Glycogen

Debranching enzyme has two independent active sites catalyzing transferase and glucosidase reactions

- (4:4 transferase activity)The transferase (glucosyl transferase) enzyme removes the three glucose residues (as a trisaccharide) adjacent to the branch point (the α-1,6 linkage) and transfers them to the end of another row. This is a 4:4 transferase because the enzyme breaks an α−1,4 bond and forms another α−1,4 bond. - (1:6 glucosidase activity)A debranching enzyme (α-1,6-glucosidase or amylo-6- glucosidase) removes the α-1,6 glucosidic bond to release one glucose molecule as glucose (NOT as glucose-1- phosphate). So some energy is needed to utilize the glucos

Von Gierke's disease (GSD I)

- A deficiency in glucose-6-phosphatase Accumulation of glycogen (normal structure) in kidney and liver cells - Diagnosis by genetic testing, enzyme assay & PE; a kidney or liver biopsy may be needed for confirmation - Liver, kidney, and intestine; hepatomegaly (fatty liver), progressive renal disease - Elevated lactate and urea Fasting hypoglycemia - Treated with nocturnal gastric infusions of glucose or uncooked cornstarch and frequent consumption of carbohydrates

McArdle Disease (GSD V)

- A deficiency of acid maltase (also called (1 4)- glucosidase) - Normal blood glucose levels - Accumulation of normal glycogen in liver & muscle - In muscles, excessive glycogen impairs normal function. - Massive cardiomegaly: early death from heart failure - Diagnosed by enzyme assay - ERT treatment: treated with MYOZYME - Future gene therapy

Glycogen Storage Disease (GSD)

- A group of inherited genetic disorders that cause glycogen to be improperly stored in the body. - In a person with a GSD, some of these enzymes are defective, deficient, or absent. This causes buildup of abnormal amounts and types of glycogen in liver and/or muscle tissues. - GSDs usually affect functioning of the liver, the muscles, or both. - General GSD common symptoms in pediatric patients are: - Low blood sugar - Enlarged liver - Slow growth - Muscle cramp -- The GSDs that mainly affect the liver are types I, III, IV, and VI. -- GSDs that mainly affect muscles are types V and VII. -- Type II affects nearly all organs, including the heart.

Lewis disease (GSD 0)

- Affects both males and female Deficiency in glycogen synthase Fasting hypoglycemia - No symptom in early infancy - Slower than expected growth (mild delay) - When exercising, tired more quickly, muscle cramps ketones in the urine - Genetic DNA testing: Testing for Type 0 GSD just recently became available - Treatment: avoiding fasting, a high in protein

Overview of Regulations of Glycogen Metabolism

- Control of glycogen metabolism involves: • Allosteric regulation of both GS & GP • Substrate availability • Enzyme-catalyzed covalent modification of both GS &GP: ØActivating the glycogen synthase ØInactivating the glycogen phosphorylase (via glycogen phosphorylase kinase

Andersen disease (GSD IV)

- Deficiency in branching enzyme - Characterized by long strands of glycogen molecules that resemble amylopectin - Infantile muscle hypotonia - Splenomegaly, and progressive hepatomegaly. - Normal at birth but fails to thrive, little weight gain, retarded growth, poor muscle tone. - Death typically occurs by two years of age. Infants who survive beyond their first birthday develop cirrhosis of the liver by age 3-5 and die as a result of chronic liver failure. - Treatments: liver transplantation, muscle and heart disease may still be a problem

Identify the sources of carbohydrates in a 24-hour period with metabolic states ranging from fed to starved states

- Glucagon is Sources of blood glucose in a 24 hour period; it is a main major storage of glucose - Although glycogen is mainly found in both liver and muscle, this storage form is used for different purposes in each tissue. (Intestine and kidney also store glycogen). -

In the liver, the glycogen is stored in the --- state to ---- as needed. It gets depleted during ----- after 12-18 hours.

- fed - maintain blood glucose - a fasted state

Molecular basis: Children with GSD I condition are unable to release glucose from liver glycogen. A deficiency in

- glucose-6- phosphatase

Utilization of Endogenously Stored Glycogen

- Step 2: Debranching enzyme (4:4 transferase) removes a tri-saccharide within 4 residues from a branch point (a-1,4 linkage) and then the last glucose at the branch. - NOTE: No energy requirements. Product (G1P) is phosphorylated Step 1: glycogen phosphorylase, removes one glucose molecule at a time and converts it to glucose-1-P.Glycogen phosphorylase breaks a-1,4 linkages

Type IV summary

- Summary: Abnormal glycogen in liver & spleen - 2◦ symptoms Failure to thrive, slow growth - Treatment: Liver transplant; ation - OutlookDeath <2 yo

steps of glycogenesis

- Step 1: glycogen synthesis begins with conversion of glucose to glucose-6- phosphate (G6P) - Step 2: G6P to glucose-1- phosphate (g1p). G6P is an intermediate in glycolysis, pentose phosphate pathway, and gluconeogenesis. Excess glucose going through glycolysis can be diverted into glycogen storage in response to insulin. Step 3: A high-energy glucose derivative is formed by UDP-glucose pyrophosphorylase. Thisconverts g-1- phosphate to a high-energy form, UDP-glucose. Step 4: α-1,4 linkages are formed. The enzyme glycogen synthase transfers the glucose in UDP-glucose to one of the growing glycogen branches. The released UDP can be reconverted to UTP by reaction with ATP. Nucleoside diphosphate kinase UDP + ATP ←→ UTP + ADP - Step 5: branches are formed. When about 11 glucose molecules are added to one growing chain of glycogen, a branching enzyme, 4:6 transferase moves a chain of 6-8 glucose molecules to form a new branch chain starting with an α-1,6 linkage at the ranch.

Sources of blood glucose in a 24 hour period

- Glycogen is a major storage form of glucose.2.Although glycogen is mainly found in both liver and muscle, this storage form is used for different purposes in each tissue. (Intestine and kidney also store glycogen): - a. In liver, the glycogen is stored in the fed state to maintain blood glucose as needed. It gets depleted during a fasted state after 12-18 hours. - b. In muscle, the glycogen is stored to provide energy during prolonged exercise. It is not affected by short periods of fasting (a few days) and moderately depleted in prolonged fasting (weeks The regulation of glycogenesis and glycogenolysis pathways is somewhat different in liver and muscle, reflecting different purposes for glycogen. 4.Overnight fasting and light work do not reduce blood sugar levels below normal levels. 5.Consumption of a large sugar meal causes an increase in blood glucose, but this is kept in check and not allowed to rise to abnormal levels. 6.Normal levels are again reached within 2 hrs after ingestion of a glucose load.

Glycogenin

- Glycogenin forms a glycosidic bond via tyrosine to a glucose reducing end via autoglycosylation *** Glycogen synthesis can only occur by extending an already existing α (1→ 4)-linked glucan chain. Therefore, how can it get started in the first place? Answer: The first step in glycogen synthesis is the attachment of a glucose residue to the -OH group on Tyr-194 of GLYCOGENIN. This attachment step is done by the enzyme TYROSINE GLUCOSYLTRANSFERASE. Glycogenin then autocatalytically extends the glucan chain by up to 7 residues long (also donated by UDPG). Glycogen synthase can then attach glucose residues to this glycogen "primer". Each molecule of glycogen is associated with ONE molecule each of glycogenin and glycogen synthase.

Allosteric Regulation by G6P (So high levels of G6P directly activate storage)

- High levels of G6P after a meal - Allosterically activates the inactive GS (phosphorylated GS) in liver and muscle

Regulation of glycogenolysis in MUSCLE

- Hormonal regulation of glycogenolysis is regulated by epinephrine (but not by glucagon) via cAMP in the same way glucagon regulates liver glycogenolysis. 2. - Allosteric regulation (increases) AMP levels activate the inactive form of glycogen phosphorylase, causing the enzyme to be more active. (increases) Calcium ions further activate the inactive form of glycogen phosphorylase kinase, causing the enzyme to be more active.Think of a pool of enzymes—some are phosphorylated, and others are not. •Initially, most are not phosphorylated, but with time, more and more become phosphorylated. •These allosteric regulators (AMP & Ca2+) act in the immediate situation arising from the muscle's energy needs, during which time the phosphorylation (covalent modification) process is happening. •Eventually, all of the enzymes will be phosphorylated, and (increases) AMP activation is no longer needed. Net result—immediate energy to the muscle.

Glycogenesis: Allosteric Activation of Glycogen Synthase by G6P

- If glucose levels in the cell, and glycolysis/TCA is going at top speed, but not keeping up with demand, - G6P Some G6P can be diverted into glycogensis.

Allosteric regulation by glucose 6-phosphate (G6P)

- In fed state, glycogen synthase is allosterically activated by elevated concentrations of G6P. - G6P allosterically activates the inactive (phosphorylated) form of glycogen synthase. This means the inactivation of glycogen synthase by phosphorylation is partially overcome by glucose-6- phosphate. - G6P is an intermediate in glycolysis. •If glucose levels (level-up) in the cell and glycolysis/TCA is going at top speed, but not keeping up with demand, G6P levels (level up). A backup occurs at glycogen synthase. - Some G6P can be diverted into glycogen for storage. So high levels of G6P directly activate synthesis of glycogen allosterically via G6P.

Insulin's effect ⇒ Insulin stimulates glycogen synthesis.

- Insulin, via an insulin factor, activates phosphatase enzyme to remove phosphate groups from glycogen phosphorylase, thus INACTIVATINGglycogen breakdown. - Insulin, via an insulin factor, activates phosphatase enzyme to remove phosphate groups from glycogen synthase, thus ACTIVATING glycogen synthesis. - Insulin stimulates the uptake of glucose by muscle (Glut-4).

Cori/ Forbes disease (GSD III) or debrancher enzyme deficiency

- Molecular basis: GSD III is a rare disease of variable severity affecting the liver, heart, and skeletal muscle. It is caused by deficiency of glycogen debranching enzyme, a key enzyme in the breakdown of glycogen, characterized by shorter branches in glycogen. Symptoms: Patients may have low blood sugar, a high level of fats in the blood and delayed growth. Symptoms related to liver disease and progressive cardiac and skeletal muscle involvement vary in age of onset, rate of disease progression and severity. Diagnosis: condition has a wide clinical spectrum. Children are often diagnosed because they have a swollen abdomen due to a very large liver.Diagnosed by liver biopsy. Treatment: Regular feeding, including a high protein diet supplemented with corn starch to help prevent muscle breakdown and maintain normal blood glucose levels

Initiation of Glycogen Synthesis

- Most of the glycogen synthesis occurs by lengthening the polysaccharide chains of a preexisting glycogen molecule. Synthesis can also begin by using the protein glycogenin which serves as a primer by glucosylating itself (autoglycosylation). That is, glycogenin via a a OH groups of tyrosine within its structure makes a bond to a glucose.The glucose on glycogenin then further elongates the polysaccharide chain.

What are the regulatory implications for the cell with regard to ATP and AMP, given that the former is generally high, and the latter is low?

- Normally, [ATP] is 5-10 mM, while [AMP] is < 0.1 mM, thus AMP is a much more sensitive indicator of a cell's energetic state. Small changes in ATP concentration are amplified into large changes in AMP concentration, hence many regulatory processes hinge on changes in the concentration of AMP. - Also, the skeletal muscle PFK-2 has no phosphorylation site and is regulated simply by substrate availability, i.e., fructose 6-phosphate (F6P). - When F6P is abundant, PFK-2 actively produces F -2,6-bis phosphate which allosterically activates PFK-1. - In the absence of F-2,6bisP, the allosteric activator, the activity of PFK-1 is reduced (inhibiting glycolysis) and the activity of FBPase-1 is enhanced (stimulating gluconeogenesis), thus enabling the liver to replenish blood glucose. In the skeletal muscle, F-2,6bisP is always present.

Glycogenolysis:

- Physiological increase in blood glucose utilization - Exercise - Pathological result of blood loss - Psychological response to acute & chronic threats

Type 0 summary

- Symptoms: Before breakfast drowsiness, tiredness, pale, vomiting - 2◦ symptoms: Muscle cramps (accumulated lactic acid - Treatment: Regular snacks, cornstarch to reduce hypoglycemia - Outlook: Good with adherence to recommended feeding.

In muscle, the glycogen is stored to provide energy during ---- . It is not affected by short periods of fasting (a few days) and moderately depleted ----

- prolonged exercise - in prolonged fasting (weeks).

What is the biological advantage of synthesizing glycogen with many branches?

1. More soluble 2. Enzymes act at the non-reducing ends 3. Branched glycogen has far more ends effectively increasing the rate of glycogen synthesis and breakdown

Initiation of glycogen synthesis

1. a preexisting glycogen molecule 2. glycogenin

During glycogenolysis, why is glycolysis active in MUSCLE? Why are the MUSCLE glycolytic enzymes active?

2021Glycogen -25 4. During glycogenolysis, why is glycolysis active in MUSCLE? Why are the MUSCLE glycolytic enzymes active?Epinephrine has the same effects as glucagon (covalent modification- phosphorylation) on the enzymes in the pathways. Why aren't these enzymes inactive? Let's look at the three regulatory enzymes in muscle glycolysis:•Hexokinase is the first regulatory enzyme and isconstitutive. Therefore, a constantphysiological level of this enzyme is always expressed . Glucose released at the branch points of glycogen would be catalyzed byhexokinase. •Phosphofructokinase-1(PFK-1) isthe second regulatory enzyme. During muscular contraction, myosin ATPase increases AMP levels from hydrolysis of ATP to ADP. This increase in AMP levels allosterically stimulates glycogenolysis. AMP is an allosteric activator of PFK-1. So AMP activates both glycogenolysis and glycolysis.

Lysosomal degradation of glycogen

Approximately 1-3% of glycogen is degraded by lysosomal enzyme α(1→ 4)-glucosidase. In lysosomes, normal α-1,4 glucosidase is involved in debranching and hydrolysis of both α-1,4-and α-1,6-glucosidic linkages at acidic PH of 5 and necessary to break down glycogen.

Fate of glucose-1- phosphate (G1P)

G1P generated from the breakdown of glycogen is converted to G6P by the action of . - Glucose-1- P (i. phosphoglucomutase) -->/<-- Glucose-6- P (phosphoglucomutaseglucose-6 - phosphatase (inducible by glucogon) -->Glucose (liver)

Glucagon effect -⇒

Inactivating glycogen synthase (by phosphorylation) and activating glycogen phosphorylase (by phosphorylation)

Regulation of glycogenolysis in LIVER

Glucagon indicates low blood glucose levels. Therefore, the body needs to break down glycogen to provide blood glucose. Glucagon acts through cAMP and activates protein kinase A which results in glycogenolysis. •Glycogen synthase ( inactive) •Glycogen phosphorylase (GP) kinase ( active) ---> phosphorylates GP --> Glycogen phosphorylase ( active).The net result is glycogen degradation by inactivating glycogenesis.

Liver- Her's disease GSD VI

Glycogen accumulation Mild hepatomegaly Symptoms like Von Geirke's, but milder

Glycogenolysis: Utilization of Endogenously Stored Glycogen

Glycogenolysis occurs when glucose levels are low; G6P from glycogen breakdown can join G6P from gluconeogenesis, resulting in higher levels of blood glucose. The major enzyme in glycogenolysis, glycogen phosphorylase, uses pyridoxal phosphate (PLP) as a cofactor. PLP can catalyze transamination reactions that are essential for providing amino acids as a substrate for gluconeogenesis.

Regulation of glycogenesis

Hormonal regulation of glycogen synthesisWhat hormone is elevated? Insulin What are the physiological effects of insulin on the cell? A phosphatase enzyme removes a phosphate group from glycogen synthase enzymes (i.e. Covalent modification by dephosphorylation) - When ↑glucose, ↑insulin, ↓glucagon, phosphatases are active

Source of glucose in a 24 hr period

Liver maintains blood [glucose] at ~5 mM; if it drops to half of this, a coma results. Upon blood [glucose] decrease, the liver releases glucose to the blood; glucose triggers pancreas to release glucagon, which causes increase [cAMP] in liver, which stimulates glycogen breakdown. Glucose diffuses freely out of liver cells, causing an increase in blood [glucose]. High blood [glucose] causes release of INSULIN from the pancreas to the blood. The rate of glucose TRANSPORT across many cell membranes increases in response to insulin

Major principles of metabolic regulation

Maximize the efficiency of fuel utilization by: 1.Preventing the simultaneous operation of opposing pathways (i.e., futile cycles). 2.Partition metabolites appropriately between alternative pathways. 3.Draw on the fuel best suited for the immediate needs of the organism. 4.Shut down biosynthetic pathways when their products accumulate

Pompe disease (GSD II)

Pompe disease is characterized by accumulation of abnormal amounts of glycogen primarily due to decreased degradation. Primarily affects muscle tissues•A deficiency of α(1→ 4)- glucosidase(lysosomal acid glucosidase or acid maltase). -- A deficiency in this enzyme results in accumulation of excess amounts of glycogen (normal structure) -- In certain tissues, especially muscles, excessive accumulated glycogen impairs their ability to function normally. -- Massive cardiomegaly: early death from heart failure -- Normal blood glucose levels -- Diagnosed by enzyme assay[Hirschhorn, 2001] -- ERT treatment: treated with MYOZYME (alglucosidase alfa); future gene therapy(currently testing on animals)

Muscle and liver- cori disease GSD III

Shorter branches Mild hypoglycemia Mild hepatomegaly Liver Transplantation

pyruvate kinase

The muscle isoenzyme of pyruvate kinase doesn't get phosphorylated. Even though the hormonal effects of epinephrine are the same as glucagon, it doesn't affect the pyruvate kinase in the muscle and pyruvate kinase remains unphosphorylated and active

Glycogen Storage Disease

There are more than 10 inherited inborn errors of metabolism that affect synthesis or breakdown of glycogen are called glycogen storage diseases (GSDs). These mostlyautosomal recessive disorders usually occur in childhood. Symptoms vary, but some are life threatening. A group of inherited genetic disorders that cause glycogen to be improperly stored in the body. In a person with a GSD, one of these enzymes is defective, deficient, or absent. This causes the buildup of abnormal amounts and types of glycogen in liver and/or muscle tissues. GSDs usually affect functioning of the liver, the muscles, or both. Common symptoms of GSDs in pediatric patients are: -Low blood sugar -Enlarged liver -Slow growth -Muscle cramps

The anomeric carbon that is not attached to another glycosyl residue (the reducing end) is attached to the protein glycogenin by

a glycosidic bond via the amino acid tyrosine.

Glycogen structure

glycogen is a branched molecule Glycogen is composed of hundreds of glucose molecules linked together. If all the glucose molecules in glycogen were lined up only in α1,4 linkages, they could only be synthesized (or degraded) one at a time, taking a long time. To make these processes quicker, glycogen has occasional branches of α-1,6 linkages in addition to the long α-1,4 strings of glucose. Having the branching means that several glucose molecules can be added (or removed) from glycogen simultaneously.

A deficiency in the enzyme glycogen synthase results in

very low amounts of glycogen stored in the liver. A person between meals can develop hypoglycemia.

McArdle's disease (GSD V)

•A deficiency of muscle phosphorylase •Most common types of GSD (1 in 100,000). •Symptoms include: -High levels of normal structure glycogen in muscle -Temporary weakness of exercising skeletal muscles; intolerance with myalgia, early fatigue, painful cramps -Rhabdomyolysis à Myoglobinuria à Red Urine -No rise in lactate during strenuous exercise -Normal renal and hepatic development