Flashcards in glycogen metabolism and storage disease type Deck (57):
what enzyme is deficient in Von Gierke disease?
Glucose 6-phosphate ( bound in ER, Ia) or glucose 6-P translocase(Ib)
what is the glycogen amount in the Von Gierke disease?
High glycogen amount in the liver and the kidney (Normal glycogen structure)
What organs are affected in the Von Gierke disease?
Liver: severe hypoglycemia
What symptoms arise from the Von Giereke disease?
Hepatomegaly, Kidney disease, Early death if untreated, treatable by uncooked corn starch meal at night or nocturnal gastric glucose infusion
what enzyme is deficient in the Pompe disease?
lysosomal (acid) glucosidase (acid maltose)
what is the glycogen amount in the pompe disease?
normal glycogen amount and normal glycogen structure in cytosol, high amount in lysosomes, lysosmes can rupture
What are the organs are affected in the Pompe disease?
Hear, Liver, Muscle
what symptoms arise from the Pompe disease?
massive cardiomegaly, treatable with enzyme infusion, normal blood glucose levels, infantile and late onset
What enzyme is defiecent in the Cori disease?
Glycogen debranching enzyme
What is the glycogen amount in the Cori disease?
high amount of glycogen in liver, heart, and muscle, abnormal glycogen structure with short branches, limit dextrinosis ( an intermediate structure accumulates)
what organs are affected in the Cori cycle?
Liver: mild hypoglycemia
Muscle, heart: weakness, hypotonia, cardiomyopathy
what symptoms come from the cori cycle?
cardiomyopathy, muscular dystrophy, hepatomegaly
What enzyme s deficient in the Anderson disease?
Glycigen branching enzyme
What is the glycogen amount in the Anderson disease?
Low amount of glycogen in liver and muscle, abnormal glycogen structure with long unbranched glucose chains (attack by immune system and scarring)
what organs are affected by Anderson disease?
Liver: Infantile cirrhosis
Muscle: infantile hypotonia
what symptoms arise from Anderson disease?
Hepatomegaly due to cirrhosis, early death
what enzyme is deficient in McArdle syndrome?
Muscle glycogen phosphorylase
what is the glycogen amount in the McArdle syndrome?
High amount of glycogen only in liver, normal glycogen structure
what organs are affected in the McArdle syndrome?
Muscle: weakness and cramping
what symptoms occur in the McArdle syndrome?
no increase of lactate in blood afte exercise, lack ATP, normal blood glucose levels
What enzyme is deficient in the Hers disease?
Liver glycogen phosphorylase
what is the glycogen amount in the Hers disease?
High amount of glycogen only in liver, normal glycogen structure
what organs are affected in the Hers disease?
Liver: mild hypoglycemia
what are the symptoms of Hers disease?
Hepatomegaly, growth retardation
What enzyme is deficient in Tarui disease?
glycolysis: muscle PFK-1, RBC PFK-1(M isoform)
what is the glycogen amount in Tarui disease?
high glycogen amount in muscle, normal glycogen structure, deficient glycolysis in muscle and RBC
what organs are affected in Tarui disease?
Muscle: lack of ATP leads to muscle cramping, nor rise in lactate
RBC: lack of ATP leads to hemolysis, normal blood glucose levels
What is the main purpose and regulation of glycogen degradation in the liver in comparison to the muscle?
In the liver: the main purpose of glycogen degradation is to release glucose into the blood at low blood glucose levels. This is under tight hormonal control and only happens after the action of glucagon or epinephrine on the hepatocyte which leads to activation of glycogen degradation by phosphorylations.
In the skeletal muscle, the main purpose of glycogen degradation is to provide glucose 6-P for glycolysis in order to generate ATP for muscle contraction.
This process can be performed without regulation by hormones, it is connected to muscle contraction. The released calcium ions allosterically activate glycogen phosphorylase kinase and high AMP levels allosterically activate glycogen phosphorylase in muscle.
In addition, during fight and flight situations, epinephrine activates protein kinase A which phosphorylates and covalently activates glycogen phosphorylase kinase. This leads to the most effective glycogen degradation in skeletal muscle.
Which pathway follows glycogen degradation in the muscle? Explain. Which pathway is inactive during glycogen degradation in the hepatocytes? Explain.
In the muscle, glycolysis follows glycogen degradation, both pathways happen at the same time. Glycogen degradation leads eventually to glucose 6-P which is used in glycolysis for ATP formation.
[aerobic glycolysis is linked to PDH and TCA cycle. Calcium ions released during muscle contraction activate glycogenolysis, PDH and TCA cycle.]
In hepatocytes, glycolysis is inactive during glycogen degradation.
The purpose of glycogen degradation in the liver is to release free glucose into the blood and the eventually formed glucose 6-P is cleaved by glucose 6-phosphatase.
Skeletal muscle Liver
Glycogen degradation Glycogen degradation///////
/// Release of glucose
Glycolysis Gluconeogenesis ///////
Which pathway is active during glycogen degradation in the liver?
Gluconeogenesis and glycogen degradation are both activated by glucagon which signals low blood glucose levels.
Which liver enzyme is used to cleave glucose 6-P formed by glycogen degradation and by gluconeogenesis?
Glucose 6-phosphatase is needed for the release of glucose into the blood as last step of both pathways in the hepatocyte.
What is the advantage of the branched structure of glycogen? Discuss glycogen degradation and glycogen synthesis.
The branched structure allows several glycogen phosphorylase enzymes to simultaneously generate many glucose 1-phosphates during glycogen degradation.
On the other hand, after a meal, glycogen synthesis is rapidly performed by action of several glycogen synthase enzymes acting at the same time. In both, liver and skeletal muscle, glycogen synthesis is activated at high insulin/glucagon ratio.
Are the reducing ends or are the non-reducing ends used for glycogen synthesis and glycogen degradation?
The nonreducing ends are used for glycogen synthase or for glycogen phosphorylase. The reducing end of glycogen is bound to glycogenin.
What is glycogenin, what is its purpose and where is it found?
Glycogen is normally not totally degraded as some branches are wanted as primer for glycogen synthase. In case that all is degraded, “genesis” of glycogen has to happen, which is performed by the protein glycogenin. It can act as an enzyme and it uses UDP-glucose and links glucose to its own tyrosine residue.
[this is very unusual for O-glycosylation, most O-glycosylations add sugars to serine or threonine residues and not to a tyrosine residue]
After elongation to a short 1-4)-glycosidic chain, glycogenin action storps and the enzyme glycogen synthase has now a primer and can act and elongate further on.
Glycogenin is found in the core of the cytosolic glycogen granule.
Where in the human body do you find the largest amount of glycogen, where do you find the highest concentration?
The largest amount is found in skeletal muscle,
and the highest concentration is found in the liver.
Glycogen stores are used in liver to provide glucose during fasting. After these stores are depleted, can you use the glycogen stores in muscle for release of glucose into the blood during severe starvation? Explain.
The glycogen stores in the muscle can never be used for release of glucose into the blood, as muscle does not contain the enzyme glucose 6-phosphatase.
Describe the action of glycogen synthase, what is special, what does it use, what does it form?
Glycogen synthase is a special enzyme that needs a primer, it cannot link UDP-glucose to a “free” glucose molecule.
It elongates glycogen at the non-reducing ends using UDP-glucose, which is the activated form of glucose. Glycogen synthase forms only 1-4)- glycosidic bonds
Describe the action of the glycogen branching enzyme. Which glycogen storage disease is due to deficiency of this enzyme?
The glycogen branching enzyme is needed to make a branched structure from the long chains with linear 1-4) glycosidic bonds. It contains two enzyme activities that work together.
The glycogen branching enzymes cleaves an 1-4) bond found about 6-8 residues from the nonreducing end and transfers this glucose chain to a non-terminal glucosyl residue.
It forms with its second enzyme activity an 1-6) glycosidic linkage.
These newly formed branch points allow glycogen synthase to form again new 1-4) linkages.
The deficiency of the branching enzyme is a rare disease. It is found in patients with Andersen disease. Hepatomegaly due to infantile cirrhosis.
Is glycogen synthase or is the glycogen branching enzyme the regulated enzyme of glycogen synthesis? How is the enzyme regulated?
The regulated enzyme of glycogen synthesis is glycogen synthase.
Glycogen synthase is active in its dephosphorylated form at high insulin/glucagon
ratio (high blood glucose)
and glycogen synthase is inactive in its phosphorylated form at low
insulin/glucagon ratio (low blood glucose) and presence of epinephrine in blood.
How is glycogen synthesis regulated? Why do you find glycogen accumulation in liver and kidney in the Von Gierke disease?
Glycogen synthesis is favored at high insulin/glucagon ratio which leads to the dephosphorylated glycogen synthase.
In addition, allosteric activation of glycogen synthase is found at high levels of glucose 6-P.
[Glucose 6-phosphatase is deficient in the Von Gierke disease in both liver and kidney. During fasting, glucose 6-P accumulates from glycogen degradation and from gluconeogenesis and allosterically activates glycogen synthase and allosterically inhibits glycogen phosphorylase. In these patients, glycogen synthesis takes place in spite of the severely hypoglycemic patients and leads to hepatomegaly!]
Describe the reaction performed by glycogen phosphorylase. What is special, what is formed, what is limit dextrin?
Glycogen phosphorylase is a special enzyme that cleaves the 1-4) glycosidic linkage using inorganic phosphate (phosphorolytic cleavage) instead of water.
This generates glucose 1-P from the nonreducing end of glycogen without input of ATP.
Glycogen phosphorylase cleaves the 1-4) bonds until it come to about 6 residues to a branch point.
There the enzyme has found its limit, and the glycogen structure with these short branches is named limit dextrin. It is an intermediate glycogen structure during glycogen degradation.
Describe the action of the glycogen debranching enzyme. Which glycogen storage disease has a deficiency of this enzyme?
The glycogen debranching enzymes contains two enzyme activities that work together.
The glycogen debranching enzyme cleaves the 1-4) linkage of short branches of limit dextrin closest to the branch point and links this glucose chain to a nonreducing end of glycogen.
With its second enzyme activity it cleaves the 1-6 ) linkage of the branch point and releases free glucose.
[in the liver this formed free glucose can be released into the blood, in the skeletal muscle this free glucose is phorphorylated to glucose 6-P by hexokinase]
Now limit dextrin is changed to normal glycogen and glycogen phosphorylase can act again until it comes close to the next branch point.
The debranching enzyme is deficient in Cori disease which is also known as limit dextrinosis,
How is glycogen phosphorylase activated in the liver? How is it activated in the muscle?
Glycogen phosphorylase can be activated in both liver and muscle by glycogen phosphorylase kinase, an enzyme which covalently activates and phosphorylates glycogen phosphorylase.
Special for the muscle: allosteric activation of glycogen phosphorylase is found at high AMP levels that can result from anaerobic exercise or severe anoxia.
What is the function of hepatic glycogen phosphorylase kinase? How is this enzyme activated?
The enzyme will phosphorylate and activate glycogen phosphorylase.
Glycogen phosphorylase kinase activity is under hormonal control and can be activated by phosphorylation by protein kinase A.
[Liver glycogen phosphorylase kinase can also be activated by high calcium ions in cytosol generated in the phosphoinositide messenger pathway. In this case glycogen phosphorylase kinase binds calcium with its own calmodulin subunits, makes a conformational change, and is active without being phosphorylated.]
What is the function of glycogen phosphorylase kinase in the skeletal muscle? How is this enzyme activated?
The enzyme will phosphorylate and activate glycogen phosphorylase.
In the skeletal muscle it is special that the calcium ions generated by muscle contraction, link muscle contraction to allosteric activation of glycogen phosphorylase kinase without the actions of hormones or phosphorylations.
After allosteric activation, glycogen phosphorylase kinase is active in its dephosphorylated form.
In addition, glycogen phosphorylase kinase itself can be phosphorylated by protein kinase A (as in the liver) in the cAMP messenger system following epinephrine and is then also active in its phosphorylated form.
Which enzyme contains 4 calmodulin subunits, glycogen phosphorylase kinase or is it glycogen phosphorylase?
Glycogen phosphorylase kinase contains 4 calmodulin subunits and is very responsive to allosteric activation by calcium and makes a conformational shift.
Summarize the allosteric regulation of glycogen degradation and discuss it related to the liver or skeletal muscle or both.
Glycogen degradation is allosterically inhibited by ATP and glucose 6-P in liver and muscle.
In the liver, free glucose allosterically inhibits glycogen degradation.
In the muscle, glycogen degradation is allosterically activated during muscle contraction by calcium ions (glycogen phosphorylase kinase) and by AMP (glycogen phosphorylase).
Which enzyme is for glycogen degradation other than in the cytosol? What is the name of the genetic defect of this enzyme and what is special?
Lysosomal acid glucosidase is needed in lysosomes. Some glycogen is taken up into lysosomes and needs to be degraded. The formed glucose is then released into the cytosol.
[This additional degradation other than in the cytosol was discovered in patients with Pompe disease. Pompe disease is grouped as Type II GSD, but it is also grouped as lysosomal storage disease (LSD) or muscular disease.]
Patients with Pompe disease show lysosomal glycogen accumulation in heart, muscle and liver with massive cardiomegaly, myopathy, hypotonia and hepatomegaly. Lysosomes can swell and rupture.
The glycogen structure in the cytosol is normal,
Pompe disease in the infantile form can lead to early death due to heart failure.
When would you expect an abnormal structure of glycogen?
Abnormal glycogen structure is expected in patients with deficient glycogen branching enzyme (Andersen disease) or with deficient glycogen debranching enzyme (Cori disease).
Describe the glycogen structure in patients with Cori disease.
Patients with Cori disease have a defect in glycogen degradation. They have a deficiency of the glycogen debranching enzyme.
The glycogen has short outer branches (limit dextrin)
[limit dextrin is a normal intermediate of glycogen degradation. In Cori disease, however, it is the main structure and leads to abnormal structure of glycogen (limit dextrinosis)]
Describe the glycogen structure in patients with Andersen disease.
Patients with Andersen disease have a defect in the synthesis of glycogen.
They have a deficiency of the glycogen branching enzyme. The glycogen structure has long unbranched glucose chains with maltose-like structure. The abnormal structure is less soluble, can be attacked by the immune system and can lead to infantile cirrhosis and eventually death in infancy.
Why do we find severe fasting hypoglycemia in patients with Von Gierke disease and milder hypoglycemia in patients with Cori disease or with Hers disease?
Patients with Von Gierke disease have a deficiency of glucose 6-phosphatase.
This enzyme generates free glucose not only from hepatic glycogen degradation, but also from gluconeogenesis. Deficiency results in severe fasting hypoglycemia.
[Patients with Cori disease (deficiency of the glycogen debranching enzyme) or Hers disease (deficiency of liver glycogen phosphorylase) have a hereditary defect in glycogen degradation, but they can still perform gluconeogenesis and release this glucose into the blood.]
Strenuous exercise raises blood lactate. In which patients do you not find the increase in blood lactate? Is it also used as test for this disease?
In patients with McArdle syndrome, we find no raise in lactate due to deficiency of muscle glycogen phosphorylase which normally provides substrates for anaerobic glycolysis and generates lactate.
[liver glycogen phosphorylase is normal in these patients]
In the rare Tarui disease, PFK-1 is deficient and we do not find lactate increase after strenuous exercise due to deficient glycolysis. In addition we find mild hemolysis as the enzyme in RBC’s is also affected.
[note: muscle weakness is also found as result of defects in the carnitine shuttle and fatty acid degradation in muscle. In these patients, a normal increase of blood lactate after exercise is found as they can access glycogen stores in muscle]
Name the glycogen storage diseases (GSD) that all lead to hepatomegaly.
Type I (von Gierke), Type II (Pompe), Type III (Cori) , Type IV (Andersen) and Type VI (Hers)
What is characteristic for Type IV GSD (Andersen disease)?
Infantile liver cirrhosis (genetic) is characteristic for Andersen disease.
[This is due to deficiency of the glycogen branching enzyme leading to abnormal glycogen structure with long unbranched glucose chains that is attacked by the immune system. Early death by 5 years of age can occur.]
What is characteristic for Type V GSD (McArdle syndrome)?
Muscle weakness and cramping after exercise. No raise of blood lactate.
Serum CK-MM is elevated. Rhabdomyolysis after exercise possible.