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Unformatted text preview: Ch. 10 Outline-The steps of a metabolic pathway can ultimately be confirmed by purifying the individual enzymes and reassembling them, along with the necessary cofactors, into a functional pathway. 1) Each step of the pathway is catalyzed by a distinct enzyme 2) The free energy consumed or released in certain reactions is transferred by molecules 3) The rate of the pathway can be controlled by altering the activity of individual enzymes-Glucose combustion requires many steps so that the cell can recover its free energy in smaller, more useful quantities. Glucose: Whence and Wherefore-Glucose is the most plentiful organic molecule on earth, produced at a rate of about 50 billion tons per year, mainly by photosynthetic organisms. Glucose is a six-carbon sugar containing an aldehyde group and five hydroxyl groups Look at Figure 10-2, 10-3- When the glucose molecule cyclizes, the hydroxyl group at C1 may point either down or up. Unless the hydroxyl group at C1 is derivatized in some way, the alpha and beta forms of glucose freely interconvert, although the beta anomeris more stable and therefore predominates. Glucose residues are linked by glycosidic bonds-Glucose is stored in polymeric form as starch in plants and as glycogen in animals. Plants also contain significant amounts of glucose polymerized in the form of cellulose, which is a major structural component of plant cell walls. -Most animals cannot digest cellulose and therefore cannot use its glucose as an energy source because the amylases that digest dietary starch and glycogen cannot cleave the glycosidic bonds between glucose residues in cellulose. -In starch and glycogen, most of the glucose residues are linked by alpha glycosidic bonds. In cellulose, glucose residues are linked by glycosidic bonds, in which the C1 oxygen has the beta configuration. Glycogenolysis-In humans, the sources of glucose include free glucose liberated from dietary starch and glycogen and glucose released by glycogenolysis in the liver. In glycogenolysis, glycogen is phosphorolyzed to yield glucose-1-phosphate. -The activity of glycogen phosphorylase is carefully regulated by a variety for mechanisms linked to hormonal signals.-The enzyme phosphoglucomutase converts glucose-1-phosphate to glucose-6-phosphate, which is then hydrolyzed by glucose-6-phosphate to release free glucose Glycolysis- Overall, glycolysis is a series of 10 enzyme-catalyzed steps in which a six-carbon glucose molecule is broken down into two three-carbon pyruvate molecules. This catabolic pathway is accompanied by the phosphorylation of two molecules of ADP (to produce 2 ATP) and the reduction of two molecules of NAD+-It is convenient to divide the 10 reactions of glycolysis into two phases. In the first, the hexose is phosphorylated and cleaved in half. In the second, the three-carbon molecules are converted to pyruvate....
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This note was uploaded on 12/31/2010 for the course CH 369 taught by Professor Kbrowning during the Spring '07 term at University of Texas.
- Spring '07