DIETARY CARBOHYDRATE

• Definition: Dietary carbohydrates are the sugars and starches found in foods.

Digestion and Absorption of Carbohydrates

• Mouth:

  • Salivary α-amylase begins the digestion of carbohydrates.

• Stomach:

  • Continues to act on polysaccharides, dextrins, sucrose, lactose, and maltose.

• Small Intestine:

  • Intestinal lining further breaks down carbohydrates.
  • Pancreatic a-amylase acts to convert carbohydrates into monosaccharides.
  • Final products are monosaccharides: glucose, galactose, and fructose.
  • Active Transport: Monosaccharides are absorbed into the bloodstream through active transport mechanisms.

Absorption Process

• INTESINAL LUMEN: Absorption occurs here for glucose and fructose.

  • Glucose Transport:
  • Uses SGLT1 (Sodium-glucose transport protein 1) for entry into enterocytes.
  • Sodium (Na+) is involved in the transport process.
  • GLUT2 transporter facilitates the glucose transfer into systemic circulation.

• FRUCTOSE Transport:

  • Facilitated by GLUT2.

• Liver Function: The liver acts as a buffer for glucose levels in the bloodstream.

Glycolysis Overview

• Glycolysis: Metabolic pathway converting glucose to pyruvate, essential for energy production.

Glycolysis Steps

1. Hexokinase (HK) - Phosphorylation of glucose to form Glucose-6-phosphate (G6P).
2. Phosphoglucose isomerase (PG) - Isomerization converting G6P to Fructose-6-phosphate (F6P) with ATP.
3. Phosphofructokinase (PFK) - Phosphorylates F6P to Fructose-1,6-bisphosphate (FBP).
4. Aldolase - Cleavage of FBP into Dihydroxyacetone phosphate (DHAP) and Glyceraldehyde-3-phosphate (GAP).
5. Triose phosphate isomerase (TIM) - Converts DHAP into GAP.
6. Glyceraldehyde-3-phosphate dehydrogenase (GAPDH) - Reduction and phosphorylation yielding 1,3-Bisphosphoglycerate (1,3-BPG).
7. Phosphoglycerate kinase (PGK) - Substrate-level phosphorylation producing ATP and 3-Phosphoglycerate (3PG).
8. Phosphoglycerate mutase (PGM) - Isomerization of 3PG to 2-Phosphoglycerate (2PG).
9. Enolase - Dehydration of 2PG to form Phosphoenolpyruvate (PEP).
10. Pyruvate Kinase (PK) - Final substrate-level phosphorylation yielding Pyruvate and ATP.

Energy Yield

• Net Products from one glucose molecule:
  1. 2 x ATP (4 produced, 2 consumed during glycolysis).
  2. 2 x NADH.
  3. 2 x Pyruvate.

Fermentation Pathways

Anaerobic Conditions

• Anaerobic Homolactic Fermentation:

  • Pyruvate is converted to lactate.
  • NADH is oxidized back to NAD+.
  • Yields: 2 Lactate from 2 NADH.

• Anaerobic Alcoholic Fermentation:

  • Pyruvate is first converted to acetaldehyde & CO2; acetaldehyde is then reduced to ethanol.
  • Yields: 2 ethanol and 2 CO2 from 2 NADH.

Importance of Glycolysis in Red Blood Cells (RBCs)

• 2,3-Bisphosphoglycerate (2,3-BPG):
  • Plays a crucial role in regulating oxygen delivery by decreasing hemoglobin's affinity for oxygen.

Entry of Other Monosaccharides into Glycolytic Pathway

• Maltase:
  • Hydrolyzes Maltose + H2O to produce 2 D-Glucose.
• Lactase:
  • Hydrolyzes Lactose + H2O to give D-Galactose and D-Glucose.
• Sucrase:
  • Hydrolyzes Sucrose + H2O to yield D-Fructose and D-Glucose.
• Galactose Metabolism:
  • Galactose is converted into Glucose-6-phosphate and enters the glycolytic pathway.

Galactose Metabolism and Galactosemia

Key Enzymatic Steps

1. Hexokinase:
   • Catalyzes the phosphorylation of Galactose producing Galactose-1-phosphate.
2. Galactose-1-Phosphate Uridylyltransferase:
   • Converts Galactose-1-phosphate into Glucose-1-phosphate.
3. Aldose Reductase:
   • Converts excess Galactose into Galactitol, depleting NADPH leading to cataracts.

Implications of Galactose Accumulation

• Accumulation can lead to tissue problems, particularly in the lens of the eye leading to cataracts.
• Galactokinase Deficiency:
  • Results in cataracts without affecting other tissues.
• Galactose-1-phosphate Uridylyltransferase Deficiency:
  • Causes severe effects leading to neonatal milk intolerance, damaging the liver, kidneys, brain, and spleen.

Transition Step

• Catalyzed by the Pyruvate Dehydrogenase (PDH) Complex:
  • Contains:
    i. Pyruvate dehydrogenase
    ii. Dihydrolipoyl transacetylase
    iii. Dihydrolipoyl dehydrogenase
• Products:
  • From 1 Pyruvate:
  1. 1 x CO2
  2. 1 x NADH
  3. 1 x Acetyl CoA

Tricarboxylic Acid (TCA) Cycle

• Cycle Products:
  • From 2 Acetyl CoA, the cycle yields:
  1. 4 x CO2
  2. 6 x NADH
  3. 2 x FADH2
  4. 2 x GTP
• Amphibolic Nature:
  • The TCA cycle is involved in both catabolic (breakdown) and anabolic (biosynthesis) processes.
• Important for the formation of intermediates that are utilized in both anabolic and catabolic pathways.

Overall ATP Yield from Glucose

• ATP Production Summary:
  • Glycolysis:
  • 2 ATP + 4 (from 2 NADH)
  • Transition Step:
  • 6 ATP (from 2 NADH)
  • TCA Cycle:
  • 2 ATP (from GTP) + 18 ATP (from 6 NADH) + 4 ATP (from 2 FADH2)
• Total ATP production from one glucose molecule = 36 ATP.