Carbohydrate Metabolism Overview

Carbohydrate Metabolism

• Carbohydrates are hydrosoluble molecules with the empirical formula (CH₂O)ₙ.
• Monosaccharides are the building blocks of larger carbohydrates.
• Monosaccharides are classified based on the carbonyl group (aldehyde or ketone) and the number of carbons.
• Examples include trioses, tetroses, pentoses, and hexoses (e.g., glucose, fructose, galactose).
• Oligosaccharides are composed of 2 to 10 monosaccharides linked by glycosidic bonds.
• Disaccharides (e.g., lactose, sucrose, maltose) are examples of oligosaccharides.
• Polysaccharides consist of more than 10 monosaccharides and differ in monomer type, chain length, and branching.
• Homopolysaccharides are composed of one type of monosaccharide (e.g., starch, glycogen, cellulose).
• Heteropolysaccharides are composed of more than one type of monosaccharide (e.g., hyaluronic acid).
• Starch is a branched glucose polymer stored in plants.
• Glycogen is a highly branched glucose polymer stored in animals.
• Cellulose is an unbranched glucose polymer forming plant cell walls.
• Hyaluronic acid is a heteropolysaccharide acting as a lubricant in joints and in the eye's vitreous humor.
• Digestion of carbohydrates involves enzymes like amylase to break down polysaccharides into monosaccharides, which are absorbed into the bloodstream.
• Key enzymes involved in carbohydrate digestion include lactase, sucrase, and a-amylase.
• Carbohydrate homeostasis maintains constant blood glucose levels.
• After a meal, insulin promotes glucose uptake and glycogen synthesis.
• During fasting, glucagon promotes glycogen breakdown and gluconeogenesis.
• Glycolysis is the central pathway for glucose breakdown, transforming glucose into pyruvate.

Glycolysis

• Glycolysis is the metabolic pathway that converts glucose into pyruvate in the cytoplasm.
• Glycolysis has two phases: preparatory and payoff.
• The preparatory phase consumes ATP.
• The payoff phase produces ATP and NADH.
• The net yield from one glucose molecule is 2 ATP and 2 NADH.
• Glycolysis is regulated by key enzymes such as hexokinase, phosphofructokinase, and pyruvate kinase.
• Activators include insulin and epinephrine, while inhibitors include ATP and glucagon.

Gluconeogenesis

• Gluconeogenesis is the process of synthesizing glucose from non-carbohydrate precursors.
• Precursors include lactate, glycerol, and amino acids.
• Gluconeogenesis, mainly in the liver and kidneys, is the reverse of glycolysis but bypasses three irreversible steps.
• This process requires ATP.
• Gluconeogenesis is important in maintaining blood glucose levels during fasting or periods with low glucose intake.

Cori Cycle

• The Cori cycle is a metabolic pathway involving the conversion of lactate produced in muscles undergoing anaerobic glycolysis to glucose in the liver.
• The lactic acid formed during anaerobic glycolysis is transported to the liver and converted to glucose.
• The glucose is then transported back to the muscles for energy.

Glycogen Metabolism

• Glycogen is the storage form of glucose in animals.
• Glycogenolysis is the breakdown of glycogen, while glycogenesis is the synthesis of glycogen.
• Glycogenolysis is a catabolic pathway activated by glucagon or epinephrine. Insulin activates glycogenesis, the anabolic pathway.
• The liver's glycogen stores maintain blood glucose levels. The muscle's glycogen stores provide fuel during exercise.

Regulation of Carbohydrate Metabolism

• Well-fed state: Insulin promotes glucose uptake and glycogen synthesis.
• Fasting state: Glucagon stimulates glycogen breakdown and gluconeogenesis.
• Exercise: Epinephrine stimulates glycogen breakdown and glycolysis.