Glycolysis & TCA Cycle Handout Document (2024) PDF

Glycolysis and TCA Cycle -Handout document (2024) Zulfiqar Ahmad, PhD, Department of Biochemistry, ATSU Learning objectives: Describe the glycolytic pathway: – Outline the major s...

Glycolysis and TCA Cycle -Handout document (2024) Zulfiqar Ahmad, PhD, Department of Biochemistry, ATSU Learning objectives: Describe the glycolytic pathway: – Outline the major steps and key enzymes involved in glycolysis. – Explain the conversion of glucose to pyruvate and the production of ATP and NADH. Understand the regulation of glycolysis: – Identify the key regulatory enzymes (hexokinase, phosphofructokinase-1, pyruvate kinase) and their roles in glycolysis. – Discuss the factors that influence the activity of these enzymes. Explain the energy yield: – Calculate the net ATP production from one molecule of glucose through glycolysis. – Describe the roles of substrate-level phosphorylation and NADH production. Differentiate between aerobic and anaerobic glycolysis: – Compare and contrast the oxidative fates of pyruvate and NADH in aerobic versus anaerobic conditions. – Explain the formation of lactate and its impact on cellular metabolism during anaerobic glycolysis. Understand the role of Glycolysis in various tissues: – Identify tissues that rely heavily on glycolysis and discuss why this reliance exists (e.g., red blood cells, muscle cells). Discuss the role of the bisphosphoglycerate shunt: – Explain the formation and function of 2,3-bisphosphoglycerate (2,3-BPG) in red blood cells and its effect on oxygen binding. Understand the TCA cycle basics – Describe the role of the TCA cycle in cellular respiration and energy production. – Identify the main substrates, products, and intermediates of the TCA cycle. Regulation of the TCA cycle – Explain the regulation of key enzymes, including citrate synthase, isocitrate dehydrogenase, and α-ketoglutarate dehydrogenase. – Describe how the TCA cycle is regulated by ATP/ADP and NADH/NAD+ ratios. Anaplerotic reactions – Define anaplerotic reactions and their importance in replenishing TCA cycle intermediates. – Identify major anaplerotic pathways, such as pyruvate carboxylase and amino acid degradation. Metabolic integration – Explain how TCA cycle intermediates contribute to biosynthetic pathways, including gluconeogenesis, fatty acid synthesis, and amino acid synthesis. – Discuss the role of TCA cycle intermediates in different tissues, such as liver, brain, and muscle. Clinical relevance – Apply knowledge of the TCA cycle to clinical scenarios involving metabolic disorders and dietary changes. – Recognize the impact of TCA cycle dysregulation on oral and systemic health. 1 Glycolysis Overview: ATP Production: o Central pathway for generating ATP in all cell types. o Produces ATP with or without oxygen (aerobic and anaerobic). Metabolic Process: o Oxidizes glucose to pyruvate, generating ATP and NADH. o Pyruvate can be further oxidized in the TCA cycle or converted to lactate under anaerobic conditions. Glucose Sources: o Derived from diet, glycogen stores, and blood. o Major carbohydrate providing over 50% of dietary calories. o Other sugars like fructose and galactose are converted to glycolytic intermediates. Glycogen as Fuel: o Stored glucose as glycogen can be used during emergencies (e.g., ischemia). o Hormones like insulin maintain blood glucose levels for ATP generation. Tissue-Specific Utilization: o After a high-carb meal, glucose is the primary fuel for most tissues except some intestinal and kidney cells. o During fasting, the brain relies on glucose due to limited capacity for fatty acid oxidation. o Cells like red blood cells use glycolysis for ATP when oxygen is limited. Anabolic Role: o Provides biosynthetic precursors for fatty acid, amino acid, and nucleotide synthesis. o In liver and adipose tissue, pyruvate from glycolysis is used for fatty acid biosynthesis. The reactions of glycolysis: Conversion of Glucose to Glucose 6-Phosphate Initial Reaction: o Glucose is phosphorylated by ATP to form glucose 6-phosphate (G6P). o This reaction is irreversible due to a high negative ΔG0′. Metabolic Commitment: o G6P cannot cross the plasma membrane, trapping glucose within the cell. o Phosphorylation does not restrict glucose to glycolysis. Metabolic Branch Point: o G6P serves as a precursor for glycolysis, the pentose phosphate pathway, and glycogen synthesis. o It can also be generated from glycogenolysis, the pentose phosphate pathway, and gluconeogenesis. Enzymatic Catalysis: o Hexokinases, tissue-specific isoenzymes, catalyze the phosphorylation. o Glucokinase, found in liver and pancreatic β-cells, has a higher Km than other hexokinases. 2  o Some hexokinases are bound to outer mitochondrial membrane porins, accessing ATP as it exits mitochondria. Conversion of Glucose 6-Phosphate to Triose Phosphates Isomerization and Phosphorylation: o Glucose 6-phosphate (G6P) is converted to fructose 6-phosphate (F6P). o F6P is phosphorylated by ATP to fructose 1,6-bisphosphate (F1,6BP) via phosphofructokinase-1 (PFK-1). o This step is the first committed and irreversible step of glycolysis. Regulation of Glycolysis: o PFK-1 regulates glucose entry into glycolysis and has tissue-specific isoenzymes. o These isoenzymes adapt to the glycolytic needs of different tissues. Cleavage to Triose Phosphates: o F1,6BP is split by aldolase into dihydroxyacetone phosphate (DHAP) and glyceraldehyde 3- phosphate (G3P). o DHAP is converted to G3P by triose phosphate isomerase. o Aldolase performs aldol cleavage and uses a lysine residue to form a covalent bond with the substrate. Outcome: o For each glucose molecule, two G3P molecules are produced for further glycolysis. Oxidation and Substrate-Level Phosphorylation in Glycolysis Glyceraldehyde 3-Phosphate Oxidation: o Catalyzed by glyceraldehyde-3-phosphate dehydrogenase. o Converts glyceraldehyde 3-phosphate to 1,3-bisphosphoglycerate (1,3-BPG). o Electrons are transferred to NAD+ to form NADH. o Involves a high-energy thioester intermediate with a cysteine residue. o Forms a high-energy acyl phosphate bond, starting substrate-level phosphorylation. ATP Formation: o 1,3-BPG transfers its phosphate to ADP via 3-phosphoglycerate kinase, forming ATP and 3- phosphoglycerate. o The energy from the acyl phosphate bond drives the formation of ATP. Conversion to High-Energy PEP: o 3-Phosphoglycerate is converted to 2-phosphoglycerate. o Water is removed to form phosphoenolpyruvate (PEP) with a high-energy enol phosphate bond. Final ATP Generation: o PEP transfers its phosphate to ADP via pyruvate kinase, forming ATP and pyruvate. o This reaction is energetically favorable and irreversible. 3 Oxidative fates of pyruvate and NADH: NADH Reoxidation: o NADH from glycolysis must be reoxidized to NAD+ for glycolysis to continue. o Two routes for NADH oxidation: aerobic and anaerobic. Aerobic Pathway: o Uses shuttles to transfer electrons to the mitochondrial ETC and oxygen. o Allows pyruvate to be oxidized to acetyl CoA and enter the TCA cycle. Anaerobic Pathway: o NADH is reoxidized by lactate dehydrogenase (LDH), reducing pyruvate to lactate. o Pyruvate is diverted from the TCA cycle. ATP Generation: o Aerobic glycolysis generates more ATP than anaerobic glycolysis. o Shuttle systems enable NADH oxidation and complete pyruvate oxidation to CO2. Mitochondrial Membrane: o Inner mitochondrial membrane is impermeable to NADH. o Shuttles are required to transfer electrons from cytosolic NADH to the ETC. Anaerobic Glycolysis: Overview: o Occurs when the cell's oxidative capacity is limited (e.g., red blood cells). o NADH is oxidized to NAD+ by converting pyruvate to lactate via lactate dehydrogenase (LDH). Net Reaction: o Glucose + 2 ADP + 2 Pi → 2 lactate + 2 ATP + 2 H2O + 2 H⁺ Acid Production: o Glycolysis produces pyruvic acid, reduced to lactic acid, dissociating into lactate and H⁺. o Excess lactate can lower blood pH, leading to lactic acidosis. Tissues Relying on Anaerobic Glycolysis: o Includes red and white blood cells, kidney medulla, eye tissues, skeletal muscles. o Tissues have high glycolytic enzymes, low ATP demand, and limited oxygen supply. Role of Anaerobic Glycolysis: o In cells with mitochondria, it supplements ATP when oxygen is limited. o Eye cells use anaerobic glycolysis to avoid opaque structures. Fate of Lactate: o Lactate is taken up by liver, heart, and muscle, converted back to pyruvate. o Pyruvate in the liver can be used for gluconeogenesis (Cori cycle). o Heart uses lactate as a fuel source due to its oxidative capacity. LDH Isoenzymes: o Composed of A (M for muscle) and B (H for heart) subunits. o Five tetramers (M4, M3H1, M2H2, M1H3, H4) with varying roles in converting lactate and pyruvate. Other Functions of Glycolysis: Biosynthesis Precursors: o Generates intermediates for nucleotide synthesis, like ribose 5-phosphate. o Produces sugars such as UDP-glucose, mannose, and sialic acid. o Synthesizes amino acids: serine from 3-phosphoglycerate and alanine from pyruvate. o Provides glycerol 3-P for triacylglycerol backbone from DHAP. 4  Liver Functions: o Major site for biosynthetic reactions. o Converts pyruvate to fatty acids. o Synthesizes glucose from lactate, glycerol 3-P, and amino acids (gluconeogenesis). o Contains isoenzymes for specialized functions. Bisphosphoglycerate Shunt: o Converts 1,3-BPG to 2,3-BPG in red blood cells. o 2,3-BPG inhibits oxygen binding to heme. o Reenters glycolysis as 3-phosphoglycerate. o Acts as a coenzyme in phosphoglyceromutase reaction. Regulation of Glycolysis by ATP Needs: Key Concepts: o Pathways are regulated at the rate-limiting (slowest) step. o Regulation ensures that products flow into alternative pathways if not needed. Major Regulatory Sites: o PFK-1 (Phosphofructokinase-1): Primary control point in glycolysis. o PDH (Pyruvate Dehydrogenase): Connects glycolysis to the TCA cycle. Regulation Mechanisms: o ATP Homeostasis: Maintained by adjusting glycolysis based on ATP needs. o Glucose 6-Phosphate Supply: Regulated through glucose transport, glycogenolysis, and hexokinase activity. Tissue-Specific Isoenzymes: o Isoenzymes: Tailor glycolysis regulation to tissue-specific conditions. o Liver Example: Pyruvate kinase isoenzyme inhibits glycolysis when gluconeogenesis is active. The tricarboxylic acid cycle (TCA Cycle): The tricarboxylic acid (TCA) cycle produces over two-thirds of ATP from fuel oxidation. It uses acetyl coenzyme A (acetyl-CoA) as its substrate, generated from: Fatty acids Glucose Amino acids Acetate Ketone bodies The TCA cycle oxidizes the acetyl group into two molecules of CO₂. Energy is conserved as: Reduced nicotinamide adenine dinucleotide (NADH) Flavin adenine dinucleotide (FAD[2H]) Guanosine triphosphate (GTP) NADH and FAD(2H) donate electrons to O₂ via the electron transport chain (ETC), producing ATP through oxidative phosphorylation. The TCA cycle is essential for energy generation in cellular respiration. Overview of TCA Cycle: Names and Origins: Also known as the Krebs cycle, after Sir Hans Krebs. Called the citric acid cycle due to the involvement of citrate. The term "tricarboxylic acid cycle" (TCA) refers to tricarboxylates like citrate and isocitrate. 5  Function: The TCA cycle generates large amounts of ATP. Acetyl-CoA is the primary substrate for the cycle, produced by fuel oxidation pathways. Key Steps: Acetyl-CoA combines with oxaloacetate (4-carbons) to form citrate (6-carbons), which is rearranged to isocitrate. Oxidative decarboxylation transfers electrons to NAD⁺, forming NADH, and releases CO₂. Generates GTP via substrate-level phosphorylation. Succinate is oxidized to regenerate oxaloacetate, producing FAD(2H) and NADH. Net Reaction: Two carbons from acetyl-CoA are oxidized to CO₂. Energy conserved as: o 3 NADH o 1 FAD(2H) o 1 GTP Required Vitamins and Minerals: Niacin (NAD⁺) Riboflavin (FAD, FMN) Pantothenic acid (coenzyme A) Thiamin Mg²⁺ Ca²⁺ Fe²⁺ Phosphate Reactions of the Tricarboxylic Acid Cycle: Overview: o The TCA cycle oxidizes the acetyl group of acetyl-CoA to two CO₂ molecules. o Conserves energy by transferring electrons to NAD⁺ and FAD, forming 3 NADH and 1 FAD(2H). o Electrons are transferred to O₂ via the ETC, generating ATP through oxidative phosphorylation. o The acetyl group is first incorporated into citrate and progresses to oxaloacetate. o Oxidation is facilitated by dehydrogenases, which transfer electrons to NAD⁺ or FAD. Energy Yield: o Generates 3 NADH, 1 FAD(2H), and 1 GTP per cycle. o GTP is produced through substrate-level phosphorylation by succinate thiokinase. o Reoxidation of NADH and FAD(2H) in the ETC yields ~2.5 ATP per NADH and ~1.5 ATP per FAD(2H). o Total energy yield is about 10 ATP per acetyl group oxidized. Formation and Oxidation of Isocitrate: o Acetyl-CoA combines with oxaloacetate to form citrate, catalyzed by citrate synthase. o Aconitase converts citrate to isocitrate. 6  o Isocitrate dehydrogenase oxidizes isocitrate, releasing CO₂ and forming α-ketoglutarate. Conversion of α-Ketoglutarate to Succinyl-CoA: o Catalyzed by the α-ketoglutarate dehydrogenase complex. o Converts α-ketoglutarate to succinyl-CoA, releasing CO₂ and forming NADH. Generation of Guanosine Triphosphate (GTP): o Succinyl-CoA synthetase catalyzes the conversion of succinyl-CoA to succinate, producing GTP. o GTP is equivalent to ATP and used for energy-requiring reactions. o This reaction is an example of substrate-level phosphorylation. o By definition, substrate-level phosphorylation is the formation of a high-energy phosphate bond where none previously existed without the use of molecular O2 (in other words, not oxidative phosphorylation). o The high-energy phosphate bond of GTP is energetically equivalent to that of ATP and can be used directly for energy-requiring reactions like protein synthesis. Oxidation of Succinate to Oxaloacetate: o Succinate is oxidized to fumarate, transferring electrons to FAD. o Fumarate is converted to malate by adding water. o Malate is oxidized to oxaloacetate, transferring electrons to NAD⁺. o Completes the cycle by regenerating oxaloacetate for the next cycle. Coenzymes of the Tricarboxylic Acid Cycle: NAD+ and FAD o Isocitrate dehydrogenase and malate dehydrogenase use NAD+. o Succinate dehydrogenase uses FAD. o Citrate synthase uses acetyl-CoA (a CoA derivative). o α-Ketoglutarate dehydrogenase complex uses TPP, lipoate, and FAD as bound coenzymes; NAD+ and CoASH are substrates. FAD vs. NAD+ as Electron Acceptors o FAD accepts single electrons (H ), forming a half-reduced intermediate, used in reactions like succinate to fumarate. o NAD+ accepts a pair of electrons as a hydride ion (H⁻), used in reactions like alcohol oxidation to ketones. o FAD must remain tightly bound to its enzyme due to the reactivity of its free-radical form. o NAD+/NADH is more like substrate/product, playing a regulatory role in energy metabolism. o NADH/NAD+ ratio controls oxidative enzymes, coordinating fuel oxidation with ATP use. Coenzyme A (CoASH) in the TCA Cycle o Forms thioester bonds with acyl groups (e.g., acetyl-CoA, succinyl-CoA). o Thioester bonds are high-energy with a large negative ΔG°' of hydrolysis. o Cleavage of these bonds provides energy for reactions, e.g., forming GTP from GDP in the succinate thiokinase reaction. o Drives the TCA cycle forward by keeping reaction ΔG°' negative. 7  α-Ketoacid Dehydrogenase Complexes o Includes α-ketoglutarate dehydrogenase, pyruvate dehydrogenase complex (PDC), and branched-chain amino acid α-keto acid dehydrogenase complex. o Each complex decarboxylates an α-keto acid, oxidizing it to a carboxylic acid and forming an acyl-CoA thioester. o Consists of three enzyme subunits: E1 (α-keto acid decarboxylase with TPP), E2 (transacylase with lipoate), E3 (dihydrolipoyl dehydrogenase with FAD). o Thiamin Pyrophosphate (TPP)  Synthesized from thiamin and rapidly turned over in the body.  Cleaves C-C bonds next to keto groups, as seen in transketolase activity in the pentose phosphate pathway.  Deficiency leads to accumulation of α-keto acids in blood. o Lipoate  Found only in α-keto acid dehydrogenase complexes.  Synthesized from carbohydrates and amino acids; no vitamin precursor needed.  Covalently bound to E2, transfers acyl groups and electrons during catalysis. o Flavin Adenine Dinucleotide (FAD) and Dihydrolipoyl Dehydrogenase  FAD accepts electrons from lipoate, transferring them to NAD+.  Bound to E3, changes reduction potential through enzyme interactions.  Facilitates oxidation of α-ketoglutarate, driving the reaction forward. Regulation of the Tricarboxylic Acid Cycle: General Overview o The TCA cycle converts acetyl-CoA into NADH and FAD(2H), which are crucial for ATP production. o The cycle’s rate is regulated to match the rate of the electron transport chain (ETC), which depends on the ATP/ADP ratio and ATP consumption. Regulatory Messengers o ATP/ADP Ratio: Reflects ATP usage and influences cycle rate. o NADH/NAD+ Ratio: Reflects NADH oxidation and impacts cycle activity. o Constant Nucleotide Pools: Total AMP, ADP, ATP, NAD+, and NADH levels remain relatively stable. A. Regulation of Citrate Synthase o Control Factors:  Oxaloacetate: Main substrate.  Citrate: Product inhibitor. o Regulation:  Low oxaloacetate concentration due to malate equilibrium.  Reduced NADH/NAD+ ratio increases oxaloacetate levels.  Decreased citrate levels reduce inhibition of citrate synthase.  In the liver, NADH/NAD+ ratio decides whether acetyl-CoA enters the TCA cycle or ketogenesis. 8  B. Allosteric Regulation of Isocitrate Dehydrogenase o Activation: By ADP. o Inhibition: By NADH. o Mechanism:  Positive cooperativity: Isocitrate binding to one subunit activates others.  ADP presence increases enzyme activity and lowers Km.  NADH and NAD+ concentrations significantly impact enzyme activity. C. Regulation of α-Ketoglutarate Dehydrogenase o Inhibition:  By NADH and succinyl-CoA.  Possibly by GTP. o Activation:  By Ca²⁺, especially during muscle contraction. o Response: Regulates to balance ATP production with consumption. D. Regulation of TCA Cycle Intermediates o Functions:  Ensures adequate NADH production for ATP homeostasis.  Regulates intermediate concentrations. o Example:  Increased citrate from reduced isocitrate dehydrogenase promotes citrate efflux.  Citrate inhibits PFK-1 and activates fatty acid synthesis in the cytosol.  Citrate efflux signals high mitochondrial energy levels. Precursors of Acetyl Coenzyme A: Entry Points into the TCA Cycle o Compounds enter the TCA cycle as acetyl-CoA or as intermediates (malate or oxaloacetate). o Acetyl-CoA is oxidized to CO₂. o Intermediates replenish those used in biosynthetic pathways (e.g., gluconeogenesis, heme synthesis) but are not fully oxidized to CO₂. Sources of Acetyl-CoA o Fatty Acids: β-oxidation generates acetyl-CoA. o Ketone Bodies: Degradation of β-hydroxybutyrate and acetoacetate forms acetyl-CoA. o Acetate: Derived from the diet or ethanol oxidation. o Glucose and Carbohydrates:  Enter glycolysis, converting to pyruvate.  Pyruvate is oxidized to acetyl-CoA by the PDC. o Amino Acids:  Alanine and serine are converted to pyruvate.  Leucine and isoleucine are directly oxidized to acetyl-CoA. o Final Oxidation: Acetyl-CoA oxidation to CO₂ in the TCA cycle is the last step in major fuel oxidation pathways. Pyruvate Dehydrogenase Complex (PDC) o Oxidizes pyruvate to acetyl-CoA, linking glycolysis and the TCA cycle. o Critical for the brain, which relies on glucose oxidation for ATP. 9 Tricarboxylic Acid Cycle Intermediates and Anaplerotic Reactions: A. TCA Cycle Intermediates as Precursors Role in Biosynthesis: o TCA cycle intermediates serve as precursors for biosynthetic pathways in different cell types, notably in the liver (open cycle due to high efflux). Pathways: o Post-High-Carbohydrate Meal: Citrate leaves mitochondria, is cleaved to acetyl-CoA for fatty acid synthesis. o Fasting: Gluconeogenic precursors convert to malate for cytosolic gluconeogenesis. o Liver: Intermediates form amino acid carbon skeletons and heme from succinyl-CoA. o Brain: α-Ketoglutarate forms glutamate and GABA (neurotransmitter). o Skeletal Muscle: α-Ketoglutarate converts to glutamine, transported to other tissues. Transporters: o Mitochondrial transporters exist for pyruvate, citrate, α-ketoglutarate, malate, ADP, ATP, and phosphate. o Acetyl-CoA is obtained in the cytosol by cleaving citrate via citrate lyase. B. Anaplerotic Reactions Purpose: Replenish TCA cycle intermediates to maintain oxaloacetate levels for continued acetyl-CoA oxidation. 1. Pyruvate Carboxylase: o Function: Converts pyruvate to oxaloacetate, using biotin, ATP, and Mg²⁺. o Location: Found in tissues like liver, brain, adipocytes, and fibroblasts. o Activation: Triggered by high acetyl-CoA levels; inhibited by high acyl-CoA derivatives. o Significance: High concentration in liver and kidney cortex for gluconeogenesis. 2. Amino Acid Degradation: o Pathways:  Alanine and serine enter via pyruvate carboxylase.  Isoleucine and valine oxidation to succinyl-CoA is a major route in most tissues.  In liver, methionine, threonine, and odd-chain fatty acids convert to succinyl-CoA.  Glutamine is converted to glutamate and then α-ketoglutarate. Limitations: o Fatty acid oxidation of even-chain lengths and ketone body oxidation do not replenish TCA cycle intermediates as they only produce acetyl-CoA. PRACTICE QUESTIONS: Q1. What is the first step in glucose metabolism within the cell? A) Isomerization to fructose 6-phosphate B) Phosphorylation to glucose 6-phosphate C) Cleavage into two three-carbon fragments D) Oxidation to pyruvate 10 Q2. Why is the phosphorylation of glucose to glucose 6-phosphate irreversible? A) It releases energy. B) It has a high positive ΔG0′. C) It has a high negative ΔG0′. D) It does not require ATP. Q3. Which enzyme catalyzes the conversion of glucose 6-phosphate to fructose 6-phosphate? Q4. The conversion of fructose 6-phosphate to fructose 1,6-bisphosphate requires: A) NADH B) ATP C) FADH2 D) CO2 Q5. Which step is considered the first committed step of glycolysis? Q6. What is the role of aldolase in glycolysis? Q7. Which of the following is a product of the aldolase reaction? A) Glucose 6-phosphate B) Fructose 6-phosphate C) Dihydroxyacetone phosphate (DHAP) D) Pyruvate Q8. Glyceraldehyde 3-phosphate dehydrogenase catalyzes the formation of which molecule? Q9. Which enzyme is responsible for the first substrate-level phosphorylation in glycolysis? Q10. The conversion of 3-phosphoglycerate to phosphoenolpyruvate involves: Q11. What is the final product of glycolysis? Q12. Which of the following reactions is catalyzed by pyruvate kinase? A) Conversion of glucose to glucose 6-phosphate B) Conversion of phosphoenolpyruvate (PEP) to pyruvate C) Conversion of fructose 1,6-bisphosphate to triose phosphates D) Conversion of glyceraldehyde 3-phosphate to 1,3-bisphosphoglycerate Q13. How many molecules of ATP are produced by substrate-level phosphorylation for each molecule of glucose in glycolysis? 11 Q14. Which high-energy intermediate is formed immediately after the oxidation of glyceraldehyde 3- phosphate? A) 3-Phosphoglycerate B) 1,3-Bisphosphoglycerate C) Phosphoenolpyruvate D) Fructose 1,6-bisphosphate Q15. What molecule is regenerated when NAD+ is reduced during glycolysis? Q18. What are the main reason shuttles are required for the oxidation of cytosolic NADH? Q21. What is the primary reason anaerobic glycolysis occurs in certain cells like red blood cells? A) Lack of oxygen supply B) Lack of mitochondria C) High ATP demand D) Presence of oxidative enzymes Q23. Which tissue primarily uses the Cori cycle to recycle lactate into glucose? A) Heart B) Brain C) Liver D) Kidney Q24. What is the consequence of excess lactate production during anaerobic glycolysis? A) Increased oxygen supply B) Lactic acidosis C) Increased blood pH D) Decreased ATP production Q26. Which intermediate of glycolysis is used to synthesize the amino acid serine? A) Pyruvate B) 3-Phosphoglycerate C) Glycerol 3-Phosphate D) Fructose 6-Phosphate Q27. What is the role of 2,3-bisphosphoglycerate (2,3-BPG) in red blood cells? A) Enhances oxygen binding to hemoglobin B) Inhibits oxygen binding to hemoglobin C) Acts as a substrate for gluconeogenesis D) Converts glucose to glycogen 12 Q29. Which compound is produced from DHAP in glycolysis and serves as a backbone for triacylglycerol synthesis? Q30. Which glycolytic intermediate is a precursor for nucleotide sugars like UDP-glucose? A) Pyruvate B) Fructose 1,6-bisphosphate C) Glucose 6-phosphate D) Ribose 5-phosphate Q31. Which process is essentially a reversal of glycolysis in the liver? Q35. Which glycolytic product can be converted into alanine? Q36. Which enzyme is considered the primary control point in glycolysis? TCA Cycle: Q1. Which molecule is the initial substrate for the TCA cycle? Q3. Which enzyme catalyzes the conversion of citrate to isocitrate? Q4. How many molecules of CO₂ are released during one turn of the TCA cycle? 5. Which molecule directly donates electrons to the electron transport chain? A. Citrate B. NADH C. Acetyl-CoA D. GTP Q6. What is the net yield of GTP from one cycle of the TCA cycle before the ETC? Q8. How many NADH molecules are produced per turn of the TCA cycle? Q9. What is the role of succinate dehydrogenase in the TCA cycle? Q10. Which high-energy compound is equivalent to ATP and is generated during the TCA cycle? Q12. Which step in the TCA cycle involves substrate-level phosphorylation? A. Conversion of citrate to isocitrate B. Conversion of succinyl-CoA to succinate C. Conversion of α-ketoglutarate to succinyl-CoA D. Conversion of malate to oxaloacetate 13 Q13. Which enzyme catalyzes the formation of citrate from acetyl-CoA and oxaloacetate? Q14. Which molecule is regenerated at the end of the TCA cycle to start another cycle? Q15. How many ATP are generated per NADH reoxidized in the electron transport chain? Q16. Which coenzyme is a part of the α-ketoglutarate dehydrogenase complex? Q17. What is the total number of carbon atoms entering the TCA cycle from one molecule of acetyl-CoA? 18. Which intermediate of the TCA cycle is involved in both the cycle and amino acid metabolism? Q19. What is the primary function of the enzyme fumarase in the TCA cycle? Q21. What is the source of the oxygen atoms in the CO₂ produced by the TCA cycle? Q22. Which intermediate of the TCA cycle is also a key precursor for gluconeogenesis? Q25. How many water molecules are consumed during one turn of the TCA cycle? Q28. Which coenzyme is regenerated in the electron transport chain to continue the TCA cycle? Q29. Which coenzyme is used by isocitrate dehydrogenase in the TCA cycle? A) FAD B) NAD+ C) CoA D) TPP Q35. Which coenzyme is found only in α-keto acid dehydrogenase complexes? A) NAD+ B) Lipoate C) FAD D) CoASH Q37. Which enzyme complex in the TCA cycle uses TPP as a coenzyme? A) Succinate thiokinase B) α-Ketoglutarate dehydrogenase C) Isocitrate dehydrogenase D) Citrate synthase Q42. Which of the following enzymes is allosterically activated by ADP? A) Citrate synthase B) α-Ketoglutarate dehydrogenase C) Isocitrate dehydrogenase D) Succinate dehydrogenase 14 Q47. What happens to citrate levels in the liver when isocitrate dehydrogenase activity decreases? Q48. What is the role of citrate efflux from mitochondria in the cytosol? A) Activates fatty acid synthesis B) Inhibits glycolysis C) Increases ATP production D) Regulates TCA cycle intermediates Q49. Which of the following pathways directly generates acetyl-CoA? A. β-oxidation of fatty acids B. Glycolysis C. Gluconeogenesis D. Heme synthesis Q50. Which compounds can be converted to pyruvate before entering the TCA cycle? A. Leucine and isoleucine B. Alanine and serine C. Acetoacetate and β-hydroxybutyrate D. Acetate and ethanol Q51. How does the brain primarily meet its ATP needs? A. By oxidizing fatty acids B. By utilizing ketone bodies C. By oxidizing glucose to CO₂ D. By synthesizing acetyl-CoA from amino acids Q52. Which enzyme complex links glycolysis and the TCA cycle by converting pyruvate to acetyl-CoA? Q53. What role do TCA cycle intermediates like malate and oxaloacetate play in cellular metabolism? A. They are fully oxidized to CO₂. B. They act as substrates for glycolysis. C. They replenish intermediates used in biosynthetic pathways. D. They are converted to ketone bodies. Q54. Which TCA cycle intermediate is involved in synthesizing fatty acids after a high-carbohydrate meal? Q55. Which TCA cycle intermediate is converted to glutamate and then to GABA in the brain? A) Citrate B) α-Ketoglutarate C) Malate D) Succinyl-CoA 15 Q56. What is the main anaplerotic enzyme that converts pyruvate to oxaloacetate? A) Citrate lyase B) Pyruvate dehydrogenase C) Pyruvate carboxylase D) Glutaminase Q59. Which of the following pathways cannot replenish TCA cycle intermediates? A) Fatty acid oxidation of even-chain lengths B) Oxidation of isoleucine and valine C) Glutamine conversion to α-ketoglutarate D) Alanine and serine through pyruvate carboxylase Q60. What is the primary function of anaplerotic reactions in the TCA cycle? Q63. Which TCA cycle intermediate is removed to form heme in liver and bone marrow cells? A) Succinyl-CoA B) α-Ketoglutarate C) Citrate D) Malate Q64. A 55-year-old male patient with diabetes visits your dental office complaining of gum inflammation and a slow-healing oral ulcer. You suspect poor blood glucose control is contributing to the delayed healing process. How does impaired glycolysis in hyperglycemic conditions affect wound healing in oral tissues? A) Decreased ATP production impairs collagen synthesis. B) Increased lactate production enhances healing. C) Excess ATP leads to overactive immune responses. D) Reduced NADH production increases oxidative stress. Q65. A 45-year-old male patient presents to your dental office with complaints of fatigue and bleeding gums. He mentions a recent dietary change to a high-fat, low-carbohydrate diet. How might this diet affect the TCA cycle and contribute to his symptoms? A) Increased production of oxaloacetate B) Decreased availability of TCA cycle intermediates C) Enhanced conversion of acetyl-CoA to oxaloacetate D) Increased gluconeogenesis 16