Biological Chemistry II Unit 2 Review: Carbohydrate Metabolism

Introduction to Carbohydrates

• Carbohydrates are organic molecules composed of carbon, hydrogen, and oxygen atoms, typically in a ratio of 1:2:1 (CH2O)n
• Classified into three main categories based on their structure and complexity: monosaccharides (glucose, fructose), disaccharides (sucrose, lactose), and polysaccharides (starch, glycogen, cellulose)
• Serve as the primary energy source for living organisms, providing 4 calories per gram when metabolized
• Play crucial roles in various biological processes, including energy storage (glycogen), structural components (cellulose in plant cell walls), and cell signaling (glycoproteins)
  • Glycoproteins are involved in cell-cell recognition, adhesion, and communication
• Carbohydrates can be linked together through glycosidic bonds, forming complex structures with diverse functions
• Stereoisomers of carbohydrates, such as D-glucose and L-glucose, have different spatial arrangements of their atoms, affecting their biological properties and interactions with enzymes

Glycolysis: Breaking Down Glucose

• Glycolysis is a central metabolic pathway that breaks down glucose into two molecules of pyruvate, releasing energy in the form of ATP and NADH
• Occurs in the cytoplasm of cells and does not require oxygen (anaerobic process)
• Consists of ten enzymatic reactions divided into two phases: the preparatory phase and the payoff phase
  • Preparatory phase consumes 2 ATP to convert glucose into fructose-1,6-bisphosphate
  • Payoff phase yields 4 ATP and 2 NADH, resulting in a net gain of 2 ATP and 2 NADH per glucose molecule
• Key enzymes involved in glycolysis include hexokinase, phosphofructokinase (PFK), and pyruvate kinase, which catalyze irreversible steps and regulate the flow of metabolites
• PFK is allosterically regulated by ATP, ADP, and AMP, allowing cells to adjust glycolytic flux based on energy needs
• Pyruvate, the end product of glycolysis, can be further metabolized in the citric acid cycle or fermented to lactate (in anaerobic conditions)
• Glycolysis is a highly conserved pathway across various organisms, highlighting its evolutionary significance in energy production

The Citric Acid Cycle

• The citric acid cycle, also known as the Krebs cycle or tricarboxylic acid (TCA) cycle, is a central metabolic pathway that oxidizes acetyl-CoA derived from carbohydrates, fats, and proteins
• Occurs in the mitochondrial matrix and is a key component of aerobic respiration, generating high-energy molecules (NADH, FADH2) for ATP production in the electron transport chain
• Consists of eight enzymatic reactions that regenerate the starting compound, oxaloacetate, allowing the cycle to continue
• Acetyl-CoA, produced from pyruvate by the pyruvate dehydrogenase complex, enters the cycle by condensing with oxaloacetate to form citrate
• Key enzymes in the citric acid cycle include citrate synthase, isocitrate dehydrogenase, and α-ketoglutarate dehydrogenase, which catalyze irreversible steps and are regulated by substrate availability and product inhibition
• The cycle generates 3 NADH, 1 FADH2, and 1 GTP (or ATP) per acetyl-CoA molecule oxidized, providing a significant source of reducing equivalents for ATP synthesis
• Intermediates of the citric acid cycle, such as α-ketoglutarate and oxaloacetate, can be used as precursors for the synthesis of amino acids and other important biomolecules (anaplerotic reactions)
• The citric acid cycle is closely linked to other metabolic pathways, such as the urea cycle and the glyoxylate cycle, allowing for the efficient utilization of various energy sources

Electron Transport Chain and ATP Synthesis

• The electron transport chain (ETC) is a series of protein complexes embedded in the inner mitochondrial membrane that transfer electrons from NADH and FADH2 to oxygen, generating an electrochemical proton gradient
• Consists of four main protein complexes (I, II, III, and IV) and two mobile electron carriers (ubiquinone and cytochrome c)
  • Complex I (NADH dehydrogenase) oxidizes NADH and transfers electrons to ubiquinone, pumping protons into the intermembrane space
  • Complex II (succinate dehydrogenase) oxidizes FADH2 and transfers electrons to ubiquinone without pumping protons
  • Complex III (cytochrome bc1 complex) transfers electrons from ubiquinone to cytochrome c, pumping protons into the intermembrane space
  • Complex IV (cytochrome c oxidase) transfers electrons from cytochrome c to oxygen, forming water and pumping protons into the intermembrane space
• The proton gradient generated by the ETC is used by ATP synthase to synthesize ATP through the process of chemiosmosis
  • ATP synthase is a rotary engine that couples the flow of protons down their electrochemical gradient to the phosphorylation of ADP, forming ATP
• The ETC and ATP synthesis are tightly coupled, with the rate of electron transfer regulated by the proton gradient and the availability of ADP and Pi
• Inhibitors of the ETC, such as rotenone (Complex I) and cyanide (Complex IV), can disrupt ATP production and lead to cell death
• Mitochondrial disorders, such as MELAS syndrome and Leigh syndrome, can arise from mutations in genes encoding ETC components, leading to impaired energy production and various clinical symptoms

Gluconeogenesis: Making Glucose

• Gluconeogenesis is the metabolic pathway that synthesizes glucose from non-carbohydrate precursors, such as lactate, amino acids (alanine, glutamine), and glycerol
• Occurs primarily in the liver and, to a lesser extent, in the kidneys during periods of fasting or prolonged exercise
• Shares several enzymes with glycolysis but requires four unique enzymes to bypass the irreversible steps of glycolysis
  • Pyruvate carboxylase converts pyruvate to oxaloacetate in the mitochondria
  • Phosphoenolpyruvate carboxykinase (PEPCK) converts oxaloacetate to phosphoenolpyruvate in the cytosol
  • Fructose-1,6-bisphosphatase converts fructose-1,6-bisphosphate to fructose-6-phosphate
  • Glucose-6-phosphatase converts glucose-6-phosphate to glucose in the endoplasmic reticulum
• Gluconeogenesis is energetically expensive, requiring 6 ATP and 2 GTP per glucose molecule synthesized
• Hormones, such as glucagon and cortisol, stimulate gluconeogenesis by increasing the expression and activity of key enzymes (PEPCK, glucose-6-phosphatase)
• Gluconeogenesis is tightly regulated to maintain blood glucose homeostasis, with reciprocal regulation to glycolysis
  • High ATP and citrate levels inhibit gluconeogenesis, while low energy states (high AMP, ADP) stimulate the pathway
• Disorders affecting gluconeogenesis, such as pyruvate carboxylase deficiency and fructose-1,6-bisphosphatase deficiency, can lead to hypoglycemia and lactic acidosis

Glycogen Metabolism

• Glycogen is a highly branched polysaccharide that serves as the primary storage form of glucose in animals, predominantly found in the liver and skeletal muscle
• Glycogen synthesis (glycogenesis) occurs when glucose is abundant, such as after a meal, and is stimulated by the hormone insulin
  • Glycogen synthase catalyzes the formation of α-1,4-glycosidic bonds between glucose monomers, while the branching enzyme introduces α-1,6-glycosidic bonds to create branch points
• Glycogen breakdown (glycogenolysis) occurs during periods of fasting or intense exercise and is stimulated by the hormones glucagon and epinephrine
  • Glycogen phosphorylase catalyzes the phosphorolytic cleavage of α-1,4-glycosidic bonds, releasing glucose-1-phosphate, while the debranching enzyme removes α-1,6-glycosidic bonds
• Glycogen metabolism is tightly regulated by allosteric effectors and covalent modifications (phosphorylation) of key enzymes
  • Glycogen synthase is inhibited by phosphorylation (by GSK-3) and activated by dephosphorylation (by PP1)
  • Glycogen phosphorylase is activated by phosphorylation (by PKA) and inhibited by dephosphorylation (by PP1)
• The liver plays a crucial role in maintaining blood glucose homeostasis by storing and releasing glucose as needed
  • During fasting, the liver breaks down glycogen and releases glucose into the bloodstream for use by other tissues
  • After a meal, the liver takes up excess glucose and stores it as glycogen, preventing hyperglycemia
• Disorders of glycogen metabolism, such as glycogen storage diseases (von Gierke disease, Pompe disease), can lead to hypoglycemia, hepatomegaly, and muscle weakness

Regulation of Carbohydrate Metabolism

• Carbohydrate metabolism is tightly regulated by hormones, allosteric effectors, and covalent modifications to maintain energy homeostasis
• Insulin, secreted by pancreatic β-cells in response to high blood glucose, promotes glucose uptake, glycolysis, and glycogen synthesis while inhibiting gluconeogenesis and glycogenolysis
  • Insulin binds to its receptor, activating a signaling cascade that leads to the translocation of GLUT4 glucose transporters to the cell surface and the activation of glycogen synthase
• Glucagon, secreted by pancreatic α-cells in response to low blood glucose, stimulates glycogenolysis, gluconeogenesis, and ketogenesis while inhibiting glycolysis and glycogen synthesis
  • Glucagon binds to its receptor, activating adenylate cyclase and increasing cAMP levels, which activates protein kinase A (PKA) and leads to the phosphorylation of key enzymes
• Allosteric regulation of enzymes allows for rapid modulation of metabolic flux based on the energy state of the cell
  • Phosphofructokinase (PFK) is allosterically inhibited by ATP and citrate, while activated by AMP and fructose-2,6-bisphosphate
  • Pyruvate kinase is allosterically inhibited by ATP and alanine, while activated by fructose-1,6-bisphosphate
• Transcriptional regulation of metabolic enzymes allows for long-term adaptations to changes in nutrient availability and hormonal signals
  • The transcription factor ChREBP (carbohydrate-responsive element-binding protein) is activated by high glucose levels and promotes the expression of glycolytic and lipogenic enzymes
  • The transcription factor FOXO1 (forkhead box protein O1) is inhibited by insulin and promotes the expression of gluconeogenic enzymes during fasting
• Dysregulation of carbohydrate metabolism, such as in diabetes mellitus, can lead to chronic hyperglycemia and associated complications (retinopathy, neuropathy, nephropathy)

Clinical Relevance and Disorders

• Diabetes mellitus is a group of metabolic disorders characterized by chronic hyperglycemia resulting from defects in insulin secretion, insulin action, or both
  • Type 1 diabetes is caused by autoimmune destruction of pancreatic β-cells, leading to absolute insulin deficiency
  • Type 2 diabetes is characterized by insulin resistance and relative insulin deficiency, often associated with obesity and sedentary lifestyle
• Glycogen storage diseases (GSDs) are a group of inherited disorders caused by defects in enzymes involved in glycogen synthesis or breakdown, leading to the accumulation of glycogen in tissues
  • von Gierke disease (GSD type I) is caused by a deficiency in glucose-6-phosphatase, resulting in hypoglycemia, hepatomegaly, and lactic acidosis
  • Pompe disease (GSD type II) is caused by a deficiency in lysosomal α-glucosidase, leading to glycogen accumulation in the lysosomes and muscle weakness
• Lactate acidosis is a condition characterized by elevated blood lactate levels (>2 mmol/L) and decreased blood pH (<7.35), often resulting from impaired oxidative phosphorylation or excessive glycolysis
  • Causes include hypoxia, sepsis, liver failure, and certain medications (metformin, nucleoside reverse transcriptase inhibitors)
• Fructose intolerance is a group of disorders caused by defects in enzymes involved in fructose metabolism, leading to the accumulation of fructose-1-phosphate and depletion of ATP
  • Hereditary fructose intolerance is caused by a deficiency in aldolase B, resulting in hypoglycemia, liver dysfunction, and kidney damage after fructose ingestion
• Galactosemia is an inherited disorder caused by defects in enzymes involved in galactose metabolism, leading to the accumulation of galactose and its metabolites
  • Classic galactosemia is caused by a deficiency in galactose-1-phosphate uridylyltransferase (GALT), resulting in cataracts, liver dysfunction, and intellectual disability if untreated
• Nutritional management, such as low-carbohydrate diets (ketogenic diet) and glucose or galactose restriction, can be effective in managing some of these disorders