Dr. Miller, 9/26 2pm. in response to a question from a student "why can't oxaloacetate from TCA cycle be used to synthesize glucose since gluconeogenesis uses oxaloacetate?" you put in a 2 C unit into TCA cycle. You lose a two carbons in the TCA cycle. The oxaloacetate in gluconeogenesis is generated from pyruvate, not acetyl CoA. GLYCOGEN SYNTHESIS very simple pathway, few reactions. honest. Glycogen is storage form of excess glucose. All the rxns in synthesis and breakdown occur in the glycogen granules inside the cell. glycogen is mainly stored in liver and skeletal muscle, tiny amt in brain and possibly other tissue. Liver stored glucose as glycogen, then when blood glucose is low it breaks down glycogen and releases glucose to the blood. NOTE all of dr. miller's discussion so far unless otherwise noted has pertained to processes within the liver. skeletal muscle at rest can use fatty acids to make acetyl coA and run TCA cycle, but under exercise conditions needs glucose. this is fortunate, because glucose metabolism is anaerobic. you get glycolytic products piling up, pyruvate converting to lactate, etc, but you woulnd't want ATP synthesis to depend on the availability of oxygen alone in the muscle, so it's good that it can use glucose. glycogen synthesis: GLUCOSE meets hexokinase and goes to GLUCOSE 6-PHOSPHATE, oxidizing an ATP to ADP. Recall that glucose 6 phosphate inhibits hexokinase. So, in the liver, there is also another enzyme called glucokinase - recall hexokinase has a very low Km, so you don't need much substrate around to work at peak effeciency. But glucokinase, which lives only in the liver, has a very high Km. When hexokinase is uninhibited, eg not much glucose 6 phosphate around, cellular [glucose] is low. therefore glucokinase doesn't work then. as hexokinase is inhibited, glucose starts building up, and then glucokinase starts working much better at high [glucose], and the glucose is converted to glucose 6 phosphate like we want. second step: glucose 6 phosphate is converted to glucose 1 phosphate by phosphoglucomutase. third: then the glucose 1 phosphate is converted to uridine diphosphate glucose by UDP-glucose pyrophosphorylase. During this rxn, UTP releases two phosphate groups hooked together - a molecule which is an acid anhydride. Note that the uridine diphosphate glucose has one phosphate from the glucose 6 phosphate and one from the UTP. This reaction should be reversible, but the cell has an enzyme which cleaves the P-Pi and leaves you with just 2Pi. this makes the rxn irreversible, because you lose the energy that was stored int he P-Pi bond, and you can't put the 2Pi back together, so you can't reverse the reaction. This helps to drive the reaction toward the production of glycogen. uridine diphosphate glucose transfers its glucose unit to glycogen, giving off UDP. this is catalyzed by glycogen synthase, with can be inhibited by hormones and cAMP. Note that this reaction increases the SIZE of glycogen by adding more carbohydrate units to the polymer. the glycogen synthase is the regulatory step, and it can be regulated by the hormone GLUCAGON in the liver or EPINEPHRINE in the muscle. Glucagon binds to liver cell membrane during low BG, releases kinase that phosphorylates and inhibits glycogen synthase. GLYCOGEN DEGRADATION: very straightforward. you remove glucose, one unit at a time, phosphorylated. you get glucose 1 phosphate from the interaction of phosphorylase a and glycogen. the glucose1 phosphate is broken into glucose 6 phosphate, which can be used in other metabolic pathways, eg glycolysis, gluconeogenesis, etc, or be broken downa by glucose 6 phosphatase into glucose and Pi. regulation - if low bg, and liver has stored glycogen, the glucagon present in the blood binds to liver cell causing synthesis of cAMP, which activates kinases, one of which changes phosphorylase B to phosphorylase A, activating it. high AMP activates phosphorylase B. Phosphorylase A is the "active" form. When you have more phosphorylase B, glycogen degradation is slowed. How are different tissues related by all of these pathways? No one has a pet liver they're going to bring you, they will have a whole aniimal that happens to have a liver inside of it. recall that pyruvate can be converted to lactate. molecular oxygen can be limiting factor in oxidative phosphorylation. the TCA cycle doesn't USE O2 but depends on it to keep going. But glycolysis is anaerobic. So, in absence of high levels of O2, glycolysis is going along but TCA can't keep up by using the acetyl coA from the pyruvate fast enough, so the pyruvate follows an alternative pathway, being reduced to lactate, and oxidizing NADH to NAD+, which is used in glycolysis (the NAD+ that is.) lactate is kind of a metabolic dead end. it can't be converted into anything except back to pyruvate (it's reversible in the liver.) so you get this lactate buildup, and when the citric acid cycle catches up w/glycolysis, it goes back to pyruvate and then to acetyl CoA and into the TCA. pyruvate can also be converted by transamination to alanine by changing glutamate to alpha ketoglutarate and using the NH2 group from the glutarate to make alanine. when muscle is at rest it uses fatty acid for energy metabolism. when skeletal muscle is exercising, glucose is the preferred substrate. again, you get buildup of pyruvate, because TCA cycle can't operate anaerobically. so in the muscle the pyruvate is then converted to lactate as before, but the muscle cna't convert lactate to pyruvate. so what happens is the CORI CYCLE: during extended muscle activity, glucose --->pyruvate---->lactate + sm amt alanine. skeletal muscle can't do anything with the lactate, so it gets dumped into the blood, which passes through the liver. the liver will grab the lactate and convert it to pyruvate and use it for gluconeogenesis, creating free glucose for use in the skeletal muscles (recall that skeletal muscle also has some glycogen storage as well.) see handout for schematic. note that the alanine can also go to the liver and be converted back into pyruvate. what determines if pyruvate goes to alanine or lactate? well, they're both happening as transaminases are pretty ubiquitous, but it seems that the high concentrations of NADH favor the formation of lactate as opposed to the transamination to alanine. now, what happens in the brain? unless you are extremely starved, brain will use only glucose as energy source. uses about 60% of the blood glucose. - whether you are thinking or not. remember that brain has no glucose 6 phosphatase and neither does muscle. these tissues don't want to lose glucose back to the blood, you see. note that glucagon is the hormone which signals low blood sugar in the liver, but epinephrine is what signals to the muscle to release *its* glycogen stores, and it works in an analogous way to glucagon, by binding to the muscle, causing synthesis of cAMP, release of kinases, etc etc. dr miller wants us to read over everything and ask questions at the review session! a couple of summation type things: make sure to look over summation page of handout.