🧬Biochemistry Unit 12 – Glycolysis and Gluconeogenesis

Key Concepts and Definitions

• Glycolysis breaks down glucose into pyruvate through a series of enzymatic reactions
• Gluconeogenesis synthesizes glucose from non-carbohydrate precursors (lactate, amino acids, glycerol)
• ATP (adenosine triphosphate) is the primary energy currency in cells
• NAD+ (nicotinamide adenine dinucleotide) is a coenzyme that accepts electrons during redox reactions
  • NADH is the reduced form of NAD+
• Enzymes are biological catalysts that lower the activation energy of reactions
  • Enzymes are highly specific to their substrates and reactions
• Substrate-level phosphorylation directly transfers a phosphate group from a substrate to ADP to form ATP
• Oxidative phosphorylation generates ATP through the electron transport chain and chemiosmosis

Glycolysis Overview

• Glycolysis is a 10-step process that occurs in the cytosol of cells
• Glucose is converted into two molecules of pyruvate
• Glycolysis can be divided into two phases:
  • Preparatory phase (steps 1-5) consumes 2 ATP to convert glucose into two triose phosphates
  • Payoff phase (steps 6-10) yields 4 ATP and 2 NADH, resulting in a net gain of 2 ATP and 2 NADH
• Glucose is phosphorylated by hexokinase in step 1, trapping it inside the cell
• Fructose-1,6-bisphosphate is split into two triose phosphates (glyceraldehyde-3-phosphate and dihydroxyacetone phosphate) in step 4
• Glyceraldehyde-3-phosphate dehydrogenase catalyzes the oxidation of glyceraldehyde-3-phosphate and reduction of NAD+ to NADH in step 6
• Pyruvate kinase catalyzes the irreversible conversion of phosphoenolpyruvate to pyruvate in step 10, a key regulatory step

Gluconeogenesis Overview

• Gluconeogenesis is the synthesis of glucose from non-carbohydrate precursors
• Key substrates for gluconeogenesis include lactate, amino acids (alanine, glutamine), and glycerol
• Gluconeogenesis occurs primarily in the liver and to a lesser extent in the kidneys
• Many steps in gluconeogenesis are reversals of glycolysis reactions
• Gluconeogenesis requires energy input in the form of ATP and GTP (guanosine triphosphate)
• Three irreversible steps in glycolysis are bypassed by alternative enzymes in gluconeogenesis:
  • Pyruvate carboxylase and phosphoenolpyruvate carboxykinase (PEPCK) bypass pyruvate kinase
  • Fructose-1,6-bisphosphatase bypasses phosphofructokinase-1
  • Glucose-6-phosphatase bypasses hexokinase
• Gluconeogenesis is important for maintaining blood glucose levels during fasting or prolonged exercise

Enzymes and Reactions

• Hexokinase phosphorylates glucose to glucose-6-phosphate in glycolysis step 1
  • Glucokinase, a liver-specific hexokinase, has a lower affinity for glucose and is not inhibited by glucose-6-phosphate
• Phosphofructokinase-1 (PFK-1) catalyzes the irreversible phosphorylation of fructose-6-phosphate to fructose-1,6-bisphosphate in glycolysis step 3
  • PFK-1 is a key regulatory enzyme in glycolysis and is allosterically inhibited by ATP and citrate
• Aldolase cleaves fructose-1,6-bisphosphate into glyceraldehyde-3-phosphate and dihydroxyacetone phosphate in glycolysis step 4
• Triose phosphate isomerase interconverts glyceraldehyde-3-phosphate and dihydroxyacetone phosphate in glycolysis step 5
• Pyruvate carboxylase catalyzes the carboxylation of pyruvate to oxaloacetate in gluconeogenesis
  • This reaction requires biotin as a coenzyme and ATP
• PEPCK catalyzes the decarboxylation and phosphorylation of oxaloacetate to phosphoenolpyruvate in gluconeogenesis
• Fructose-1,6-bisphosphatase hydrolyzes fructose-1,6-bisphosphate to fructose-6-phosphate in gluconeogenesis
• Glucose-6-phosphatase hydrolyzes glucose-6-phosphate to glucose in gluconeogenesis, allowing glucose to be released into the bloodstream

Energy and Regulation

• Glycolysis has a net yield of 2 ATP and 2 NADH per glucose molecule
  • ATP is generated through substrate-level phosphorylation in steps 7 and 10
  • NADH is produced in step 6 and can be oxidized in the electron transport chain to yield additional ATP
• Gluconeogenesis requires 6 ATP and 2 GTP per glucose molecule synthesized
• Glycolysis and gluconeogenesis are reciprocally regulated to prevent futile cycles
• Fructose-2,6-bisphosphate is a key allosteric regulator of glycolysis and gluconeogenesis
  • High levels of fructose-2,6-bisphosphate stimulate glycolysis by activating PFK-1 and inhibit gluconeogenesis by inhibiting fructose-1,6-bisphosphatase
• Hormones such as insulin, glucagon, and cortisol regulate glycolysis and gluconeogenesis
  • Insulin stimulates glycolysis and inhibits gluconeogenesis in the fed state
  • Glucagon and cortisol stimulate gluconeogenesis and inhibit glycolysis during fasting or stress
• AMP-activated protein kinase (AMPK) is a cellular energy sensor that responds to changes in AMP/ATP ratio
  • AMPK activation stimulates catabolic pathways (glycolysis) and inhibits anabolic pathways (gluconeogenesis) to restore energy balance

Metabolic Pathways and Integration

• Glycolysis is a central metabolic pathway that feeds into other pathways (citric acid cycle, pentose phosphate pathway, lipid synthesis)
• Pyruvate from glycolysis can be oxidized in the citric acid cycle or converted to lactate under anaerobic conditions
• Gluconeogenesis is interconnected with other metabolic pathways:
  • Lactate from anaerobic glycolysis can be converted back to glucose in the Cori cycle
  • Amino acids from protein breakdown can be converted to glucose via glucogenic amino acids
  • Glycerol from lipolysis can be converted to glucose
• Pentose phosphate pathway generates NADPH and ribose-5-phosphate for biosynthetic reactions
  • Glucose-6-phosphate from glycolysis can enter the pentose phosphate pathway
• Glycerol-3-phosphate from glycolysis can be used for triglyceride synthesis
• Citrate from the citric acid cycle can be exported to the cytosol for fatty acid synthesis

Clinical Significance

• Inborn errors of metabolism can affect enzymes in glycolysis and gluconeogenesis
  • Pyruvate kinase deficiency leads to hemolytic anemia due to reduced ATP production in red blood cells
  • Von Gierke disease (glucose-6-phosphatase deficiency) results in severe hypoglycemia and hepatomegaly
• Diabetes mellitus is characterized by impaired glucose utilization and increased gluconeogenesis
  • In type 1 diabetes, insulin deficiency leads to uncontrolled gluconeogenesis and hyperglycemia
  • In type 2 diabetes, insulin resistance and relative insulin deficiency result in increased gluconeogenesis and hyperglycemia
• Warburg effect describes the increased reliance on glycolysis for ATP production in cancer cells, even in the presence of oxygen
  • Many cancer cells exhibit upregulated glycolytic enzymes and increased lactate production
• Lactic acidosis can occur due to increased anaerobic glycolysis and lactate production
  • Causes include hypoxia, sepsis, and certain medications (metformin, nucleoside reverse transcriptase inhibitors)
• Hypoglycemia can result from impaired gluconeogenesis or excessive insulin action
  • Causes include liver disease, alcohol abuse, insulin overdose, and certain medications (sulfonylureas)

Study Tips and Exam Prep

• Create a visual map or flowchart of glycolysis and gluconeogenesis to understand the sequence of reactions and key enzymes involved
• Use mnemonics to remember the order of glycolysis steps (e.g., "Goodness Gracious Father Franklin, Pray Pray Pray, Seek Seek Seek")
• Practice drawing the structures of key intermediates in glycolysis and gluconeogenesis
• Understand the role of ATP, NAD+/NADH, and other cofactors in each reaction
• Compare and contrast the regulation of glycolysis and gluconeogenesis by key allosteric effectors and hormones
• Integrate knowledge of glycolysis and gluconeogenesis with other metabolic pathways (citric acid cycle, pentose phosphate pathway, lipid metabolism)
• Review clinical examples and case studies to understand the relevance of glycolysis and gluconeogenesis in health and disease
• Solve practice problems involving energy calculations, enzyme kinetics, and regulation of metabolic pathways
• Participate in study groups or discussions to reinforce concepts and clarify any questions
• Utilize online resources (videos, animations, quizzes) to supplement textbook and lecture material