Monosaccharides Classification Quiz

Monosaccharides

• Classified by the number of carbon atoms:
  • Trioses (3 carbons)
  • Tetroses (4 carbons)
  • Pentoses (5 carbons, e.g., ribose)
  • Hexoses (6 carbons, e.g., glucose, fructose)
• Functional groups:
  • Aldoses (contain an aldehyde group, e.g., glucose)
  • Ketoses (contain a ketone group, e.g., fructose)
• Exhibit chirality, allowing existence in different stereoisomers due to asymmetric carbons.

D- and L-Isomers

• Classified based on the orientation of the hydroxyl group (-OH) on the farthest asymmetric carbon from the carbonyl group.
• Most naturally occurring sugars are in D-form.

Cyclization of Monosaccharides

• In aqueous solutions, monosaccharides like glucose and fructose can cyclize:
  • Aldoses form hemiacetals, often creating six-membered rings (pyranose).
  • Ketoses form hemiketals, typically resulting in five-membered rings (furanose).
• Cyclization introduces an anomeric carbon with two configurations:
  • Alpha (α) if the hydroxyl group on the anomeric carbon is down.
  • Beta (β) if it is up.

Disaccharides

• Comprised of two monosaccharides linked by a glycosidic bond:
  • Sucrose (glucose + fructose): Table sugar, α(1→2) glycosidic bond.
  • Lactose (galactose + glucose): Found in milk, β(1→4) glycosidic bond.
  • Maltose (glucose + glucose): Product of starch digestion, α(1→4) glycosidic bond.

Polysaccharides

• Long chains of monosaccharides linked by glycosidic bonds.
• Types include:
  • Homopolysaccharides (one type of monosaccharide)
  • Heteropolysaccharides (more than one type)

Pyruvate Dehydrogenase Complex (PDC)

• Converts pyruvate from glycolysis to acetyl-CoA before entering the TCA cycle.
• Involves:
  • Decarboxylation: Removal of carbon from pyruvate, releasing CO₂.
  • Oxidation: Electrons transferred to NAD⁺, forming NADH.
  • Formation of acetyl-CoA by attaching the remaining two-carbon unit to coenzyme A.
• Composed of three enzymes and requires five cofactors: TPP, lipoic acid, CoA, FAD, and NAD⁺.

Regulation of PDC

• Inhibited by phosphorylation when energy levels are high (high ATP, NADH, and acetyl-CoA).
• Activated by dephosphorylation when energy levels are low (high ADP and pyruvate).

TCA Cycle

• A series of eight reactions that oxidize acetyl-CoA to CO₂, generating energy molecules (NADH, FADH₂, GTP).
• Key steps include:
  • Citrate formation from acetyl-CoA and oxaloacetate (catalyzed by citrate synthase).
  • Sequential transformations producing isocitrate, α-ketoglutarate, succinyl-CoA, succinate, fumarate, and malate.

Gluconeogenesis

• Energy-intensive process to synthesize glucose from two pyruvate molecules, requiring:
  • 4 ATP, 2 GTP, 2 NADH.
• Regulated to prevent simultaneous activation of glycolysis:
  • Hormonal control by insulin (inhibits), glucagon (stimulates), and cortisol (stimulates).

Glycogenolysis

• Breakdown of glycogen into glucose.
• Key steps:
  • Glycogen phosphorylase cleaves glucose units into glucose-1-phosphate.
  • Conversion of glucose-1-phosphate to glucose-6-phosphate by phosphoglucomutase.
  • In the liver, glucose-6-phosphatase converts glucose-6-phosphate to free glucose.

Regulation of Glycogen Metabolism

• Insulin promotes glycogenesis and inhibits glycogenolysis; glucagon stimulates glycogenolysis and inhibits glycogenesis.
• Allosteric regulation via glucose-6-phosphate and AMP for glycogen phosphorylase and synthase.

Glycogen Storage Diseases

• Genetic disorders due to enzyme deficiencies affecting glycogen metabolism.
• Example: von Gierke Disease, a deficiency of glucose-6-phosphatase, causing hypoglycemia and liver glycogen accumulation.

Pentose Phosphate Pathway

• Involves interconversion of ribose-5-phosphate using transketolase and transaldolase, linking to glycolysis.
• Produces NADPH for biosynthesis and detoxification, affecting cellular function and metabolic balance.

Clinical Relevance of the Pentose Phosphate Pathway

• G6PD deficiency leads to reduced NADPH, increasing oxidative stress risk.
• Tumor cells may exhibit increased PPP activity for rapid growth support.
• Altered PPP activity can influence glucose metabolism and complications in diabetes.