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Carbohydrates
This set of lectures will describe the metabolism of carbohydrates. Metabolism off fats and proteins will be covered later in the course. Carbohydrates have the general formula (CH2O)n, carbon + water. Monosaccharides (simple sugars) can have 3 to 8 carbons, and contain multiple hydroxyl groups as well as either an aldehyde group (aldoses) or a ketone group (ketoses).
Glucosefructose
(6-carbon aldose) (6-carbon ketose)
In aqueous solution, one of the hydroxyl groups tends to react with the aldehyde or ketone group, closing the linear molecule into a ring. In an equilibrium mixture, only about 1% of the sugar exists in the open chain form, but the two forms interconvert rapidly. For more information, see PEDS 207, Unit 2.
In the closed ring form, the hydroxyl groupon the carbon that carries the aldehyde or ketone can lie either below the plane of the ring or above it. The two positions are called a and b. They also rapidly interconvert, but once one sugar is linked to another, the a or b form is frozen.
The a or b hydroxyl group can react with any hydroxyl group on a second sugar molecule to form a glycosidic bond, releasing water. Sucrose is adisaccharide formed by an a linkage between carbon 1 of glucose and carbon 2 of fructose. Lactose is formed by ab linkage between carbon 1 of galactose and carbon 4 of glucose. When the hydroxyl group on a carbon that carries a carbonyl is available for such a reaction, the sugar is said to be “reducing”. Sucrose is a nonreducing sugar.
Oligosaccharides and polysaccharides are formed from multiple sugarslinked in a chain by a series of glycosidic bonds. The chemical nature of a polysaccharide is determined by its constituent subunits and the types of linkages between them. For example, a chain of glucose molecules held together by b 1-4 glycosidic bonds is cellulose, a major component of plant cell walls. A chain of glucose subunits held together by a 1-4 glycosidic bonds is a-amylose, or starch.When carbohydrate-containing food is consumed, polysaccharides and disaccharides are hydrolyzed to their component monosaccharide subunits by digestive enzymes in the saliva in the lumen of the small intestine. The enzymes are stereospecific. a-amylase in the saliva and secreted by the pancreas into the intestine will hydrolyze a 1-4 bonds but not b 1-4 bonds, therefore we can digest starch butnot cellulose. A variety of specific enzymes on the surface of the intestinal cells hydrolyze various disaccharides. Lactose in milk is a major source of nutrition for young mammals including humans, and it is split by b-galactosidase (also called lactase) in the intestine into galactose and glucose. Most adult mammals, including the majority of adult humans, stop secreting this enzyme, and becomelactose intolerant. Undigested lactose is osmotically active, drawing water into the large intestine and causing diarrhea. Lactose is readily fermented by the bacteria living in the large intestine, generating CO2 and H2gas, resulting in bloating and flatulence.
Two types of glucose carriers enable gut epithelial cells to transfer glucose and other monosaccharides across the gut lining into thebloodstream. Glucose is actively transported into the cell by Na+-driven cotransporters at the apical surface, a process that requires energy (in the form of the Na+ gradient) since glucose is moving from a region where its concentration is low, in the lumen, to a region where its concentration is high, in the intestinal cell cytoplasm. Glucose is released by the cell into the bloodstream bypassive transport down its concentration gradient, mediated by a different glucose transporter. Most other cells take up glucose from the bloodstream via passive transport, since their cytoplas














Los hidratos de carbono
Esta serie de conferencias describirá el metabolismo de los hidratos de carbono. Los hidratos de carbono tienen la fórmula general ( CH2O ) n, Carbono +...