Carbohydrate Metabolism Overview

Carbohydrate Metabolism

• Carbohydrates are the most abundant organic molecules on Earth.
• They are the primary structural component of plants and provide food energy as starch and sugars.
• Carbohydrates provide half or more of the worldwide human food energy intake.
• Carbohydrates also function as metabolic intermediates, RNA and DNA components, structural elements in cells and tissues, and energy storage molecules in the body.
• Carbohydrates are combinations of carbon, hydrogen, and oxygen atoms (C-H₂O)n.

Carbohydrate Categories

• Carbohydrates are categorized into simple and complex forms.
• Simple carbohydrates include monosaccharides, like glucose, fructose, and galactose, and disaccharides, like sucrose, lactose, and maltose.
• Complex carbohydrates include oligosaccharides (3-10 sugar units) and polysaccharides (>10 sugar units).

Carbohydrate Structure (Examples)

• Maltose: α(1-4) bond
• Lactose: β(1-4) bond
• Sucrose: α(1-2) bond
• Trehalose: α(1-1) bond

Complex Carbohydrates (Details)

• Complex carbohydrates are polymers of linked saccharide units by glycosidic bonds.
• Oligosaccharides typically have 3–10 saccharide units, and polysaccharides contain more than 10, usually thousands, of units.
• Glucose is the most common type of saccharide in complex carbohydrates.
• Oligosaccharides are commonly conjugated to proteins and lipids in cell membranes.
• On cell surfaces, these conjugated oligosaccharides act to modulate cell function in various ways
• Examples of complex carbohydrates include starch (amylose and amylopectin) and glycogen.

Digestion of Complex Carbohydrates

• Salivary a-amylase hydrolyses a(1-4) glycosidic bonds to break down amylose and amylopectin (in the mouth).
• Gastric juice halts further digestion in the stomach.
• Pancreatic a-amylase further hydrolyses carbohydrates in the small intestine (further breaking down amylose and amylopectin)
• Maltose, maltotriose, and limit dextrins are further broken down into glucose by maltase and a-dextrinase.

Small Intestine Absorption

• The small intestine's structural features (folds of Kerckring, villi, and microvilli) increase surface area for absorption.
• Enterocytes (specialized absorptive cells) and microvilli are present on the apical (brush border) membrane.
• Nutrients are absorbed into capillaries and lacteals.

Monosaccharides Absorption and Transport

• Monosaccharides (glucose, galactose, and fructose) must cross the plasma membrane of the enterocyte twice to enter the bloodstream.
• Monosaccharides first enter cells on the brush border side and exit on the basolateral side.
• They're then delivered to the liver.
•   Two major transport protein families facilitate this: SGLTs (sodium-glucose cotransporters) and GLUTs (glucose transporters).
• SGLTs (e.g. SGLT1) are sodium-dependent and utilize ATP hydrolysis.
• GLUTs (e.g. GLUT2 and GLUT5) are facilitated diffusion transporters.
•   GLUT isoforms have different tissue distribution, substrate properties, and regulation.
• GLUT4 is insulin-dependent in muscle and adipose tissue; GLUT2 is found in the liver.

Glucose Homeostasis

• Normal blood glucose levels are a crucial homeostatic balance.
• The liver and skeletal muscle are key to maintaining glucose balance.
• Insulin lowers blood glucose and acts as the primary anabolic hormone.
• Insulin promotes glucose and nutrient uptake by cells.
• Glucagon opposes insulin and is the primary catabolic hormone, promoting glycogen breakdown.
• Other factors like glucocorticoids also regulate blood glucose.

Glycogenesis

• Glycogenesis is the synthesis of glycogen from glucose.
• It starts with phosphorylation of glucose into glucose-6-phosphate by glucokinase (primarily in the liver).
• The process then involves creating UDP-glucose for transfer, forming a glycogen primer via glycogenin, and then adding glucose molecules to the chain by glycogen synthase.
• A branch-inducing enzyme (also called amylo(1-4→1-6)-transglycosylase) is required to create branch points in the glycogen.
• Glucokinase in the liver can effectively process large amounts of glucose.

Glycogenolysis

• Glycogenolysis is the breakdown of glycogen into glucose-1-phosphate.

• Glycogen phosphorylase helps carry out this process.

• It acts on the non-reducing ends of the glycogen chains and is especially adapted for linear segments.

• The debranching enzyme handles branched portions of glycogen.

• Glucose-6-phosphatase acts only in liver and kidney cells to release free glucose.

Explore the fundamentals of carbohydrate metabolism in this quiz. Learn about the different categories of carbohydrates, their structures, and their roles in providing energy and supporting cellular functions. Test your knowledge on monosaccharides, disaccharides, and polysaccharides.