Carbohydrates metabolism Dr.Ehsan Hassan AL-Dabbagh

Chemistry of carbohydrates All carbohydrates contain C double bond O and hydroxyl groups and are classified into; 1. monosaccharides or simple sugars that can not hudrolyzed to a simpler form, it may contain three,four,five,six or more carbon atoms known respectively as trioses,tetroses,pentoses,hexoses,so on. Monosaccharides may be aldoses or ketoses depending upon whether they have an aldehyde or ketone groups respectively. Most important monosaccharides are hexoses like glucose,galactoseand fructose which are reducing substances because they contain aldehyde or ketone groups.

Monosaccharides have stereoisomer property which could be D(common) or of its mirror image L( D on right and L on left) depending on position of hydroxyl group at carbon atom adjescent to terminal alcohol carbon (C5 in glucose).

2. disaccharides they are products of chemical reaction between two mono saccharides with loss of a molecule of water (can be hydrolyzed) the linkage between two monosaccharides known as glycosidic link or bond. Examples of disaccharides are maltose , lactose & sucrose If the glycosidic link between aldehyde or ketone group of one monosaccharide & the hydroxyl group of another monosaccharide the produced disaccharide have reducing property as in maltose (glucose + glucose) & lactose (glucose + galactose)

While if the glycosidic link between aldehyde or ketone group of the two molecules of monosaccharide the produced disaccharide have no reducing property as in sucrose (glucose + fructose) 3.oligosaccharides they are products of condensation of 3-10 molecules or units as maltotriose. 4.polysaccharides they are products of condensation of more than 10 units;examples are:

Starch ; is apoly saccarides of plant origin consist of amylose (one unbranched chain of glucose molecules linked by α 1-4 glucosidic linkages with only terminal aldehyde is free) and amylopectin (contain α1-4 glucosidic linkages + α1-6 branched glucosidic linkages of glucose molecules). Glycogen is a polysaccharide of animal origin ,it has structure similar to amylopectin except that branching is more extensive.

Fate of carbohydrates Carbohydrates account for a large proportion of daily intake ,dietary digestible carbohydrates include mainly starch ,sucrose and to less extent lactose. In order to be absorbed it should be converted into monosaccharides The absorbed monosaccharides from small intestine reach the liver through portal vein, Glucose is the only carbohydrate to be used for energy or stored as glycogen while galactose and fructose are mainly converted to glucose before they can be used.

Pentoses as xylose ,arabinose , and ribose are important in nucleotides,nucleic acids and several coenzymes. Carbohydrates (mainly glucose ) is the main source of human energy and it is a unique source of energy to some tissues nervous system including brain and red blood cells,therefore; we concern with its metabolism.

After absorbtion of glucose it is converted to glucose 6- phosphate inside the cells which may pour into one of the following pathways depending on energy requirement,type of tissue and state of glycogen storage. 1.glycolysis ……..produces energy 2.hexose monophosphate shunt (pentose phosphate shunt)……nucleotide biosynthesis 3.glycogenesis ……..storage.

Glycolysis Glycolysis is the major pathway for glucose metabolism it takes place outside mitochondria in the cytosole of all cells through Embden-Meyerhof pathway .it is unique in that it can function either aerobically or an aerobically ,however ,an aerobic condition limits the amount of energy liberated per molecule of glucose ,therefore; more glucose is needed.

To oxidize glucose beyond pyruvate (the end product of glycolysis) requires oxygen ,mitochondrial enzyme system ,the citric acid cycle and the respiratory chain. The ability of glycolysis to provide ATP in aerobic pathway are especially important in RBC which lack mitochondria and completely depend on glucose as their metabolic fuel ,also in skeletal muscle in anoxic episodes.

However , in heart muscle ,which is adapted for aerobic performance , has relatively low glycolytic activity and then poor survival under a condition of ischemia. The steps of glycolysis(Embden-Meyerhof pathway) are the followings:

Reaction 1 phosphate ester synthesis Reaction 2 isomerization Reaction 3 phosphate ester synthesis Reaction 4 split molecule in half Reaction 5 oxidation Reaction 6 phosphate ester hydrolysis ,synthesis of ATP Reaction 7 isomerization Reaction 8 alcohol dehydration or enolation Reaction 9 phosphate ester hydrolysis, synthesis of ATP

Reaction 1 phosphate ester synthesis In all body tissues except the liver ,brain and pancreatic β islet cells, the transport of glucose into the cell is regulated by insulin. Folowing entry of glucose into the cells,phosphate is added to the glucose present in the cytoplasm at the C 6 position using ATP as the phosphate donor in the presence of magnesium ion ,this reaction is irreversible inhibited by its product (glucose 6 phosphate)and it is catalyzed by the enzymes hexokinase or glucokinase

Hexokinase has affinity for its substrate i Hexokinase has affinity for its substrate i.e glucose and it even acts at lower speed on other hexoses, in the liver and pancreatic β islet cells hexokinase is saturated under all normal condition ,therefore; both the liver and pancreatic β cells also contain an isoenzyme of hexokinase called glucokinase, which has lower affinity for its substrate i.e specific for glucose so it act at higher glucose level.

The function of glucokinase in liver is to remove glucose from the blood following a meal,providing glucose 6 phosphate in excess of requirements for glycolysis,which will be used for glycogen synthesis and lipogenesis. In pancreas ,the glucose 6 phosphate formed by glucokinase signals increased glucose level and this stimulates insulin synthesis.

Glucose 6 phosphate is a corner stone,or grey-zone compound at the junction of several metabolic pathways (glycolysis, gluconeogenesis , pentose phosphate shunt , glycogenesis and glycogenolysis).