Sugars, such as galactose, fructose, and glycogen, are catabolized into new products in order to enter the glycolytic pathway.
You have learned about the catabolism of glucose, which provides energy to living cells. But living things consume more than glucose for food. How does a turkey sandwich end up as ATP in your cells? This happens because all of the catabolic pathways for carbohydrates, proteins, and lipids eventually connect into glycolysis and the citric acid cycle pathways. Metabolic pathways should be thought of as porous; that is, substances enter from other pathways, and intermediates leave for other pathways.
These pathways are not closed systems. Many of the substrates, intermediates, and products in a particular pathway are reactants in other pathways. Like sugars and amino acids, the catabolic pathways of lipids are also connected to the glucose catabolism pathways.
Glycogen Pathway : Glycogen from the liver and muscles, hydrolyzed into glucosephosphate, together with fats and proteins, can feed into the catabolic pathways for carbohydrates. Glycogen, a polymer of glucose, is an energy-storage molecule in animals. When there is adequate ATP present, excess glucose is shunted into glycogen for storage.
Glycogen is made and stored in both the liver and muscles. The glycogen is hydrolyzed into the glucose monomer, glucosephosphate GP , if blood sugar levels drop.
The presence of glycogen as a source of glucose allows ATP to be produced for a longer period of time during exercise. Glycogen is broken down into GP and converted into glucosephosphate GP in both muscle and liver cells; this product enters the glycolytic pathway. Glycogen Structure : Schematic two-dimensional cross-sectional view of glycogen: A core protein of glycogenin is surrounded by branches of glucose units.
The entire globular granule may contain around 30, glucose units. Galactose is the sugar in milk. Infants have an enzyme in the small intestine that metabolizes lactose to galactose and glucose. In areas where milk products are regularly consumed, adults have also evolved this enzyme. Galactose is converted in the liver to GP and can thus enter the glycolytic pathway.
Fructose is one of the three dietary monosaccharides along with glucose and galactose which are absorbed directly into the bloodstream during digestion. Fructose is absorbed from the small intestine and then passes to the liver to be metabolized, primarily to glycogen.
The catabolism of both fructose and galactose produces the same number of ATP molecules as glucose. Fructose Metabolism : Although the metabolism of fructose and glucose share many of the same intermediate structures, they have very different metabolic fates in human metabolism. In the first reaction, galactokinase enzyme phosphorylates galactose at first carbon changing it into galactose 1-P. Now, these glucose 1-P molecules change into glucose 6-P by the action of the phosphoglucomutase enzyme.
Mannose enter into glycolytic pathway via a two-step reaction, in the first step hexokinase phosphorylates mannose to forming mannose 6-P by hydrolysis of ATP, then mannose 6-P isomerize into fructose 6-P by the action of phosphomannose isomerase. The affected person can use predigested milk. Galactosemia is due to a deficiency in any of the three enzymes of galactose catabolism, first enzyme galactokinase, second enzyme UDP glucose: galactose 1-P uridylyltransferase and third is glucose 4-epimerase.
Galactose concentration is high in blood and urine, and galactose metabolite galactitol, deposit in eye lens causes cataract in infancy. Save my name, email, and website in this browser for the next time I comment. However, our diet contains several other sugars in significant amounts. The guiding motif in the metabolism of these sugars is economy: instead of completely separate degradative pathways, there are short adapter pathways which merge into the main pathway of carbohydrate degradation, that is, glycolysis.
Lactose and sucrose are disaccharides. Degradation of both sugars begins with hydrolytic cleavage, which releases glucose and galactose or glucose and fructose, respectively.
Fructose is also found in the diet as a monosaccharide. We already know how glucose is degraded, so we here only need to concern ourselves with the remaining monosaccharides. The degradation of sorbitol will be discussed as well, whereas ribose and deoxyribose will be covered in later chapters.
The hydrolytic cleavage of sucrose, like that of of maltose, occurs at the surface of the intestinal epithelial cells. Both sugars are then taken up by specific transport: Glucose by the SGLT1 transporter, and fructose by the GLUT5 transporter, which is named after glucose but actually transports fructose more effectively than glucose. Fructose degradation, also called fructolysis , runs mostly in the liver. In the first step, fructose is phosphorylated by fructokinase 1 , which uses ATP as a cosubstrate.
This yields fructosephosphate. The latter is then cleaved by aldolase B 2. The products of this reaction are dihydroxyacetone phosphate, which is already a metabolite in glycolysis, and glyceraldehyde, which can enter glycolysis after phosphorylation by glyceraldehyde kinase 4. Glyceraldehyde can alternately be utilized by conversion to glycerol and then to glycerolphosphate. The latter is a substrate in the synthesis of triacylglycerol, that is, fat.
Fructose and sucrose appear to promote obesity more strongly than equivalent amounts of starch or glucose, and it has been suggested that its utilization via glycerolphosphate, with subsequent triacylglycerol synthesis, may be among the reasons.
Fructose intolerance is a hereditary disease caused by a homozygous defect in the aldolase B gene. In this condition, fructose is still phosphorylated by fructokinase. The resulting fructosephosphate, however, cannot be processed further, and therefore the phosphate tied up in it cannot be reclaimed.
Accordingly, the disease is characterized by potentially severe liver failure. Fructose, alone or in combination with glucose, has been used in the past in the intravenous nutrition of intensive care patients; the perceived advantage of this treatment was the insulin-independent utilization of fructose. However, large intravenous dosages of fructose can significantly deplete liver ATP [ 8 ] ; it appears that, under heavy load, aldolase B is unable to keep up with fructose kinase.
Fructose is no longer a major component of intravenous nutrition schemes. A defect in the gene encoding fructokinase leads to a condition named fructosemia or fructosuria. As these names suggest, fructose levels are increased both in the blood 16 and the urine.
Since fructose is not phosphorylated, no phosphate depletion occurs, and the liver cells do not incur any damage. The disease is therefore quite benign.
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