Carbohydrate metabolism

Carbohydrate metabolism denotes the various biochemical processes responsible for the formation, breakdown and interconversion of carbohydrates in living organisms.

The most important carbohydrate is glucose, a simple sugar (monosaccharide) that is metabolized by nearly all known organisms. Glucose and other carbohydrates are part of a wide variety of metabolic pathways across species: plants synthesize carbohydrates from atmospheric gases by photosynthesis, which can then be consumed by other organisms and used as fuel for cellular respiration. Oxidation of one gram of carbohydrate yields approximately 4 kcal of energy. Energy obtained from carbohydrate metabolism is usually stored in the form of ATP. Organisms capable of aerobic respiration metabolize glucose and oxygen to release energy with carbon dioxide and water as byproducts.

All carbohydrates share a general formula of approximately C_nH_2nO_n; glucose is C₆H₁₂O₆. Monosaccharides may be chemically bonded together to form disaccharides such as sucrose and longer polysaccharides such as starch and cellulose.

Carbohydrates are a superior short-term energy reserve for organisms, because they are much simpler to metabolize than fats or proteins. In animals, all dietary carbohydrates are delivered to cells in the form of glucose. Carbohydrates are typically stored as long polymers of glucose molecules with Glycosidic bonds for structural support (e.g. chitin, cellulose) or energy storage (e.g. glycogen, starch). However, the strong affinity of carbohydrates for water makes storage of large quantities of carbohydrates inefficient due to the large molecular weight of the solvated water-carbohydrate complex. In some organisms, excess carbohydrates are catabolised to form Acetyl-CoA, where they enter the fatty acid synthesis pathway. Fatty acids, triglycerides, and other lipids are commonly used for long-term energy storage. The hydrophobic character of lipids makes them a much more compact form of energy storage than hydrophilic carbohydrates.

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Catabolism

Main article: Carbohydrate catabolism

Oligo/polysaccharides are cleaved first to smaller monosaccharides by enzymes called Glycoside hydrolases. The monosaccharide units can then enter into monosaccharide catabolism.

Metabolic pathways

• Carbon fixation, whereby CO₂ is reduced to carbohydrate.
• Glycolysis - the breakdown of the glucose molecule in order to obtain ATP and Pyruvate
  • Pyruvate from glycolysis enters the Krebs cycle in aerobic organisms.
• The Pentose phosphate pathway, which acts in the conversion of hexoses into pentoses and in NADPH regeneration.
• Glycogenesis - the conversion of excess glucose into glycogen in order to prevent excessive osmotic pressure buildup inside the cell
• Glycogenolysis - the breakdown of glycogen into glucose, in order to provide a steady level of glucose supply for glucose-dependent tissues.
• Gluconeogenesis - de novo synthesis of glucose molecules from simple organic compounds

Glucoregulation

Glucoregulation is the maintenance of steady levels of glucose in the body; it is part of homeostasis, and keeps a constant internal environment around cells in the body.

The hormone insulin makes the body convert glucose into glycogen and puts it into the liver; the insulin is made in the pancreas, and is secreted when the blood sugar is too high. Insulin also promotes the use of glucose by the muscles.

The hormone glucagon, on the other hand, acts in the opposite direction and promotes the conversion of glycogen to glucose in response to low blood sugar.

Human diseases of carbohydrate metabolism

• Diabetes mellitus
• Lactose intolerance
• Fructose intolerance
• Galactosemia
• Glycogen storage disease