Glycolysis and Carbohydrate Digestion Quiz

Glycolysis Overview

• Glycolysis is a cytoplasmic pathway converting glucose to two pyruvates
• Releases energy in two substrate-level phosphorylation and one oxidation reaction
• Aerobic glycolysis occurs in cells with mitochondria and oxygen
• Anaerobic glycolysis occurs in the absence of mitochondria or oxygen (e.g., erythrocytes, exercising muscle)

Carbohydrate Digestion

• Salivary amylase hydrolyzes starch polymers to dextrin
• Acid in the stomach destroys salivary amylase
• Intestinal enzymes (maltase, isomaltase, lactase, sucrase) further break down disaccharides to monosaccharides
• Monosaccharides are absorbed by sodium/glucose transporters in mucosal cells

Glucose Transport

• Four glucose transporters (GLUTs) exist with different affinities for glucose
• GLUT 1 & 3 have high affinity for basal glucose uptake in tissues like brain, nerves, and red blood cells
• GLUT 2 is a low-affinity transporter in hepatocytes, initially capturing excess glucose for storage
• GLUT 4 is in adipose tissue and muscle and responds to blood glucose concentration; insulin increases its membrane presence

Glycolysis (Step 1): Hexokinase/Glucokinase

• Trapping glucose in cells via phosphorylation using ATP
• Hexokinase: widely distributed in tissues. Inhibited by glucose-6-phosphate
• Glucokinase: only in hepatocytes and pancreatic β-islet cells. Induced by insulin

Glycolysis (Step 2): Phosphofructokinase (PFK-1 and PFK-2)

• Rate-limiting enzyme in glycolysis
• Fructose 6-phosphate phosphorylated to fructose 1,6-bisphosphate using ATP
• Activated by AMP
• Inhibited by ATP and citrate
• Insulin activates PFK-2, increasing fructose 2,6-bisphosphate (F2,6-BP), which activates PFK-1
• Glucagon inhibits PFK-2, decreasing F2,6-BP and inhibiting PFK-1

Glycolysis (Step 3): Glyceraldehyde 3-phosphate Dehydrogenase

• Catalyzes oxidation and addition of inorganic phosphate to its substrate
• Produces a high-energy intermediate 1,3-bisphosphoglycerate
• NADH is produced
• Aerobic: NADH reoxidized in the mitochondria
• Anaerobic: NADH reoxidized by reducing pyruvate to lactate

Glycolysis (Step 4): 3-Phosphoglycerate Kinase

• Transfers a phosphate from 1,3-bisphosphoglycerate to ADP, forming ATP and 3-phosphoglycerate
• Substrate-level phosphorylation; does not need oxygen

Glycolysis (Step 5): Pyruvate Kinase

• Last enzyme in aerobic glycolysis
• Catalyzes substrate-level phosphorylation of ADP using phosphoenolpyruvate (PEP) to pyruvate.
• Activated by fructose 1,6-bisphosphate (feed-forward activation)

Glycolysis (Step 6): Lactate Dehydrogenase

• Only used in anaerobic glycolysis
• Reoxidizes NADH to NAD, preventing glycolysis from halting
• Lactate dehydrogenase reduces pyruvate to lactate

Pyruvate Kinase Deficiency

• Second most common genetic deficiency causing hemolytic anemia
• Characterized by chronic hemolysis, decreased HbA's oxygen affinity, absence of Heinz bodies
• Loss of ion balance and osmotic fragility leads to swelling and lysis

Galactose Metabolism

• Galactose is transported to the liver and phosphorylated
• Galactose-1-phosphate is converted to glucose-1-phosphate
• Important enzymes: galactokinase, and galactose 1-phosphate uridyltransferase

Galactosemia

• Autosomal recessive trait
• Defective gene encoding galactokinase or galactose 1-phosphate uridyltransferase
• Symptoms include cataracts, jaundice, and hyperbilirubinemia
• Can result in decreased glucose synthesis and hypoglycemia

Lactose Intolerance

• Deficiency of lactase (enzyme responsible for digesting lactose)
• Symptoms include bloating, diarrhea, and cramps after lactose ingestion
• Treated with dietary restriction of lactose or lactase pills

Fructose Metabolism

• Fructose is found in fruit, honey, and sucrose
• Phosphorylated to fructose-1-phosphate
• Cleaved into glyceraldehyde and DHAP
• Deficiency of fructokinase is generally benign
• Deficiency of fructose-1-phosphate aldolase (aldolase B) (Hereditary fructose intolerance) is severe

Pyruvate Dehydrogenase Complex

• Catalyzes the conversion of pyruvate to acetyl-CoA
• Essential for aerobic metabolism
• Cofactors and coenzymes: TPP, lipoic acid, CoA, FAD, NAD

Test your knowledge on glycolysis, carbohydrate digestion, and glucose transport mechanisms. This quiz covers cytoplasmic pathways, enzyme functions, and glucose transporter characteristics. Perfect for students studying biochemistry or cellular metabolism.