Glycolysis: Metabolic Pathway

• Glycolysis is a metabolic pathway that converts glucose into pyruvate or lactate and produces ATP
• It occurs in the cytoplasm of cells
• Glycolysis does not require oxygen, making it an anaerobic process
• It is a fundamental pathway in most organisms for energy production

Phases of Glycolysis

• Glycolysis is divided into two main phases: the energy investment phase and the energy payoff phase

Energy Investment Phase

• This phase consumes ATP to phosphorylate glucose and its intermediates
• Glucose is phosphorylated by hexokinase or glucokinase to form glucose-6-phosphate (G6P), consuming one ATP
• G6P is isomerized to fructose-6-phosphate (F6P) by phosphoglucose isomerase
• F6P is phosphorylated by phosphofructokinase-1 (PFK-1) to form fructose-1,6-bisphosphate (F-1,6-BP), consuming another ATP. PFK-1 is a key regulatory enzyme in glycolysis
• F-1,6-BP is cleaved by aldolase into two 3-carbon molecules: dihydroxyacetone phosphate (DHAP) and glyceraldehyde-3-phosphate (G3P)
• DHAP is isomerized to G3P by triose phosphate isomerase, resulting in two molecules of G3P

Energy Payoff Phase

• This phase produces ATP and NADH
• G3P is oxidized and phosphorylated by glyceraldehyde-3-phosphate dehydrogenase (GAPDH) to form 1,3-bisphosphoglycerate (1,3-BPG). This reaction also reduces NAD+ to NADH
• 1,3-BPG transfers a phosphate group to ADP, forming ATP and 3-phosphoglycerate (3PG) via phosphoglycerate kinase. This is substrate-level phosphorylation
• 3PG is isomerized to 2-phosphoglycerate (2PG) by phosphoglycerate mutase
• 2PG is dehydrated to phosphoenolpyruvate (PEP) by enolase
• PEP transfers a phosphate group to ADP, forming ATP and pyruvate via pyruvate kinase. This is another substrate-level phosphorylation

Net Reaction of Glycolysis

• Glucose + 2 NAD+ + 2 ADP + 2 Pi → 2 Pyruvate + 2 NADH + 2 ATP + 2 H2O + 2 H+
• For each molecule of glucose, glycolysis yields a net gain of 2 ATP molecules, 2 NADH molecules, and 2 pyruvate molecules

Fate of Pyruvate

• Depending on the presence of oxygen, pyruvate can undergo different fates
• Under aerobic conditions, pyruvate is transported into the mitochondria and converted to acetyl-CoA, which enters the citric acid cycle for further oxidation
• Under anaerobic conditions, pyruvate is reduced to lactate by lactate dehydrogenase, regenerating NAD+ for glycolysis to continue. This process is called fermentation
• In yeast, pyruvate can be converted to ethanol and CO2 in a process called alcoholic fermentation

Regulation of Glycolysis

• Glycolysis is tightly regulated to meet the energy needs of the cell
• The key regulatory enzymes are hexokinase, phosphofructokinase-1 (PFK-1), and pyruvate kinase
• Hexokinase is inhibited by glucose-6-phosphate
• PFK-1 is the most important regulatory enzyme. It is allosterically activated by AMP and fructose-2,6-bisphosphate (F-2,6-BP) and inhibited by ATP and citrate
• Pyruvate kinase is activated by fructose-1,6-bisphosphate and inhibited by ATP and alanine

Isozymes of Hexokinase

• There are four isozymes of hexokinase (I-IV)
• Hexokinase I, II, and III have high affinity for glucose and are inhibited by glucose-6-phosphate
• Hexokinase IV (glucokinase) is found in the liver and pancreatic β-cells. It has a lower affinity for glucose and is not inhibited by glucose-6-phosphate. Glucokinase helps regulate blood glucose levels

Energetics of Glycolysis

• The energy investment phase consumes 2 ATP molecules
• The energy payoff phase produces 4 ATP molecules and 2 NADH molecules
• The net ATP production is 2 ATP molecules per glucose molecule
• The 2 NADH molecules can be used in the electron transport chain to produce additional ATP under aerobic conditions

Importance of Glycolysis

• Provides ATP for cellular energy needs
• Generates pyruvate, which can be further oxidized in the citric acid cycle under aerobic conditions
• Generates NADH, which can be used in the electron transport chain to produce ATP under aerobic conditions
• Provides precursors for other metabolic pathways, such as amino acid synthesis and lipid synthesis

Glycolysis in Erythrocytes

• Red blood cells rely solely on glycolysis for ATP production because they lack mitochondria
• In erythrocytes, the first step in glycolysis is Glucose -> Glucose-6-Phosphate, by glucokinase
• A unique step in erythrocytes is the conversion of 1,3-BPG to 2,3-BPG by bisphosphoglycerate mutase
• 2,3-BPG binds to hemoglobin and reduces its affinity for oxygen, facilitating oxygen release in tissues
• 2,3-BPG is converted back to 3-PG

Clinical Significance

• Cancer cells rely heavily on glycolysis for energy, even in the presence of oxygen (Warburg effect)
• Glycolytic enzymes are potential targets for cancer therapy
• Genetic defects in glycolytic enzymes can cause hemolytic anemia due to impaired ATP production in red blood cells
• Understanding glycolysis is crucial for managing metabolic disorders such as diabetes

Anaerobic Glycolysis

• In the absence of oxygen, pyruvate is converted to lactate in a process called anaerobic glycolysis or fermentation
• Lactate dehydrogenase (LDH) catalyzes the reduction of pyruvate to lactate, regenerating NAD+ for glycolysis to continue
• Anaerobic glycolysis allows ATP production to continue when oxygen is limited, such as during intense exercise
• However, anaerobic glycolysis is less efficient than aerobic respiration and leads to the accumulation of lactate, which can cause muscle fatigue and acidosis

Cori Cycle

• The Cori cycle is a metabolic pathway in which lactate produced by anaerobic glycolysis in muscles is transported to the liver
• In the liver, lactate is converted back to glucose through gluconeogenesis, which requires ATP
• The glucose is then released back into the bloodstream and transported back to the muscles
• The Cori cycle helps to recycle lactate and maintain blood glucose levels during intense exercise