Carbohydrate Metabolism
- Glucose is called
is called 'energy currency' of carbohydrates.
- The formation of glycogen from glucose is called "glycogenesis". Glycogen is the stored form of glucose and the liver is the main storage house.
- The formation of glucose from glycogen is called "glycogenolysis". It takes place mainly in liver.
- The glycogen in liver is converted into glucose during fasting conditions. Glycogen in muscle cannot be converted into glucose due to the absence of the enzyme “Glucose-6-phosphatse” (Liver is the only viscera which can synthesize glycogen from monosaccharides other than glucose).
- Instead of it, glycogen from muscle is broken down to pyruvic acid or lactic acid. This is reabsorbed and converted into glycogen and glucose by liver and recirculated into blood for use.
- The events that
connect muscle glycogen to liver glycogen is called "Cori cycle".
- The synthesis of
glucose and glycogen from non-carbohydate source is called "gluconeogenesis"
or "glyconeogenesis". Blood sugar level during prolonged starvation
is maintained by gluconeogenesis.
- To get maximum available energy from glucose, it has to be completely oxidised to carbohydrate and water.
- Pyruvic acid (lactic acid) produced by muscle or liver is oxidised to acetate which combines with a molecule of oxaloacetate to form citric acid.
- The citric acid runs through series of events where the acetate molecule is oxidised to carbon dioxide and water with production of large amounts of energy and oxaloacetate is regenerated. This is called "citric acid cycle".
- Glucose is also metabolised through certain pathways which are not important for energy production, but various intermediate products are useful for synthesis for various substances is produced. These are "hexose monophosphate pathway" and the "uronic acid pathway".
- Normal levels of fasting glucose are 60-90 mg %, post prandial level of 100-140 mg% and random is 90-120 mg%.
- Tubular maximum for glucose reabsorption (Tmg) is 250-350 mg, of glucose per minute.
- The Level of blood glucose above which the kidney starts excreting in urine is called "renal threshold of glucose" (140-180 mg/100 ml).
- For glucose to enter any metabolic process be it catabolism or anabolism, it has to be phosphorylated first. This reaction is mediated by glucokinase or hexokinase. first glucokinase is present in liver but absent in muscles. Magnesium and molybdenum are required for activation this enzyme.
- Phosphorylation
of glucose is the first step mediated by glucokinase in glycogenesis.
- This product formed by phosphorylation is glucose-6-phosphate. It is a "locking mechanism by which glucose is kept within the cell".
- Last step in glycolysis is formation of pyruvate to oxaloacetate.
- The reactions involved in the conversion of glucose or glycogen to pyruvic acid is called "Embden-Meyerhof" pathway. This pathway of glucose takes place in anaerobic conditions and yields lactic acid.
- All enzymes required for glycolysis are cytoplasmic (extra-mitochondrial).
