and UDP precursor of