Biochemistry 2280 Topic 12.docx

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Biochemistry 2280A
Christopher Brandl

Topic 12Carbohydrate metabolismReadings p425435 445451By the end of this topic you should be able toIdentify the different metabolic fates of glucose6phosphate and how these pathways are relatedExplain in general terms how glycogen is synthesize and degradedState the net reactions of glycolysis and gluconeogenesis including stoichiometriesState the metabolic circumstances under which glycogen synthesis glycogen degradation glycolysis and gluconeogenesis are favoured without memorizing individual regulatorsDescribe the purpose of fermentation identify circumstances under which it is used and give the products of the two main fermentation pathwaysExplain the purposes of the pentose phosphate pathway and identify the ultimate fates of carbon atoms that enter this pathwayExplain in general terms how pathways that carry out opposite processes eg glycolysis and gluconeogenesis can both be energetically favourableCarbohydrates are an important source of energy and reducing power for the cell In this topic we will consider the fates of glucose an important monosaccharide and more particularly its phosphorylated form glucose6phosphateGlucose is absorbed from the digestive tract and circulates in the blood from which it can enter cells via glucosespecific transport proteins These proteins move glucose passively ie without input of energy across the membrane from the side of higher glucose concentration to the side of lower concentration Inside the cell glucose is converted to glucose 6phosphate which can be used in several different ways Glycogen synthesis and breakdown Glucose is oxidized by glycolysis to provide energy to the cell glycolysis is discussed later in this topic When the available energy is sufficient to meet an animals needs some glucose is stored as glycogen a polysaccharide composed of glucose monomers see Topic 9 Glycogen is stored in the liver and striated muscleTo make glycogen the cell first converts glucose6phosphate to glucose1phosphate Glucose1phosphate is added to the glycogen chain in a twostep reaction that involves the net hydrolysis of UTP to UDP and P The second step of this twostep reaction is icatalyzed by glycogen synthaseGlycogen n residuesG1PUTP Glycogen n1 residuesUDP2 PiHydrolysis of UTP to UDP and P is energetically similar to hydrolysis of ATP to ADP iand P Coupling the reaction to hydrolysis of UTP makes it energetically favourable iUTP can be considered an ATP equivalent in this reaction In fact NTPs are readily interconverted by the enzyme nucleoside diphosphate kinaseNTPADPNDPATP
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