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Chemistry 2223B Note

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Western University
Chemistry 2223B
Paul Ragogna

Topic 12 Carbohydrate storage as glycogen • Glucose is a good energy source – we oxidize it to get ATP • Excess glucose is saved for later – makes glycogen out of it • Liver and muscle are two main places that take up glycogen • The first part of the reaction has equilibrium arrows because the ΔG is close to 0 so the direction the reaction is going to go is dependent on the relative concentrations of the molecules • Glycogen synthase is used for synthesizing glycogen by adding glucose monomers to each other o This requires energy so it uses energy from the hydrolysis of UTP to UDP and 2Pi  Uses UTP not ATP’, it has the same sort of structure  Hydrolysis of UTP is favourable • Overall this reaction has a negative ΔG because of this hydrolysis • When it comes time to hydrolyze glycogen we can’t use the same enzyme because it would be energetically unfavourable, so we used glycogen phosphorylase which breaks off glucogen molecules one at a time from glycogen o Goes on its own (ΔG is –ve) • Glycogen synthase makes glycogen – glycogen phosphorylase breaks it apart so it can be used for energy Allosteric regulation in skeletal muscle • Glucose-6-phosphate is an allosteric regular of both of these enzymes • When G6P concentration is high we don’t want to make more so it would make sense to inhibit glycogen phosphorylase, and we want to store it so glycogen synthase is activated • When ATP is high it is going to inhibit glycogen phosphorylase,, but if you start working your muscles they will complain that AMP is increasing (get it with the breaking down of ATP) so it needs more energy so it activates glycogen phosphorylase o ATP indicates energy is high so inhibits glycogen phosphorylase o AMP indicates energy is low so activates glycogen phosphorylase • Muscle only cares about muscle, lookin out for number 1… contrast that with the liver Allosteric regulation in liver • Same thing is going on in liver • The enzymes are slightly different – have different isoforms – located in different tissues so different indicators • Instead of ATP concentration regulating it, glucose regulates it • Lots of glucose around the liver will deactivate glycogen phosphorylase (don’t need more glucose) • Liver unlike muscle is selfless – will give up energy to help the rest of the body o When glycogen is released by the liver it is not always for its own use – exports it into the blood stream to help out other niggas Control by phosphorylation in liver & muscle • These enzymes are also regulated by phosphorylation • Phosphorylate glycogen synthase and it is inactivated • Phosphorylate glycogen phosphorylase and it is activated • When phosphorylated the allosteric regulation tends to not matter • Insulin is released when blood glucose is high – leads to dephosphorylation o Glycogen synthase is active – makes sense because we have lots of glucose  Happens in both liver and muscle • Glucagon is released when blood glucose is low – leads to phosphorylation in liver o Muscles don’t care about glucose concentration – just cares about itself o The brain loves glucose – its main energy source – the liver is concerned about the brain • Epinephrine leads to phosphorylation in liver and muscle Glycogen metabolism: influence of insulin • When insulin is high it is going to bind to the receptor which leads to “cellular events” • Results in dephosphorylation of these enzymes which makes glycogen synthase active • Insulin doesn’t actually go in the cell – it is mediated by a receptor • If glucagon is high we have another receptor and we get phosphorylation and glycogen phosphorylase is activated Glycolysis: net reaction • Glucose is oxidized to pyruvate • Make 2 ATP for every glucose that goes through • Pyruvate goes on and gets oxidized in the mitochondria • ATP is used for energy • NADH is carried onto the electron transport chain • Whole process is geared towards making ATP • Glycolysis turned on when we want ATP o Know what goes in and what goes out of the reaction as well as the numbers • Other sugars can be broken down to give you ATP as well ΔG values for glycolysis reactions • Steps 1, 3 and 10 are targets for control of flux • Glucose is compound 1, pyruvate is compound 11 Control of flux through glycolysis Step 1 – regulate pathway with hexokinase, first step is a good place to regulate because it’s the very beginning of your pathway Step 3 – might want to not have glycolysis to go but have glycogen to be made so there is a regulator at this branching point Step 10 – last step is regulated which may seem counter intuitive but he is going to leave it with us and explain it later…. Okay thanks guy • Phosphofructokinase (PFK) o First thing that inhibits this enzyme is ATP  High concentration shuts down glycolysis by inhibiting this enzyme o When citrate concentration is high it is an indication that the energy status of the cel is good so we want to shut down glycolysis o When AMP, ADP, Pi are high it means ATP is low so PFK is activated o Fructose-2,6-biphosphate is outside the pathway but when insulin is high in the blood one of the effects of this signalling is F26BP is also high  Signal that lots of glucose is around so lets make some ATP • Comes a point in every runners marathon where they have used up all of their glycogen… o Girl wants her boyfriend to stand on the side lines with energy packets… what kind of carbohydrates are in these energy packets?  Mostly starch – just enough sugar to feed what glycolysis needs to start,
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