Carbohydrates and Glycolysis

• Carbohydrates are organic compounds containing carbon, hydrogen, and oxygen, often in a 1:2:1 ratio, hence the general formula (CH₂O)ₙ.
• They are the most abundant biomolecules on Earth and serve various functions, including energy storage and structural components.
• Monosaccharides, such as glucose and fructose, are the simplest carbohydrates and cannot be further hydrolyzed into smaller units.
• Disaccharides, like sucrose and lactose, consist of two monosaccharides linked by a glycosidic bond.
• Polysaccharides, such as starch and cellulose, are polymers composed of many monosaccharide units.

Glycolysis Overview

• Glycolysis is a metabolic pathway that converts glucose into pyruvate or lactate, producing ATP and NADH.
• It occurs in the cytoplasm of cells and does not require oxygen (anaerobic).
• The glycolytic pathway consists of ten enzymatic reactions.
• Glycolysis can be divided into two phases: the energy investment phase and the energy payoff phase.

Energy Investment Phase

• This phase consumes ATP to phosphorylate glucose, preparing it for subsequent reactions.
• Glucose is phosphorylated to glucose-6-phosphate by hexokinase (or glucokinase in the liver), using one ATP.
• Glucose-6-phosphate is then isomerized to fructose-6-phosphate by phosphoglucose isomerase.
• Fructose-6-phosphate is phosphorylated to fructose-1,6-bisphosphate by phosphofructokinase-1 (PFK-1), using another ATP. This is a key regulatory step.
• Fructose-1,6-bisphosphate is cleaved into two three-carbon molecules: glyceraldehyde-3-phosphate (GAP) and dihydroxyacetone phosphate (DHAP), by aldolase.
• DHAP is isomerized to GAP by triose phosphate isomerase, ensuring that both products of the aldolase reaction can proceed through the second half of glycolysis.
• For each molecule of glucose, two molecules of glyceraldehyde-3-phosphate are formed at the end of the investment phase

Energy Payoff Phase

• This phase generates ATP and NADH.
• Glyceraldehyde-3-phosphate is oxidized and phosphorylated by glyceraldehyde-3-phosphate dehydrogenase, producing 1,3-bisphosphoglycerate and NADH.
• 1,3-bisphosphoglycerate transfers its high-energy phosphate to ADP, forming ATP and 3-phosphoglycerate, catalyzed by phosphoglycerate kinase (substrate-level phosphorylation).
• 3-phosphoglycerate is isomerized to 2-phosphoglycerate by phosphoglycerate mutase.
• 2-phosphoglycerate is dehydrated to phosphoenolpyruvate (PEP) by enolase.
• PEP transfers its high-energy phosphate to ADP, forming ATP and pyruvate, catalyzed by pyruvate kinase (substrate-level phosphorylation).

Net ATP Production

• Glycolysis produces a net of 2 ATP molecules per glucose molecule.
• Two ATP are consumed in the energy investment phase.
• Four ATP are produced in the energy payoff phase.
• Glycolysis also generates 2 NADH molecules per glucose molecule.

Regulation of Glycolysis

• Glycolysis is tightly regulated to meet the energy needs of the cell.
• The key regulatory enzymes are hexokinase (or glucokinase), phosphofructokinase-1 (PFK-1), and pyruvate kinase.
• Hexokinase is inhibited by its product, glucose-6-phosphate.
• Glucokinase is only active at high glucose concentrations, and it is not inhibited by glucose-6-phosphate.
• PFK-1 is the most important regulatory enzyme in glycolysis.
• PFK-1 is allosterically activated by AMP and fructose-2,6-bisphosphate and inhibited by ATP and citrate.
• Pyruvate kinase is activated by fructose-1,6-bisphosphate and inhibited by ATP and alanine.

Fate of Pyruvate

• The fate of pyruvate depends on the presence or absence of oxygen.
• Under aerobic conditions, pyruvate is transported into the mitochondria and converted to acetyl-CoA, which enters the citric acid cycle.
• Under anaerobic conditions, pyruvate is converted to lactate by lactate dehydrogenase, regenerating NAD+ for glycolysis to continue.
• In some microorganisms, pyruvate can be converted to ethanol and carbon dioxide in a process called fermentation.
• The conversion of pyruvate to lactate or ethanol regenerates NAD+, which is essential for glycolysis to continue under anaerobic conditions.

Clinical Significance

• Glycolysis is important in red blood cells, which lack mitochondria and rely solely on glycolysis for ATP production.
• Cancer cells often have high rates of glycolysis, even in the presence of oxygen (Warburg effect).
• Genetic defects in glycolytic enzymes can cause various diseases, such as hemolytic anemia (e.g., pyruvate kinase deficiency).
• Understanding glycolysis is important for understanding metabolic disorders and developing new therapies.
• Glycolysis provides precursors for other metabolic pathways, such as the pentose phosphate pathway and amino acid synthesis.