Chapter 24

Chapter 24: Carbohydrate Metabolism

Overview of Carbohydrate Metabolism

Carbohydrate metabolism encompasses the biochemical processes responsible for the formation, breakdown, and interconversion of carbohydrates in living organisms. The primary pathways include glycolysis, gluconeogenesis, glycogenolysis, and glycogenesis, which are essential for energy production and storage.

• Glycolysis: The breakdown of glucose to pyruvate, yielding ATP and NADH.

• Gluconeogenesis: The synthesis of glucose from non-carbohydrate precursors.

• Glycogenolysis: The breakdown of glycogen to glucose-1-phosphate and glucose.

• Glycogenesis: The synthesis of glycogen from glucose.

Digestion and Absorption of Carbohydrates

• Primary Site of Digestion: The small intestine is the main site where carbohydrate digestion occurs in the human body.

• Enzymes Involved: Amylases, maltase, sucrase, and lactase break down polysaccharides and disaccharides into monosaccharides.

• Absorption: Monosaccharides are absorbed through the intestinal mucosal cells and transported to the liver via the portal vein.

Glycolysis

Glycolysis is a ten-step metabolic pathway that converts glucose into pyruvate, generating ATP and NADH in the process. It occurs in the cytoplasm of all cells and is anaerobic (does not require oxygen).

• Key Functions:

  • Converts glucose to pyruvate

  • Generates ATP and NADH

  • Provides intermediates for other metabolic pathways

• Net Reaction:

  • $\text{Glucose} + 2\,\text{NAD}^+ + 2\,\text{ADP} + 2\,\text{P}_i \rightarrow 2\,\text{Pyruvate} + 2\,\text{NADH} + 2\,\text{ATP} + 2\,\text{H}_2\text{O} + 2\,\text{H}^+$

• Key Intermediates:

  • Glucose-6-phosphate

  • Fructose-6-phosphate

  • Fructose-1,6-bisphosphate

  • Glyceraldehyde-3-phosphate and dihydroxyacetone phosphate

  • 1,3-Bisphosphoglycerate

  • Phosphoenolpyruvate

  • Pyruvate

• Phosphorylated Intermediates: Most glycolytic intermediates are phosphorylated, which helps retain them within the cell and increases their reactivity.

• ATP Yield: Net gain of 2 ATP per glucose molecule (4 produced, 2 consumed).

• Regulation: Key regulatory enzymes include hexokinase/glucokinase, phosphofructokinase-1 (PFK-1), and pyruvate kinase.

Fate of Pyruvate

• Aerobic Conditions: Pyruvate is converted to acetyl-CoA by the pyruvate dehydrogenase complex, entering the citric acid cycle.

• Anaerobic Conditions: Pyruvate is reduced to lactate (in animals) or ethanol (in yeast) to regenerate NAD+ for glycolysis.

• Fermentation: The process by which cells regenerate NAD+ under anaerobic conditions.

Gluconeogenesis

Gluconeogenesis is the synthesis of glucose from non-carbohydrate precursors such as lactate, amino acids, and glycerol. It primarily occurs in the liver and, to a lesser extent, in the kidney.

• Key Steps: Many steps are the reverse of glycolysis, but three irreversible glycolytic steps are bypassed by unique gluconeogenic enzymes.

• Bypass Enzymes:

  • Pyruvate carboxylase and phosphoenolpyruvate carboxykinase (PEPCK)

  • Fructose-1,6-bisphosphatase

  • Glucose-6-phosphatase

• Energy Requirement: Gluconeogenesis is an energy-consuming process, requiring ATP and GTP.

Glycogen Metabolism

• Glycogenesis: The synthesis of glycogen from glucose, primarily in the liver and muscle.

• Glycogenolysis: The breakdown of glycogen to glucose-1-phosphate and free glucose.

• Key Enzymes:

  • Glycogen synthase (glycogenesis)

  • Glycogen phosphorylase (glycogenolysis)

• Regulation: Glycogen metabolism is regulated by hormones such as insulin and glucagon, as well as by allosteric effectors.

ATP Yield from Glucose Oxidation

• Complete Oxidation: The complete aerobic oxidation of one glucose molecule yields approximately 30–32 ATP molecules.

• Glycolysis: Net gain of 2 ATP and 2 NADH per glucose.

• Citric Acid Cycle and Oxidative Phosphorylation: Additional ATP is generated from NADH and FADH2 produced in these pathways.

Key Pathways and Their Interconnections

Pathway	Main Function	Key Substrates/Products	Location
Glycolysis	Glucose breakdown to pyruvate	Glucose → Pyruvate, ATP, NADH	Cytoplasm
Gluconeogenesis	Glucose synthesis from non-carbohydrates	Lactate, amino acids, glycerol → Glucose	Liver, kidney
Glycogenolysis	Glycogen breakdown	Glycogen → Glucose-1-phosphate, Glucose	Liver, muscle
Glycogenesis	Glycogen synthesis	Glucose → Glycogen	Liver, muscle

Regulation of Blood Glucose

• Hormonal Control: Insulin lowers blood glucose by promoting uptake and storage; glucagon and epinephrine raise blood glucose by stimulating glycogenolysis and gluconeogenesis.

• Allosteric Regulation: Key enzymes are regulated by metabolites such as ATP, AMP, citrate, and fructose-2,6-bisphosphate.

Special Topics

• Cori Cycle: The cycle of lactate produced by anaerobic glycolysis in muscles being transported to the liver, converted to glucose, and returned to the muscles.

• Substrate-Level Phosphorylation: Direct synthesis of ATP from ADP and a phosphorylated intermediate, as seen in glycolysis.

• Key Intermediates: Compounds such as fructose-1,6-bisphosphate, 1,3-bisphosphoglycerate, and phosphoenolpyruvate play central roles in energy transfer and regulation.

Examples and Applications

• Example: Glycolytic ATP Yield

  • Net ATP from glycolysis per glucose: 2 ATP (substrate-level phosphorylation)

  • Additional ATP from NADH via oxidative phosphorylation (aerobic conditions)

• Example: Gluconeogenesis Precursors

  • Lactate (from muscle), glycerol (from fat), and certain amino acids can be converted to glucose in the liver.

Summary Table: Key Glycolytic Intermediates and Phosphorylation

Intermediate	Number of Phosphate Groups
Glucose-6-phosphate	1
Fructose-1,6-bisphosphate	2
Glyceraldehyde-3-phosphate	1
1,3-Bisphosphoglycerate	2
Phosphoenolpyruvate	1

Additional info:

• Some questions in the file refer to the regulation of glycolysis and gluconeogenesis, the role of specific enzymes, and the fate of metabolic intermediates under different physiological conditions.

• Understanding the interconversion of metabolic intermediates and the regulation of these pathways is essential for comprehending metabolic diseases such as diabetes.