Carbohydrates are major sources of energy for living cells. Glucose is central molecule in carbohydrate metabolism. Carbohydrate metabolism begins with digestion in small intestine where monosaccharides are absorbed into blood stream. It refers to various biochemical processes responsible for formation, breakdown, and interconversion of carbohydrates in living organisms.
The important metabolites involved in regulation of carbohydrate metabolism includes ATP, NADH, glucose-6- phosphate, citrate, and fructose-2,6-bisphosphate.
The major pathways of carbohydrate metabolism are:
- Glycolysis (Embden-Meyerhof pathway): Oxidation of glucose to pyruvate and lactate. It is series of enzyme catalyzed reactions for conversion of glucose to pyruvate of lactic acid with the production of ATP.
- Citric acid cycle (Krebs cycle or Tri-carboxylic acid cycle): Oxidation of acetyl Co-A to CO2. Krebs cycle is the final common oxidative pathway for carbohydrates, fats or amino acids through Acetyl Co-A. It is a cyclic process which involves sequence of compounds interrelated by oxidation-reduction and other reactions which finally produces CO2 and H2O with release of energy.
- Gluconeogenesis: Synthesis of glucose or glycogen from non-carbohydrate precursors (e.g. amino acids, glycerol etc.). It meets demand of glucose in the body when carbohydrates are not available in sufficient amounts from diet & clear the products of metabolism of other tissues (lactic acid, glycerol) from the blood.
- Glycogenesis: Formation of glycogen from glucose. Excess of glucose is converted to glycogen in various tissues specially liver and skeletal muscle & stored for future needs. Glycogen is synthesized depending on demand for glucose and ATP (energy).
- Glycogenolysis: Breakdown of glycogen stored in liver cells and muscles to glucose for providing immediate energy and to maintain blood glucose levels in body (e.g. during fasting). Glycogen are broken down by sequential removal of glucose monomers via phosphorolysis, by the enzyme glycogen phosphorylase.
- Hexose monophosphate shunt (Pentose phosphate pathway or Direct oxidative pathway):
This pathway is an alternative to glycolysis and TCA cycle for the oxidation of glucose (directly to
carbon dioxide and water) and provides:
- NADPH, used for reductive synthesis.
- Pentoses utilized for nucleic acids synthesis.
It is not meant for the production of energy.
- Uronic acid pathway: Glucose is converted to glucuronic acid, pentose and, in some animals to ascorbic acid (not in human).This pathway is also an alternative oxidative pathway for glucose. It is alternative pathway for oxidation of glucose & provides D-glucuronic acid which is used for synthesis of mucopolysaccharides and conjugation reactions.
- Galactose metabolism: The pathways concerned with the conversion of galactose to glucose and the synthesis of lactose. A component part of glycolipids; galactose required for synthesis of lactose (milk sugar) in lactating mammary gland, chondromucoids and mucoproteins.
- Fructose metabolism: The oxidation of fructose to pyruvate and the relation between fructose and glucose metabolism. E.g. In Seminal fluid fructose is required for the metabolism of spermatozoa, which is produced from glucose by sorbitol (polyol) pathway.
- Amino sugar and mucopolysaccharide metabolism: The synthesis of amino sugars and other sugars for the formation of mucopolysaccharides and glycoproteins. Mucopolysaccharides are produced from D-glucuronic acid formed by uronic acid pathway.
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