biochem-carbohydrate metabolism 2-glycogenolysis glycogenesis

BIOCHEM: Carbohydrate Metabolism 2 (Glycogenesis and Glycogenolysis)

Glycogenolysis

Breakdown of glycogen to glucose in the form of G6P Glycogenesis

Synthesis of glycogen Only occurs in the muscles & liver

The formation of glucose from glycogen in the form of glucose-1-phosphate

Glycogen = the storage form of glucose in humans - Composed of both α-1,4-glycosidic linkages and α -1,6-glycosidic linkages - Highly branched

Starch = the storage form of glucose in plants - Composed of α-1,4-glycosidic linkages - Longer than glycogen

Glycogenesis will only be occurring in the muscles and liver while Gluconeogenesis can occur in any organ, but only the muscles and liver will supply glycogen. The muscles supply glycogen only for its own use while the liver will be more generous because it can supply any other organs in the body. Glycogen is stored in the form of granules. That is why when we do the dissection in anatomy, we can see glycogen granules. This is abundant in the liver during the well-fed state. But it will be absent for about 24 hours. So that’s why during the night, after eating and during sleeping, the brain still needs glucose, which came from the process of glycogenolysis in the liver. When exercising, the glycogen in the muscles will be utilized. After 24 hours of fasting, there will be few glycogen granules. These granules contain enzymes that catalyze these granules.

STORAGE: The polymeric nature of glycogen allows energy to be sequestered without the problems of osmotic effects that glucose would cause.

- Glucose is hydrophilic which attracts electrolytes and water. So that is why when we have diabetes, we have lots of glucose in blood vessels and goes to kidneys and draws water and sodium—causing polyuria and fatigue because no ATP is produced.

- Primarily stored in the MUSCLE and LIVER. o Liver has the most glycogen supply. o Muscles have the highest glycogen content because muscle is

more abundant than the liver. - In humans, liver glycogen stores are typically adequate for up to 12 hours.,

without the support of gluconeogenesis. o Between 12 hours between 24 hours, we will have glycogenolysis

and gluconeogenesis, but after 24 hours, all of it will be gluconeogenesis.

Glycogen

- storage form of fuel - Composed of glucosyl residues, mostly linked together by - 1,4 –

glycosidic linkages. Branches arise from frequent - 1,6 – glycosidic linkages

SUBJECT: BIOCHEMISTRY

TOPIC: CARBOHYDRATE METABOLISM 2 (GLYCOGENESIS AND GLYCOGENOLYSIS)

LECTURER: DRA. UY

DATE: DECEMBER 2010


Cellulose - Has β-1,4 glycosidic linkages that is why we don’t have the enzyme for that, and we will not be able to digest it.

Glycogen “Tree”

- Branches at every 4th glucosyl residue within the more central core (glycogenin) of the molecule and less in the outer region

o The branches, 1,6-glycosidic bond and α-1,4-glycosidic bonds. - Imagine glycogen as the tree and at the middle is the glycogenin

Muscle glycogen = fuel reserved for ATP production within a particular tissue Liver glycogen = glucose reserve for the maintenance of blood concentration

After breakfast, there is an elevation of glycogen in the liver (well-fed state), and it goes down during lunch. Before lunch, there is a decrease of glycogen supply, hence the giddiness, so the body has to undergo glycogenesis, and for this to occur, one must eat. By 4 o’clock, it starts to go down again. The most important part is breakfast because you have to break the fast. :>

G6P in glycolytic pathway or gluconeogenesis has to be converted to G1P by the enzyme phosphoglucomutase.

Then, you invest ATP. And then by the glucokinase in the liver, glucose will be converted to G6P. G6P can either go to glycolysis or to other alternate pathway like pentose phosphate pathway. By the enzyme phosphoglucomutase, we convert G6P to G1P because the enzyme, UDP glucosephosphorylase only reacts on G1P. If there is a mutation in this enzyme, there will be no glycogenesis occurring. Body will depend on glycolysis and pentose phosphate pathway as supplier of ATP, so we feed them every now and then but these children will not last long and die before the age of 2. G1P is the molecule that is the basis of glycogenesis. This G1P will enter a pathway with the help of a cofactor which is UTP (Uridine Triphosphate, a high energy compound) and UDP-glucose pyrophosphorylase to form UDP-glucose, liberating 2 PPi. UDP-glucose will be attached to a glycogenin (protein primer of glycogenesis) in a (14) glucosydic bond and the UDP will be liberated. This is due to the enzyme glycogen synthase. More UDP-glucose are added in the glycogenin in the non-reducing ends with the help of glycogen synthase. Glycogen synthase transfers the activated glucosyl moiety of UDP – glucose to the carbon 4 of a glucosyl residue of the growing chain to form a new glycosidic bond at the hydroxyl group of C1 of the activated sugar. The


reducing end of glucose (C1) is always added to the non- reducing end (C4 of a glucosyl residue) of the glycogen chain to form 14 glycosidic bond. Glycogen synthase should have the assistance of branching enzyme (46 transferase) because glycogen synthase will only form 14 glucosyl units. We have to have a branching enzyme so that it will also add 16 glucosyl residue so that it’s not linear. Glycogen synthase is regulated by insulin, glucagon and ephinephrine. In glucose or carbohydrate metabolism, memorize insulin and glucagon. Glucagon and epinephrine in the muscles. Linear glycogen, hence, decrease solubility and will not be utilized. Glycogen synthase cannot form the - 1,6- glycosidic linkages Once an amylose chain of at least 11 residues has been formed, a “branching enzyme” called glucosyl (-4:6) transferase removes a block of about 7 glucosyl residues from a growing chain of amylase chain and transfer it to another chain to produce an - 1,6 – linkage until we form the glycogen tree. How does the branching enzyme work?

- Transfers approximately 5-8 glucosyl residues from the non-reducing end of the glycogen chain to another residue within the chain and attaches the residues via an α-1,6 linkage.

- If there is a mutation or deficiency in the enzyme, there will be hepatomegaly because everything is linear. It will further develop into fibrosis.

Glycogenesis is an activity of 2 important enzymes: glycogen synthase and branching enzyme. The new branch has to be introduced at least 4 glucosyl residues to the nearest branch points What is the purpose of branching of glycogen molecules?

- To facilitate the breakdown of glycogen and aid in solubility - When used as a fuel during strenuous exercise in muscle and in

liver in the fasted state (of only about 12 hours to 24 hrs, there will be glycogenolysis and gluconeogenesis[mostly after 24 hours])

Glycogenin is Required as a Primer for Glycogen Synthesis

- protein - It is self-glucosylating, it will form and add glucose - We have concerted effort of glycogen synthase and branching

enzyme with cofactor UTP to form UDP-glucose. And UDP will be liberated

- Then we have the glycogenin-glycogen complex


Phosphoglucomutase is needed in glycogenesis, glycogenolysis, glycolysis, gluconeogenesis, glycogenolysis, pentosephosphate pathway. We already have a glycogen chain, now we have our glycogen phosphorylase. This enzyme phosphorylates, to form G1P, releasing it from the glycogen molecule. Through the help of phosphoglucomutase it will turn G1P to form G6P. From the glycolytic pathway, G6P can be utilized to form glucose or glycolysis to form pyruvate, and later on lactate to glucose in gluconeogenesis. The control of enzyme glycogen phosphorylase and glycogen synthase: The glycogen phosphorylase is allostericaly inhibited by glucose and ATP, and G6P because we don’t need the energy anymore and will have a feedback on the enzyme. Glycogen synthase is activated by G6P in the liver. What inhibits the degradation activates the formation.

In the muscle on the other hand, it’s the same principle but except in the muscles, we have the calcium and the AMP which are activators of glycogen phosphorylase. The calcium and AMP works by activating the contraction of muscles. G6P activates glycogen synthase. The hydrolytic action of the glycogen phosphorylase is in the non-reducing end (C4). ACTION OF GLYCOGEN PHOSPHORYLASE

- The hydrolytic action will always be at the non-reducing end - The α 1-4 glycosidic link is cleaved by phosphorolysis. ACTION OF DEBRANCHING ENZYME

- Debranching enzyme is required for glycogenolysis - Glycogen phosphorylase is specific -1,4 glycosidic linkages - It stops attacking -1,4 - glycosidic linkages 4 glucosyl residues from an -1,6- branch point - Phosphorylase-limit dextrin is the glycogen molecule that has been degraded to the limit.

o If there is no debranching enzyme, there will be lactase deficiency. - Allows phosphorylase to continue to degrade glycogen - Bifunctional enzyme

o 1,4-1,4-glucan transferase activity o -1,6 – glucosidase activity

- The cooperative and repetitive action of phosphorylase and debranching enzyme result in the complete breakdown of glycogen into G1P and glucose. Glucose in the liver, while G6P in the muscles.

Special Features of Glycogenolysis & Glycogenesis Why store glucose as glycogen and not fat? 1. Fat cannot be mobilized nearly as rapidly as glycogen

- We need 19 cycles in fat metabolism, in glycogen to glucose, we only need 2 enzymes 2. Fat cannot be used as a source of energy in the absence of O2

3. Fat cannot be converted to glucose to maintain blood glucose levels required by the brain

Why not Store it as Free Glucose? Why Waste ATP Making a Polymer Out of Glucose? - It would cost ATP to “pump” glucose into a cell against a concentration gradient, and its concentration would have to

reach about 400 mm in liver cells to match the “glucose reserve” provided by the usual liver glycogen content. - Unless balanced by outward movement of some other osmotically active compound accumulation of glucose would

cause considerable uptake of water with osmotic lysis of the cell. - Patients with diabetes have so much free glucose circulating in the blood vessels. They are not absorbed

because transporters such as GLUT4 are downregulating and insulin resistance. This results in so much free glucose and a corresponding hyponatremia.

Glycogen Synthesis and Degradation are Highly Regulated Glycogen synthase and glycogen phosphorylase are the regulatory enzymes of glycogen synthesis and degradation

respectively. Both catalyze non-equilibrium reactions, & both are subject to control by allosteric effectors and covalent modification


Regulation of Glycogen Synthase and Glycogen Phosphorylase

- Glycogen Synthase Main regulatory enzyme for synthesis

- Glycogen Phosphorylase Main regulatory enzyme for degradation

- Both will catalyze the non-equilibrium - Both are subjected to allosteric effectors and covalent modification - In the liver and muscles, the 2 enzymes important - The glucagon, (for glycogenolysis) - G-protein is a receptor for glucagon in the liver

Has 7-membrane spanning domain - Adenylyl Cyclase

2nd messenger Helps to form cAMP

cAMP acts on cAMP-dependent protein kinase A cAMP-dependent protein kinase A has 2 ends -- the catalytic and regulatory portion. When we have the abundance of cAMP, this will activate cAMP-dependent protein kinase A

(which cleaves the regulatory and catalytic domain). The catalytic portion activates the glycogen phosphorylase kinase B.

Kinase B inactive Kinase A active Phosphorylase kinase, once phosphate is added, it becomes active Synthase – opposite of kinase

Glycogen phosphorylase B is activated by addition of ATP. Glycogen phosphorylase kinase A will now again activate glycogen phosphorylase B into

glycogen phosphorylase A and glycogen is degraded. (Regulation of glycogenolysis in the liver) The same is true in the muscles and liver if there are a lot of epinephrine. cAMP activation, and

then we will have glycogen phosphorylase B is activated further to degrade glycogen. In the muscles, we have conditions of extreme anoxia (ATP depletion). AMP activates

phosphorylase B without it being phosphorylated. In the muscles, we have phosphorylation and at the same time addition of AMP, activating

phosphorylase B without it being phosphorylated. When we have muscle contraction, Ca is released from sarcoplasmic reticulum and calcium

binds to calmodulin. Calmodulin is a subunit of phosphorylase kinase. Always remember that calmodulin in the muscles is associated with phosphorylase kinase

(refers to breakindg down). Phosphorylase kinase is activated without phosphorylation in the muscles. Muscles are endowed with so many regulation to help it in contraction. Phosphorylase kinase can then activate glycogen phosphorylase, and so break down proceeds.

In the liver, the activation of phosphorylase is through phosphorylation through ATP. In muscles on the other hand, activation is without phosphorylation through calcium calmodulin and AMP activation.

Soda and coffee You will also have an enzyme, phosphodiesterase

cAMP is rapidly degraded to 5’-AMP (not useful in contraction of muscles) by phosphodiesterase

Phosphodiesterase inhibitors: Antiasthma drugs:

Theophilin inhibits phosphodiesterase so that there is cAMP elevation Viagra is a phosphodiesterase inhibitor, it inhibits cAMP degradation so that

cAMP continues to elevated and will form relaxation of muscles of blood vessels of the penis to have perfect erection.

AMPLIFICATION OF HORMONE REGULATION: 1 mole of glucagon is being secreted by pancreatic alpha cells activates

hundreds of cAMP. Hundreds of cAMP activates thousands of protein kinase A. Active protein kinase A activates tens of thousands of phosphorylase kinase B. Phosphorylase kinase A activates hundreds of thousands of molecules of glycogen degradation.

When you have glycogen breakdown, your glycogen synthesis has to be inhibited, otherwise we will have a futile cycle.


When we have glucagon, adenylyl cyclase to form cAMP from ATP, inactive cAMP-dependent protein kinase will be activated so that we will have the catalytic portion. Glycogen synthase A is active, this will be phosphorylated and will be inactive.

A glycogen synthase is inactive when it is phosphorylated. Phosphorylase is active when it is phosphorylated. Synthase, when we have a phosphate attached to it is inactive. A phosphorylase when a phosphate is attached to it, is active. So when we have synthase B, which is inactive, glycogen synthesis is

inhibited and glycogen breakdown is activated. Phosphorylase kinase B is inactive, and once it is phosphorylated it will be

activated. It is activated by protein kinase A which will split into 2—a regulatory and catalytic domain. Once the catalytic domain is activated, this will now catalyze and cleave glycogen phosphorylase kinase B to add a phosphate group to phosphorylase kinase to be activated. When it is activated, glycogen phosphorylase B is activated again and glycogen is degraded.

What is the role of insulin? Glycogen synthase B (inactive and phosphorylated) will be acted upon by a

protein phosphatase so that Pi is released and by addition of water, it will be transformed into glycogen synthase A (active)

Insulin is an allosteric activator of protein phosphatase. Insulin acts on the protein phosphatase so that when it is activated,

glycogen synthase will be activated so glycogen synthesis proceeds. Activates the protein phosphatase It will also somehow activates phosphodiesterase Regulation of glycogen breakdown In a few minutes, all the substrates will be available: 1. We have allosteric activators an inhibitors 2. covalent modification of enzymes 3. synthesis of new molecules (gluconeogenesis)

Glycogenesis, glucogenesis,

ENZYMES ACTIVE IN DEPHOSPHORYLATED STATE: Irreversible They are for the synthesis and glycolysis For the production of ATP

1. Glycogen synthase a. Formation of glycogen from UDP-glucose

2. Phosphofructokinase 2 3. Pyruvate kinase 4. Pyruvate dehydrogenase 5. Acetyl CoA Carboxylase

ENZYMES INACTIVE IN DEPHOSPHORYLATED STATE OR ACTIVE IN PHOSPHORYLATED STATE:

1. Glycogen phosphorylase kinase 2. Glycogen phosphorylase 3. Hormone sensitive lipase

In the liver:

- Will have a high glucose levels by increasing phosphorylation of glucose by glucokinase - Glucokinase has very high Km for glucose.


Abundance of insulin-independent GLUT2 - GLUT2 are insulin independent. Whether we have insulin or not, it will continuously transport glucose enzyme inside the

liver - GLUT4 in the muscles and adipose tissues will not transport glucose inside the cell if there is no insulin. - In diabetics, they have the down-regulation of GLUT4 transporters that’s why even if they have insulin secretion, it will

not be transported inside the cell. - Diabetes Type 1 – no secretion of insulin at all, these glucose will not be transported inside the cell because there is no

insulin. So patient will be starving, hence thin appearance. Glucose cannot be absorbed by the muscles and adipose tissues because they do not have insulin, GLUT are dependent of insulin.

- Amino acids from the gut, glucose from the gut, chylomicron remnants serve as fuel on the liver - Adipose on the other hand, we have Insulin-dependent GLUT4 transporters, by hexokinase to form glucose trapped

inside as a source of energy - We eat, and in the intestines, this will be degraded. Final digestive products will be glucose or monossaccharides for

carbohydrates, amino acids for proteins, chylomicrons or fatty acids for fats. - The role of insulin which is secreted by pancreas is an anabolic signal which will promote glycogen synthesis, protein

synthesis, and triacylglycerol synthesis - Immediately after eating, an hour or two, we have a high glucose content of the liver. The hormone insulin is then

elevated. The hormone insulin allows glycolysis (activation of pyruvate kinase). If there is too much of ATP is present, there will be glycogenesis.

- Glycogen will now be utilized as a source of fuel in the starving state. In 24 hours, an individual undergoes gluconeogenesis.

- First thing the patient will feel if we have controlled diabetes person will gain weight o Because there is a signal for the synthesis of glycogen, protein, and triacylglycerol o Insulin is not just for glucose, it will also have the effect of synthesis of glycogen, protein, and triacylglycerol o Insulin is not just a drug for glucose metabolism, it affects all the intermediary metabolism of fats, glucose,

proteins o Glycogen will now be utilized as a source of fuel in the starving state. In 24 hours, an individual undergoes

gluconeogenesis. - Amino acids, glycerol, lactate as a source of glucose GLUCONEOGENESIS - Fatty acids utilized to form acetyl coA. If acetyl coA is utilized it gluconeogenesis, then we will form ketone bodies, and it

will be utilized by the body. There will also be an elevation of acetoacetic acid, acetone, beta-hydroxybuterate. Among the 3 ketone bodies, the acetoacetic acid is the most abundant. But the hydroxybutyric acid is detected in the test.

- Umbilical vein which carries oxygenated blood - Release of insulin and decrease of glucagon, leading to synthesis - Tissues involved in metabolism - In the fasting state, there will be no nutrients absorbed, decreased glucose, decreased insulin, increased counter

regulatory hormone which is glucagon so that there will be release of fatty acids for hydrolysis, release of glucose in gluconeogenesis, release of fatty acids and ketone bodies to provide for the starving tissues

- There are 12 glycogen diseases. 1. Von Gierke’s Disease or Type 1

- most common - deficiency of liver, intestinal mucosa &

kidney G-6 – Phosphatase except muscles because the muscles don’t have G6P. Muscles sequester its own glucose phosphate for energy for its contraction

- diagnosis possible by intestinal biopsy - Manifestation:

a. fasting hypoglycemia b. lactic acadosis

- hyperlipidemia, & hyperuricemia (nucleic acid metabolism is affected) with

- gouty arthritis

2. Pompe’s Disease – Type II - caused by the absence of - 1,4 –

glucosidase (or acid maltase), normally found in lysosomes

- Not a defect on glycogenesis or glycogenolysis, but rather a defect in lysosomes

i. Lysosomes are abundant in the GIT (there is also in kidneys and muscles)

- accumulation of glycogen mostly in lysosomes in virtually every tissue

- Severe hypoglycemia, massive cardiomegaly & cardiomyopathy occur & death results from heart failure

i. There is severe hypoglycemia because there is no formation of glucose

ii. Cardiac muscles are abundant of lysosomes

- Not that severe and not common.


3. Cori’s Disease or Type III (not the PNoy’s disease :> ) - caused by deficiency of glycogen

debranching enzyme, α-1,6 glucosidase. - Glycogen accumulates because only the

outer branches can be removed by - phosphorylase. - hepatomegally & other clinical

manifestation are similar to, but milder than those in Von Gierke’s disease

4. Anderson’s Disease or Type IV - Opposite of Cori’s disease - Deficiency of the branching enzyme,

glucosyl 4:6 transferase - Normal amount of glycogen but with very

long outer branches - Liver cirrhosis and death before 2 years of

age

5. McArdle’s Disease or Type V - caused by the absence or deficiency of

skeletal muscle glycogen phosphorylase - The liver enzyme is normal

- patients suffer from painful muscle cramps and are unable to perform strenuous exercise because muscle glycogen stores are not available to the exercising muscle

- the normal increase in plasma lactate following exercise is absent

6. Her’s Disease or Type VI

- Deficient enzyme is liver phosphorylase - Mild hepatomegaly and hyperlipidemia - Mild hypoglycemia or no symptoms at all

7. Tarui’s disease or Type VII - Enzyme deficient is phosphofructokinase - Patients have painful muscle cramps with

exercise 8. GSD Type VIII

- Phosphorylase kinase (liver) is the enzyme deficient

- Mild hepatomegaly and hypoglycemia - Cerebral palsy, growth retardation,

delayed motor development and increased blood lipids

biochem-carbohydrate metabolism 2-glycogenolysis glycogenesis

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