Carbohydrate Metabolism Overview

Carbohydrate Metabolism

• Carbohydrates are hydrosoluble molecules with the empirical formula (CH₂O)ₙ.
• Monosaccharides (simple sugars) are the building blocks of larger carbohydrates.
• Aldoses have an aldehyde group, ketoses have a ketone group.
• Monosaccharides can be classified based on the number of carbons (trioses, tetroses, pentoses, hexoses) and their functional group (aldoses, ketoses). Examples include glucose, fructose, galactose, and ribose.
• Oligosaccharides are formed by the linkage of 2-10 monosaccharides via glycosidic bonds. Maltose is an example of a disaccharide (2 monosaccharides). Sucrose and lactose are also disaccharides.
• Polysaccharides are formed by more than 10 monosaccharides linked by glycosidic bonds. They differ in the identity of the repeating units, the length of chains, the type of bonds, and the degree of branching. Examples include starch and glycogen (both glucose polymers).
• Starch is a branched polymer found in plants, glycogen is a branched glucose polymer in animals.
• Cellulose is a structural polysaccharide in plants, unbranched polymer.
• Heteropolysaccharides contain more than one type of monosaccharide and include hyaluronic acid.
• Hyaluronic acid is found in synovial fluid for lubrication and in the vitreous humor of the eye for its jelly-like consistency

Glucose Homeostasis

• Glucose is an essential metabolic fuel for the brain and other tissues.
• Homeostasis ensures constant blood glucose levels.
• After a meal: Blood glucose rises, insulin released, glucose uptake stimulated, glycogen formation.
• During fasting: Blood glucose lowers, glucagon stimulates, glycogen breakdown, gluconeogenesis starts.

Glycolysis

• Glycolysis is the catabolic pathway of glucose breakdown into pyruvate.
• Glycolysis is a universal central pathway.
• Glycolysis occurs in the cytoplasm.
• Stages are preparatory and payoff phases, consuming 2 ATP and producing 2 ATP and 2 NADH.

Gluconeogenesis

• Gluconeogenesis is the biosynthesis of glucose from non-carbohydrate precursors like pyruvate, glycerol, lactate and some amino acids.
• It's an anabolic pathway, and largely takes place in the liver and sometimes the renal cortex.
• It's important during fasting and after exercise to provide glucose.
• Gluconeogenesis bypasses the 3 irreversible steps of glycolysis via different enzymes.
• Requires 4 ATP and 2 GTP per glucose molecule formed from pyruvate.

Cori Cycle

• The Cori cycle describes the metabolic pathway that converts lactate from muscle tissues into glucose in the liver.
• During intense exercise, lactate produced in muscles is transported to the liver.
• In the liver, lactate is converted back to glucose, which is returned to the muscles for use.

Glycogen Metabolism

• Glycogen is a highly branched polysaccharide, the storage form of glucose in animals.
• Glycogenolysis is the breakdown of glycogen to glucose for energy. Activated by epinephrine or glucagon.
• Glycogenesis is the process of converting glucose to glycogen to be stored. Insulin activates the process.
• Glycogen in muscles is important for muscle contraction, in liver for blood glucose homeostasis.

Regulation of Carbohydrate Metabolism

• Regulation of carbohydrate metabolism involves various hormones such as insulin, glucagon, and adrenaline, along with differing states of energy availability.
• During well-fed state: Insulin released, glucose increases, glycogen synthesis, glycolysis.
• During fasting state: Glucose decreases, glucagon released, glycogen breakdown, gluconeogenesis.