Biochemistry, 4th Edition P72. Continuing Garrett and Grisham's innovative conceptual and organizing framework, "Essential Questions," BIOCHEMISTRY guides students through course concepts in a way that reveals the beauty and usefulness of biochemistry in the everyday world. Streamlined for increased clarity and readability, this edition also includes new photos and illustrations that show the subject matter consistently throughout the text. New end-of-chapter problems, MCAT practice questions, and the unparalleled text/media integration with the power of CengageNOW round out this exceptional package, giving you the tools you need to both master course concepts and develop critical problem-solving skills you can draw upon. | How Are Glycogen and Starch Catabolized in Animals 673 hibition by fructose-2 6-bisphosphate whereas phosphofructokinase is allosterically activated by fructose-2 6-bisphosphate. The combination of these effects should permit either phosphofructokinase or fructose-1 6-bisphosphatase but not both to operate at any one time and should thus prevent futile cycling. For instance in the fasting state when food that is glucose intake is zero phosphofructokinase and therefore glycolysis is inactive due to the low concentration of fructose-2 6-bisphosphate. In the liver gluconeogenesis operates to provide glucose for the brain. However in the fed state up to 30 of fructose-1 6-bisphosphate formed from phosphofructokinase is recycled back to fructose-6-P and then to glucose . Because the dependence of fructose-1 6-bisphosphatase activity on fructose-1 6-bisphosphate is sigmoidal in the presence of fructose-2 6-bisphosphate see Figure substrate cycling occurs only at relatively high levels of fructose-1 6-bisphosphate. Substrate cycling in this case prevents the accumulation of excessively high levels of fructose-1 6-bisphosphate. How Are Glycogen and Starch Catabolized in Animals Dietary Starch Breakdown Provides Metabolic Energy As noted earlier well-fed adult human beings normally metabolize about 160 grams of carbohydrates each day. A balanced diet easily provides this amount mostly in the form of starch. If too little carbohydrate is supplied by the diet glycogen reserves in liver and muscle tissue can also be mobilized. The reactions by which ingested starch and glycogen are digested are shown in Figure . The enzyme known as a-amy-lase is an important component of saliva and pancreatic juice. -Amylase is found in plants. The a- and -designations for these enzymes serve only to distinguish the two and do not refer to glycosidic linkage nomenclature. -Amylase is an endoglycosidase that hydrolyzes 1 4 linkages of amylopectin and glycogen at random .
