Feeder Pathways for Glycolysis

• Endogenous glycogen starch

  1. Phosphorylase phosphorylates it to glucose 1-phosphate
  2. Glucose 1-phosphate is converted to glucose 6-phosphate through phosphoglucomutase
  3. Glucose 6-phosphate is converted to fructose 6-phosphate
  4. Fructose 5-phosphate is converted to fructose 1,6-bisphosphate
  5. Fructose 1,6-bisphosphate is converted DHAP or glyceraldehyde 3-phoshate by fructose 1,6-bisphosphate aldolase
  6. DHAP is converted to glyceraldehyde 3-phosphate by triose phosphate isomerase.
• Lactose

  1. Lactose is converted to glucose or galactose by lactase.
  2. Galactose is converted to glucose 1-phosphate using ATP and continues down that pathway to eventually make glyceraldehyde 3-phosphate.
  3. Glucose is converted to glucose 6-phosphate by hexokinase and ATP and continues down that pathway to eventually make glyceraldehyde 3-phosphate.
• Dietary glycogen starch

  1. Starch is converted to glucose by alpha-amylase and water. It then continues down the rest of the pathway to make glyceraldehyde 3-phosphate.
• Trehalose

  1. Trehalose is converted to glucose by trehalase. It then continues down the rest of the pathway to make glyceraldehyde 3-phosphate.
• Sucrose

  1. Sucrose is converted to glucose and fructose by sucrase.
  2. Glucose is converted to glucose 6-phosphate by hexokinase and ATP. It then continues down the rest of the pathway to glyceraldehyde 3-phosphate.
  3. Fructose is converted to fructose 6-phosphate by hexokinase and ATP. This then continues down the pathway to make glyceraldehyde 3-phosphate.
  4. Fructose can also be converted to fructose 1-phosphate by fructokinase and ATP.
  5. Fructose 1-phosphate is converted to DHAP and glyceraldehyde by fructose 1-phosphate aldolase.
• Sucrose, part 2

  6. DHAP is converted to glyceraldehyde 3-phosphate by triose phosphate isomerase

  7. Glyceraldehyde is converted to glyceraldehyde 3-phosphate by triose kinase and ATP.
• Mannose

  1. Mannose is converted to mannose 6-phosphate by hexokinase and ATP.
  2. Mannose 6-phosphate is converted to fructose 6-phosphate by phosphomannose isomerase. This then continues down the pathway to make glyceraldehyde 3-phosphate.
• Endogenous glycogen and starch are degraded by phosphorolysis.
• Glycogen phosphorylase mobilizes glycogen stored in animal tissues and microorganisms by a phosphorolytic reaction to yield glucose 1-phosphate.
• Starch phosphorylase mobilizes starch by a phosphoryltic reaction.
• Phosphoglycomutase catalyzes the reversible reaction of glucose 1-phosphate to glucose 6-phosphate.
• Glucose 6-phosphate can continue through glycolysis or enter another pathway.
• Mutase is an enzyme that catalyzes the transfer of a functional group from one position to another in the same molecule. It is a subclass of isomerases.
• Isomerases are enzymes that interconvert stereoisomers or structural or positional isomers.
• Alpha-amylase is a salivary and small intestine enzyme that hydrolyzes the internal (alpha 1-->4) glycosidic linkages of starch and glycogen, producing di and trisaccharides.
• Pancreatic alpha-amylase yields mainly maltose, maltotriose, and limit dextrins.
• Dextrins are fragments of amylopectin containing (alpha 1-->6) branch points, which are removed by limit dextrinases.
• Dietary glycogen

  • effectively produces an extra ATP when its glucose goes through glycolysis
  • is broken down to glucose by a different group of enzymes than is dietary starch
  • is broken down to glucose 6-phosphate, which is then absorbed by intestinal cells
  • Is broken down in the intestines to limit dextrins, which are then degraded to glucose.


  is broken down in the intestine to limit dextrins, which are then degraded to glucose.
• Membrane bound hydrolases in the intestinal brush border hydrolyze disaccharides.
• Dextrin + nH2O --(dextrinase)-> n glucose
• Maltose + H2O --(maltase)-> 2 glucose
• Lactose + H2O --(lactose)-> galactose + glucose
• Sucrose + H2O --(sucrase)-> fructose + glucose
• Trehalose + H2O --(trehalase)-> 2 glucose
• Monosaccharides pass through intestinal cells to the bloodstream, which transports them to the liver or other tissues.
• Cellulase attacks the (beta 1-->4) glycosidic bonds of cellulose.

  • Absent in most animals
  • Microorganisms produce it
• Lactase converts lactose to glucose and galactose. It is present in infants, but is often absent in adults, producing lactose intolerance.
• Lactose persistence phenotype means there is the production of lactase into adulthood.
• Lactose intolerance is the inability to digest lactose due to the disappearance of lactase in adulthood. It causes abdominal cramps and diarrhea.
• Galactose is a product of lactose hydrolysis and is an important component in the infant diet.
• Galactokinase uses ATP to phosphorylate galactose at C-1.

  • Galactose + ATP --(Mg2+)-> galactose 1-phosphate + ADP
• The conversion of galactose to glucose 1-phosphate proceeds through the UDP galactose and UDP glucose intermediates.
• Galactosemia is caused by a genetic defect in enzymes on the galactose to glucose 1-phosphate pathway. Treatment involves carefully controlling dietary galactose.
• Galactose to glucose 1-phosphate

  1. Galactose is converted to galactose 1-phosphate by galactokinase, ATP, and Mg2+
  2. Galactose 1-phosphate is converted to UDP-galactose by UDP-glucose galactose 1-phosphate uridylyltransferase
  3. UDP-galactose is converted to UDP-glucose by UDP-glucose 4-epimerase, and 2 NAD+.
  4. UDP-glucose is converted to glucose 1-phosphate
• Which statement about carbohydrate metabolism is false?

  • The overall strategy for metabolizing the common monosaccharides is to convert them into glycolytic intermediates.
  • Galactose metabolism requires a nucleotide
  • Mannose is phosphorylated by hexokinase
  • Galactosemia results from defective or absent lactase in the intestine


  Galactosemia results from a defective or absent lactase in the intestine. Instead, it results from a defect in any of the enzymes in the pathway responsible for converting galactose to glucose 1-phosphate. Defective/absent intestinal lactase causes lactose intolerance.
• Fructose and mannose can be phosphorylated and funneled into glcolysis.
• Hexokinase phosphorylated fructose in the small intestine

  • Fructose + ATP --(Mg2+)-> fructose 6-phosphate + ADP
• Fructose 1-phosphate aldolase phosphorylates fructose in the liver

  • Fructose + ATP --(Mg2+)-> fructose 1-phosphate + ADP
• Fructose 1-phosphate aldolase cleaves fructose 1-phosphate to glyceraldehyde and dihydroxyacetone
• The products of fructose 1-phosphate hydrolysis enter glycolysis as glyceraldehyde 3-phosphate.