Lecture 15 [virg Notes].ppt

Lecture 15. Metabolism of Glycogen, Fructose and Galactose • Glycogen Metabolism • Fructose Metabolism • Galactose Metabolism

In animals We store as glycogen Glycogen is the storage form of glucose in all cells. However, it is especially because of osmosis and abundant in liver and muscle. because ATP is not needed Glycogen is present in cytosol in the form of granules ranging in diameter to pump the glucose and from 10 to 40 nm and consists of ~ 55,000 glucose molecules.- very large because water would rush polymer into the cell and cause lysis Muscle Goes from high 1-2% glycogen by weight, 400g glycogen/ 35 kg muscle concentration to low Liver concentration 10% glycogen by weight, 100g glycogen/ 1.6 kg liver Pressure is generated Why store glucose as glycogen? Why not store as free glucose? between the 2 solutions and Review it’s independent of the type Osmosis is a colligative property. That is, it only depends on the number of and size of solutes, just the solute (e.g., glucose) molecules. Osmosis is independent of the type or size number of solutes of the solute molecule.

Glycogen Metabolism

The concentration of glycogen in the liver is .00001 mM. However, it is a polymer of glucose. Therefore you would need 400 mM glucose to match glycogen. This would require using ATP to pump glucose into the cell against the concentration gradient. More importantly, water would rush into the cells and cause lysis.

Why do we need a storage form of glucose? Answer depends on the type of tissue. Muscle glycogen serves as a fuel reserve for ATP synthesis during muscle contraction. – when ATP is needed the muscle glycogen is broken down and goes to GCP and then glycolysis to generate more ATP

Different tissues store glycogen differently

Liver glycogen serves as a source of blood glucose. Glycogen is broken down to make GCP and a phosphate group is made and the glucose is transported across the membrane in order to maintain the blood glucose Red blood cells and the brain absolutely require glucose for energy metabolism. They consume ~80% of the 200g of glucose metabolized in the body/day. There is only ~10g of glucose in the plasma so the blood glucose must be replenished constantly. Otherwise, hypoglycemia develops leading to confusion and coma.

3 primary sources of blood glucose A. food- sporadic and not reliable because it depends on the diet B. Gluconeogenesis- slow to respond to a falling blood glucose level C. Glycogen degradation- rapidly mobilized form of glucose

C- It reacts much faster than gluconeogenesis Glycogen content in liver after meals, the glucose stores in the form of glycogen At night the glycogen starts to be broken down to glucose

Review of the Structure of Glycogen Branch points every 4 to 6 glucose molecules. (caused by 16 glycosidic bond)

Branching provides numerous sites for synthesis and degradation

Biosynthesis of glycogen occurs very fast because many sites can happen at the same time

Glycogen Breakdown (Glycogenolysis) - Glucose to become G6P then pyruvate- this is glycolysis - Pyruvate to glucose is gluconeogenesis - Glycogen breakdown to G6P then goes to glucose or glyclysis to become pyruvate - Glycogen synthesis- starts from G6P - Pyruvate is key intermediate

Glycogen Breakdown (Glycogenolysis) NOTE: Anaerobic glycolysis from glycogen produces 3 ATP for each glucose residue because the hexokinase reaction is not required. - Left side- glycogen break down- glycogen goes to G1P then G6P and it’s 3 enzymes involved - First 2 enzymes break it down to G1P and the last moves it to be G6P - Breakdown is much faster than gluconeogenesis - Right side- glycogen synthesis- 4 enzymes work to make it- it’s anaerobic and produces 3 ATP Glycogen phosphorylase is the key enzyme

Glycogen Phosphorylase

Phosphoglucomutase

- Involved in glycogen breakdown - The first one catalyzes the cleavage of the 1,4 glycosidic bond - And then phosphate is added to make G1P - Then the 2nd enzyme catalyzes the transfer of the phosphate group to the 6th carbon to make it G6P and then it can enter into glycolysis - Don’t have to know the mechanism Phosphoglucomutase: G1P to G6P

Debranching Enzymes Has 2 activities 1. Transferase activity- transports 3 residues from the branch to the main chain 2. Break the alpha 1,6 glycosidic bond Theses are the debranching enzymes When you reach the last 4 residues, the debranching comes in and takes the last 3 and then cleaves the last one

Glucose- 6- phosphatase

Glucoses are cleaved 1 by 1 G6P can enter into glycolysis Glucose is then delivered to blood G6P cannot be transported across the plasma membrane because there’s a negative phosphate group so it must be dephosphorylated to become glucose to be transported across the membrane In the ER, there is glucose 6 phosphatase- this cleaves the phosphate and makes glucose GLUT2 delivers glucose to the blood circulation

Glycogen Synthesis – not the opposite of glycogen break down How was it discovered that there was a separate pathway for glycogen synthesis? In other words, what made us think that glycogen synthesis is not simply the reverse of glycogen breakdown. McArdle’s Disease •Glycogen storage disease •Inherited disorder, results in painful muscle cramps upon strenuous exercise •Patients have no glycogen phosphorylase activity and thus no glycogen breakdown. •However, muscles contain high quantities of glycogen. •Thus, there must be separate pathways for synthesis and breakdown

UDP-glucose pyrophosphorylase

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- So if they do heavy exercise, the muscle contraction requires a huge amount of ATP but they can’t break down glycogen so there isn't’ a sufficient amount of ATP to be generated so they get muscle cramps - The glycogen synthesis in their bodies was normal but the degradation was abnormal so they have to use different pathways - Understand McArdle’s disease- this is proof that synthesis and breakdown and completely different reactions - UDP- know the name- this is the intermediate in glycogen synthesis and it’s related to galactose metabolism - G1P- interacts with the UTP and diphosphate is the leaving group to form UDP

Glycogenin Glycogenin is a 349 amino acid protein with glucosyltransferase activity. Extends chain up to 7 glucose residues. There is one glycogenin molecule/glycogen molecule. It is needed because glycogen synthase cannot use free glucose as a substrate.

Reaction repeats 6 times so 7 glucose residues are added to the chain

Glycogen Synthase

This is the enzymes that catalyzes this reaction Everytime 1 glucose is added

Branching Enzyme

Transfer the residues from the main chain to the branch to make an alpha 1-6 glycosidic bond

Regulation of Glycogen Metabolism • Glycogen synthesis and breakdown are both exergonic. • If both pathways operated simultaneously, all that would be achieved would be wasteful hydrolysis of UTP. • Thus, glycogen phosphorylase and glycogen synthesis are under stringent control. Allosteric control Allosteric control of glycogen phosphorylase and glycogen synthase allow adjustment of enzyme activity to meet the needs of the cell in which enzymes are expressed.

Thus, when the demand for ATP is high (ie. low ATP, low glucose-6-P and high AMP), glycogen phosphorylase is stimulated, and glycogen synthase is inhibited. Conversely, when ATP and glucose6-P are high, glycogen phosphorylase is inhibited and glycogen synthesis is favored.

- Red negative means it’s an inhibitor - When ATP is high, you don’t need to make more glucose - G6P is an activator because when it is high, it should be converted to glycogen - Calcium and AMP are activators in muscle - When AMP is high, ATP is low - Calcium is an activator because when it’s high, there is activity for the muscle that requires energy and turns on the phosphorylase

Regulation of Glycogen Metabolism Hormonal Control In contrast to allosteric control, hormonal control allows glycogen metabolism to adjust to the needs of the entire organism. Covalent modification of glycogen phosphorylase and glycogen synthase. Glycogen phosphorylase is activated by phosphorylation (b → a), while glycogen synthase is inactivated by phosphorylation (a →b). Dephosphorylation inactivates glycogen phosphorylase and activates glycogen synthase.

Regulation of Glycogen Metabolism Control of Glycogen Phosphorylase Activity

Don’t know detail

Regulation of Glycogen Metabolism How is phosphorylase kinase regulated?

Protein kinase A is activated by glucagon and then it phosphorylated kinase and makes it active and that phosphorylates the glycogen phos and makes it active

Regulation of Glycogen Metabolism Protein kinase A (cAMP-dependant protein kinase) Glucagon can activate the formation of cAMP and then it can activate the enzymes

Regulation of Glycogen Metabolism Where does cAMP come from?

Regulation of Glycogen Metabolism What regulates adenylate cyclase?

Hormones (especially glucagon) binds to receptors and active the adenyl cyclase and it makes cAMP

Regulation of Glycogen Metabolism What hormones start this process?

Peptide hormone made in the pancreas used for maintaining the steady-state level of blood glucose. Thus, it only activates glycogenolysis in liver, not muscle. Synthesized in and secreted from the adrenal medulla. Fight or flight hormone used for generating energy very rapidly (i.e., muscles) and throughout the organism (i.e. liver)

Regulation of Glycogen Metabolism Putting it all together

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Glucagon work in coordination and actives the cAMP formation and then it binds to protein kinase A and it’s now activated and phosphor. The enzymes glycogen phosphorylase to make it activate and can go into the glycogen break down So glucagon increases glyocgen break down

Calcium regulates phosphorylase kinase via calmodulin.

Calmodulin is a ubiquitous eukaryotic Ca2+ binding protein that participates in numerous cellular regulatory processes. Calcium binding to calmodulin induces a conformational change that exposes a patch of hydrophobic amino acids. This patch allows calmodulin to bind to phosphorylase kinase and activate it.

- Caused by the calcium calmodulin - When they bind there is a conformational chain so it can bind to phos. Kinase and activates it and then glycogen phosphoylase is phosphorylated and it’s active - 3 different things that active this 1. ATP hydrolysis – AMP directly activates the enzyme- allosteric 2. Neuron cells signal release of calcium and then it binds to calmodulin and then binds to kinase 3. cAMP activates protein kinase A 3 pathways all work together to degrade glycogen

How do glucagon and epinephrine affect glycogen synthesis? Note: Insulin is antagonistic to glucagon and can reverse all of the mechanisms of glucagon. The mechanism is still under invesitgation. However, two mechanisms are:

1. Insulin promotes glucose transport into the cell which inhibits glycogen phosphorylase activity. 2. Insulin stimulates protein phosphatase activity.- can activate glycogen synthesis

***To remember whether a particular enzyme has been activated or inhibited by cAMP-dependent phosphorylation, consider whether it makes sense for the enzyme to be active or inhibited under fasting conditions (In a PHast, PHosphorylate).***

Why is the mechanism of regulation so complicated? Because you have a small number of hormone molecules, such as epinephrine. However, in a fight or flight situation, you need a massive response. Thus, the cascade system allows for amplification of a small signal. High conc of AMP and calcium promote glycogen breakdown

Glycogen Storage Diseases

Only know McArdile

Inherited disorders that result in glycogen that is abnormal either in quantity or quality. Glycogen storage diseases that affect the liver cause heptomegaly (enlarged liver) and hypoglycemia (low blood sugar). Disorders that affect the muscles result in cramps and weakness.

This is the regulation for glycogen degratation and synthesis Glycogen phosphorlyase is key to degradation Glycogen synthase is key to synthesis G6P is the activator of synthesis Inhibition of glucagon activates the glycogen synthase and inhibit the glycogen phosphorylase and inhibits the degradation In fasting state, the glucose is low so inhibits insulin and releases glucagon and increases protein kinase and phosphorlyates the enzymes and activates glycogen breakdown

Metabolism of Fructose Fructose

10% of the calories in the Western diet are supplied by fructose (~50 gms/ day) Sources: Sucrose high fructose corn syrup (55% fructose/ 45% glucose) fruits honey

2 Different Pathways of Fructose Metabolism On the right is glycolysis Galactose is related to G6P Fructose goes through 2 different pathways, in the muscle it goes to F6P and in the liver it goes to GAP

Fructose Metabolism

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- In muscle and liver it’s different - In muscle it’s simple, fructose becomes F6P- phosphorylation occurs in the carbon 6 and it’s the second compound in glycolysis and the enzymes is hexokinase (for phosphorylation of glucose and fructose) - Hexokinase has higher affinity for glucose so it’s the major in muscles - Fructose has lower activity for hexokinase - In the liver fructose goes through complicated reactions fructose kinase phosphorylates fructose at carbon 1 so it becomes F1P and then it can be cleaved into 2 fragments (Glyceraldehyde) which becomes 2 GAP and it enters glycolysis - Don’t need to know the reaction, just have an idea that in liver it’s more complicated and then it goes to GAP to enter glycolysis - In muscle hexokinase catalyzes the phosphorylation

Fructose metabolism • The metabolism of fructose occurs primarily in the liver because the Km of hexokinase for fructose is 20 times higher than for glucose. Thus, the V/K or catalytic efficiency of hexokinase for fructose is much less than for glucose. The muscle pathway is only important when fructose is high • Note: fructose metabolism in the liver bypasses phosphofructokinase that can lead to lipogenesis. This suggests a link between high fructose soft drinks and obesity.

Hexokinase activity is lower for fructose and higher for glucose Fructose usually can’t be metabolized in muscles so the major metabolism is in liver

Fructose can be synthesized by the Polyol Pathway Fructose is the principal energy source for spermatozoa in the seminal fluid.

In nerves, the accumulation of sorbitol contributes to neuropathy in peripheral nerves.

Fructose can be synthesized by the Polyol Pathway Fructose is the principal energy source for spermatozoa in the seminal fluid.

Don’t know detail

In nerves, the accumulation of sorbitol contributes to neuropathy in peripheral nerves.

Galactose Metabolism The major dietary source is from hydrolysis of lactose (milk sugar). Unlike glucose and fructose, no catabolic pathway exists to metabolize galactose. Therefore, galactose is converted into a metabolite of glucose by the Leloir pathway. First galactose becomes G1P Then G1P interacts with UDP glucose to become UDP galactose And then it becomes G6P to enter glycolysis UDP galactose can be converted to UDP glucose by an enzyme

Homework Questions • Why is glycogen the storage form of energy instead of free glucose in animal cells? – Higher molecular concentration of glucose to match glycogen would increase osmosis, and would require using ATP to pump glucose into the cell against the concentration gradient

• Describe three primary sources of blood glucose to maintain blood glucose content. – Food, gluconeogenesis, and glucogenolysis

• How was it discovered that there was a separate pathway for glycogen synthesis from glycogen degradation? – McArdle’s Disease: patients have no glycogen phosphorylase activity and thus no glycogen breakdown. However, muscles contain high quantities of glycogen

• How does the secretion of glucagon affect glycogen degradation? – Glucagon secretion will activate cAMP-dependent protein kinase A, resulting in active glycogen phosphorylase, leading to glycogen degradation