Carbohydrate Metabolism Overview

CHO Metabolism

• CHO metabolism encompasses the fate of food molecules after digestion and absorption.
• It involves chemical enzymatic reactions within the body focused on building (synthesis) and breaking down (breakdown) various substances.

Metabolic Pathways

• Anabolic pathways: These build complex molecules from simpler ones. This process requires energy (endergonic). Protein synthesis is an example.
• Catabolic pathways: These break complex molecules into simpler ones. This process releases energy (exergonic). Oxidative processes are examples.
• Amphibolic pathways: These act as links between anabolic and catabolic pathways, serving as common metabolic routes. The citric acid cycle is an example.

Carbohydrate Metabolism

• Carbohydrates provide 50% of daily calories.
• Complete oxidation of 1 gram of carbohydrates yields 4 kcal.
• Key carbohydrate sources in food include starch (around 50% of dietary carbohydrates, e.g., potatoes), sucrose and lactose, and fructose and glucose (from fruits and honey).

Digestion of CHO

• Polysaccharides and disaccharides must be broken down into monosaccharides for absorption.
• Enzymes involved in CHO digestion include:
  • Salivary amylase (converts starch and glycogen into dextrins)
  • Pancreatic amylase (converts dextrins into maltose)
  • Intestinal disaccharidases (e.g., maltase, sucrase, lactase) further break down disaccharides into monosaccharides (glucose, fructose, galactose).

Lactose Intolerance

• Definition: A condition where the body lacks the lactase enzyme.
• Cause: Deficiency of Intestinal lactase.
• Effects: Undigested lactose accumulates in the intestine, leading to fermentation by intestinal bacteria and producing acids and gases.
• Symptoms: Abdominal distension, abdominal cramps, and diarrhea.
• Treatment: Lactose-free milk formula.

Metabolic Pathways of Carbohydrates

• Catabolic pathways: These include oxidative pathways like glycolysis, the hexose monophosphate shunt, uronic acid pathway, and glycogenolysis.
• Anabolic pathways: These include gluconeogenesis and glycogenesis.
• Amphibolic pathways: The only amphibolic pathway discussed is the citric acid cycle.

Glycolysis

• Definition: The oxidation of glucose to pyruvate in the presence of oxygen or to lactate in the absence of oxygen.
• Site: Occurs in the cytoplasm of all tissue cells, but particularly important in tissues lacking mitochondria (e.g., red blood cells, cornea, lens) or experiencing frequent oxygen deprivation (e.g., skeletal muscles during exercise).
• Stages:
  • Stage 1 (energy requiring): Glucose is converted to 2 molecules of glyceraldehyde-3-phosphate, requiring energy input.
  • Stage 2 (energy producing): The 2 glyceraldehyde-3-phosphate molecules are converted to pyruvate (or lactate) producing energy.

Importance of Glycolysis

• Energy production: Anaerobic glycolysis yields 2 ATP, while aerobic glycolysis gives 6-8 ATP.
  • Provides a major pathway for the metabolism of fructose and galactose.
  • Crucial for good oxygenation of tissues through the formation of 2,3-BPG, altering hemoglobin's oxygen affinity.
  • Provides intermediates for other metabolic pathways (e.g., dihydroxyacetone phosphate for lipogenesis, pyruvate for alanine synthesis).

Energy Production in Glycolysis

• Aerobic: Generating 8-10 ATP through substrate-level phosphorylation and oxidative phosphorylation.
• Anaerobic: Generating 2 ATP through substrate-level phosphorylation.

Glycolysis in RBCs

• Mature red blood cells (RBCs) lack mitochondria, so they rely solely on glycolysis for energy.
• The end product is lactate, and net energy produced is 2 ATP.
• Uptake of glucose by RBCs doesn't rely on insulin.
• RBCs produce 2,3-biphosphoglycerate (2,3-BPG).

Regulation of Glycolysis

• Key enzymes (irreversible) are: hexokinase/glucokinase, phosphofructokinase-1 (PFK-1), and pyruvate kinase.
• Hormonal regulation: Insulin stimulates the synthesis of these enzymes, while glucagon inhibits it.
• Allosteric regulation: Different metabolites regulate the activity of these enzymes influencing glycolytic activity.

In Vitro Inhibition of Glycolysis

• Inhibitors: Arsenate (competes with inorganic phosphate), Iodoacetate (inhibits glyceraldehyde-3-phosphate dehydrogenase), and Fluoride (inhibits enolase).
• Clinical significance: Hemolytic anemia (RBCs lysis) can stem from defects in glycolytic enzymes like pyruvate kinase.

Additional Notes

• Glycolysis is a fundamental metabolic pathway, crucial for energy production in various tissues and crucial to overall body function
• Substrate-level phosphorylation is also important
• Regulation mechanisms ensure that glycolysis adjusts to the body's metabolic needs.
• These notes provide a general overview of CHO metabolism and glycolysis. It is essential to refer to more detailed texts for a comprehensive understanding.