Glycogen phosphorylase
Encyclopedia
Glycogen phosphorylase is one of the phosphorylase
enzyme
s . Glycogen phosphorylase catalyzes the rate-limiting step in the degradation of glycogen
in animals by releasing glucose-1-phosphate
from the terminal alpha-1,4-glycosidic bond. Glycogen phosphorylase is also studied as a model protein regulated by both reversible phosphorylation
and allosteric
effects.
(α-1,4 glycogen chain)n + Pi ↔ (α-1,4 glycogen chain)n-1 + D-glucose-1-phosphate.
Glycogen phosphorylase breaks up glycogen
into glucose
subunits. Glycogen
is left with one fewer glucose
molecule
, and the free glucose
molecule is in the form of glucose-1-phosphate
. In order to be used for metabolism
, it must be converted to glucose-6-phosphate
by the enzyme phosphoglucomutase
.
Although the reaction is reversible in solution, within the cell the enzyme only works in the forward direction as shown above because the concentration of inorganic phosphate is much higher than that of glucose-1-phosphate.
Glycogen phosphorylase can act only on linear
chains of glycogen
(α1-4 glycosidic linkage). Its work will immediately come to a halt four residues away from α1-6 branch
(which are exceedingly common in glycogen). In these situations, a debranching enzyme is necessary, which will straighten out the chain in that area. In addition, the enzyme transferase shifts a block of 3 glucosyl residues from the outer branch to the other end, and then a α1-6 glucosidase enzyme
is required to break the remaining (single glucose) α1-6 residue that remains in the new linear chain. After all this is done, glycogen phosphorylase can continue. The enzyme is specific to α1-4 chains, as the molecule contains a 30-angstrom-long crevice with the same radius as the helix formed by the glycogen chain; this accommodates 4-5 glucosyl residues, but is too narrow for branches. This crevice connects the glycogen storage site to the active, catalytic site.
Glycogen phosphorylase has a pyridoxal phosphate (PLP, derived from Vitamin B6
) at each catalytic site. Pyridoxal phosphate links with basic residues (in this case Lys680) and covalently forms a Schiff base
. Once the Schiff base linkage is formed, holding the PLP molecule in the active site, the phosphate group on the PLP readily donates a proton to an inorganic phosphate molecule, allowing the inorganic phosphate to in turn be deprotonated by the oxygen forming the α-1,4 glycosidic linkage. PLP is readily deprotonated because its negative charge is not only stabilized within the phosphate group, but also in the pyridine ring, thus the conjugate base resulting from the deprotonation of PLP is quite stable. The protonated oxygen now represents a good leaving group
, and the glycogen chain is separated from the terminal glycogen in an SN1
fashion, resulting in the formation of a glucose molecule with a secondary carbocation at the 1 position. Finally, the deprotonated inorganic phosphate acts as a nucleophile
and bonds with the carbocation, resulting in the formation of glucose-1-phosphate and a glycogen chain shortened by one glucose molecule.
There is also an alternative proposed mechanism involving a positively charged oxygen in a half-chair conformation.
in muscle cells. While the enzyme can exist as an inactive monomer or tetramer, it is biologically active as a dimer
of two identical subunits.
The glycogen phosphorylase dimer has many regions of biological significance, including catalytic sites, glycogen binding sites, allosteric sites, and a reversibly phosphorylated serine residue. First, the catalytic sites are relatively buried, 15Å from the surface of the protein and from the subunit interface. This lack of easy access of the catalytic site to the surface is significant in that it makes the protein activity highly susceptible to regulation, as small allosteric effects could greatly increase the relative access of glycogen to the site.
Perhaps the most important regulatory site
is Ser14, the site of reversible phosphorylation
very close to the subunit interface. The structural change associated with phosphorylation, and with the conversion of phosphorylase b to phosphorylase a, is the arrangement of the originally disordered residues 10 to 22 into α helices. This change increases phosphorylase activity up to 25% even in the absence of AMP, and enhances AMP activation further.
The allosteric site of AMP
binding on muscle isoforms of glycogen phosphorylase are close to the subunit interface just like Ser14. Binding of AMP at this site, corresponding in a change from the T state of the enzyme to the R state, results in small changes in tertiary structure at the subunit interface leading to large changes in quaternary structure. AMP binding rotates the tower helices (residues 262-278) of the two subunits 50˚ relative to one another through greater organization and intersubunit interactions. This rotation of the tower helices leads to a rotation of the two subunits by 10˚ relative to one another, and more importantly disorders residues 282-286 (the 280s loop) that block access to the catalytic site in the T state but do not in the R state.
The final, perhaps most curious site on the glycogen phosphorylase protein is the so-called glycogen storage site. Residues 397-437 form this structure, which allows the protein to covalently bind to the glycogen chain a full 30 Å from the catalytic site . This site is most likely the site at which the enzyme binds to glycogen granules before initiating cleavage of terminal glucose molecules. In fact, 70% of dimeric phosphorylase in the cell exists as bound to glycogen granules rather than free floating.
In mammals, the major isozyme
s of glycogen phosphorylase are found in muscle, liver, and brain. The brain type is predominant in adult brain and embryonic tissues, whereas the liver and muscle types are predominant in adult liver and skeletal muscle, respectively.
is a known inhibitor of HLPG and stabilizes the less active T-state. These glucose derivatives have had some success in inhibiting HLPG, with predicted Ki values as low as 0.016 mM.
Mutations in the muscle isoform of glycogen phosphorylase (PYGM) are associated with McArdle's Disease (glycogen storage disease type V
). More than 65 mutations in the PYGM gene that lead to McArdle disease have been identified to date. Symptoms of McArdle disease include muscle weakness, myalgia
, and lack of endurance, all stemming from low glucose levels in muscle tissue.
Mutations in the liver isoform of glycogen phosphorylase (PYGL) are associated with Hers' Disease (glycogen storage disease type VI
). Hers' disease is often associated with mild symptoms normally limited to hypoglycemia
, and is sometimes difficult to diagnose due to residual enzyme activity.
The brain isoform of glycogen phosphorylase (PYGLB) has been proposed as a biomarker for gastric cancer
.
.
Hormones such as epinephrine
, insulin
and glucagon
regulate glycogen phosphorylase using second messenger amplification systems that are linked to G proteins. Epinephrine activates adenylate cyclase through a seven transmembrane receptor
coupled to Gs
which, in turn, activates adenylate cyclase
to increase intracellular concentrations of cAMP. cAMP binds to and releases an active form of protein kinase A (PKA). Next, PKA phosphorylates phosphorylase kinase
, which, in turn, phosphorylates glycogen phosphorylase b, transforming it into the active glycogen phosphorylase a. This phosphorylation is added onto the glycogen phosphorylase b serine 14. In the liver, glucagon
activates another G-protein-linked receptor that triggers a different cascade, resulting in the activation of Phospholipase C (PLC). PLC indirectly causes the release of calcium from the hepatocytes' endoplasmic reticulum into the cytosol. The increased calcium availability binds to the calmodulin
subunit and activates glycogen phosphorylase kinase. Glycogen phosphorylase kinase activates glycogen phosphorylase in the same manner mentioned previously.
Glycogen phosphorylase b is not always inactive in muscle, as it can be activated allosterically by AMP. An increase in AMP concentration, which occurs during strenuous exercise, signals energy demand. AMP activates glycogen phosphorylase b by changing its conformation from a tense to a relaxed form. This relaxed form has similar enzymatic properties as the phosphorylated enzyme. An increase in ATP concentration opposes this activation by displacing AMP from the nucleotide binding site, indicating sufficient energy stores.
Upon eating a meal, there is a release of insulin
, signaling glucose availability in the blood. Insulin indirectly activates PP-1 and phosphodiesterase. The PP-1 directly dephosphorylates glycogen phosphorylase a, reforming the inactive glycogen phosphorylase b. The phosphodiesterase converts cAMP to AMP. This activity removes the second messenger (generated by glucagon and epinephrine) and inhibits PKA. In this manner, PKA can no longer cause the phosphorylation cascade that ends with formation of (active) glycogen phosphorylase a. These modifications initiated by insulin end glycogenolysis in order to preserve what glycogen stores are left in the cell and trigger glycogenesis (rebuilding of glycogen).
Phosphorylase a and phosphorylase b each exist in two forms a T (tense) inactive state and R (relaxed) state. Phosphorylase b is normally in the T state, inactive due to the physiological presence of ATP and Glucose 6 phosphate, and Phosphorylase a is normally in the R state (active).
An isoenzyme of glycogen phosphorylase exists in the liver sensitive to glucose concentration, as the liver acts as a glucose exporter. In essence, liver phosphorylase is responsive to glucose, which causes a very responsive transition from the R to T form, inactivating it; furthermore, liver phosphorylase is insensitive to AMP.
. In 1943, with the help of Arda Green, the pair illustrated that glycogen phosphorylase existed in either the a or b forms depending on its phosphorylation state, as well as in the R or T states based on the presences of AMP.
Phosphorylase
Phosphorylases are enzymes that catalyze the addition of a phosphate group from an inorganic phosphate to an acceptor.They include allosteric enzymes that catalyze the production of glucose-1-phosphate from a glucan such as glycogen, starch or maltodextrin. Phosphorylase is also a common name used...
enzyme
Enzyme
Enzymes are proteins that catalyze chemical reactions. In enzymatic reactions, the molecules at the beginning of the process, called substrates, are converted into different molecules, called products. Almost all chemical reactions in a biological cell need enzymes in order to occur at rates...
s . Glycogen phosphorylase catalyzes the rate-limiting step in the degradation of glycogen
Glycogen
Glycogen is a molecule that serves as the secondary long-term energy storage in animal and fungal cells, with the primary energy stores being held in adipose tissue...
in animals by releasing glucose-1-phosphate
Glucose-1-phosphate
Glucose 1-phosphate is a glucose molecule with a phosphate group on the 1'-carbon.-Catabolic:In glycogenolysis, it is the direct product of the reaction in which glycogen phosphorylase cleaves off a molecule of glucose from a greater glycogen structure.To be utilized in cellular catabolism it must...
from the terminal alpha-1,4-glycosidic bond. Glycogen phosphorylase is also studied as a model protein regulated by both reversible phosphorylation
Phosphorylation
Phosphorylation is the addition of a phosphate group to a protein or other organic molecule. Phosphorylation activates or deactivates many protein enzymes....
and allosteric
Allosteric regulation
In biochemistry, allosteric regulation is the regulation of an enzyme or other protein by binding an effector molecule at the protein's allosteric site . Effectors that enhance the protein's activity are referred to as allosteric activators, whereas those that decrease the protein's activity are...
effects.
Mechanism
The overall reaction is written as:(α-1,4 glycogen chain)n + Pi ↔ (α-1,4 glycogen chain)n-1 + D-glucose-1-phosphate.
Glycogen phosphorylase breaks up glycogen
Glycogen
Glycogen is a molecule that serves as the secondary long-term energy storage in animal and fungal cells, with the primary energy stores being held in adipose tissue...
into glucose
Glucose
Glucose is a simple sugar and an important carbohydrate in biology. Cells use it as the primary source of energy and a metabolic intermediate...
subunits. Glycogen
Glycogen
Glycogen is a molecule that serves as the secondary long-term energy storage in animal and fungal cells, with the primary energy stores being held in adipose tissue...
is left with one fewer glucose
Glucose
Glucose is a simple sugar and an important carbohydrate in biology. Cells use it as the primary source of energy and a metabolic intermediate...
molecule
Molecule
A molecule is an electrically neutral group of at least two atoms held together by covalent chemical bonds. Molecules are distinguished from ions by their electrical charge...
, and the free glucose
Glucose
Glucose is a simple sugar and an important carbohydrate in biology. Cells use it as the primary source of energy and a metabolic intermediate...
molecule is in the form of glucose-1-phosphate
Glucose-1-phosphate
Glucose 1-phosphate is a glucose molecule with a phosphate group on the 1'-carbon.-Catabolic:In glycogenolysis, it is the direct product of the reaction in which glycogen phosphorylase cleaves off a molecule of glucose from a greater glycogen structure.To be utilized in cellular catabolism it must...
. In order to be used for metabolism
Metabolism
Metabolism is the set of chemical reactions that happen in the cells of living organisms to sustain life. These processes allow organisms to grow and reproduce, maintain their structures, and respond to their environments. Metabolism is usually divided into two categories...
, it must be converted to glucose-6-phosphate
Glucose-6-phosphate
Glucose 6-phosphate is glucose sugar phosphorylated on carbon 6. This compound is very common in cells as the vast majority of glucose entering a cell will become phosphorylated in this way....
by the enzyme phosphoglucomutase
Phosphoglucomutase
Phosphoglucomutase is an enzyme that transfers a phosphate group on an α-D-glucose monomer from the 1' to the 6' position in the forward direction or the 6' to the 1' position in the reverse direction....
.
Although the reaction is reversible in solution, within the cell the enzyme only works in the forward direction as shown above because the concentration of inorganic phosphate is much higher than that of glucose-1-phosphate.
Glycogen phosphorylase can act only on linear
Linear
In mathematics, a linear map or function f is a function which satisfies the following two properties:* Additivity : f = f + f...
chains of glycogen
Glycogen
Glycogen is a molecule that serves as the secondary long-term energy storage in animal and fungal cells, with the primary energy stores being held in adipose tissue...
(α1-4 glycosidic linkage). Its work will immediately come to a halt four residues away from α1-6 branch
Branching (chemistry)
In polymer chemistry, branching occurs by the replacement of a substituent, e.g., a hydrogen atom, on a monomer subunit, by another covalently bonded chain of that polymer; or, in the case of a graft copolymer, by a chain of another type...
(which are exceedingly common in glycogen). In these situations, a debranching enzyme is necessary, which will straighten out the chain in that area. In addition, the enzyme transferase shifts a block of 3 glucosyl residues from the outer branch to the other end, and then a α1-6 glucosidase enzyme
Enzyme
Enzymes are proteins that catalyze chemical reactions. In enzymatic reactions, the molecules at the beginning of the process, called substrates, are converted into different molecules, called products. Almost all chemical reactions in a biological cell need enzymes in order to occur at rates...
is required to break the remaining (single glucose) α1-6 residue that remains in the new linear chain. After all this is done, glycogen phosphorylase can continue. The enzyme is specific to α1-4 chains, as the molecule contains a 30-angstrom-long crevice with the same radius as the helix formed by the glycogen chain; this accommodates 4-5 glucosyl residues, but is too narrow for branches. This crevice connects the glycogen storage site to the active, catalytic site.
Glycogen phosphorylase has a pyridoxal phosphate (PLP, derived from Vitamin B6
Vitamin B6
Vitamin B6 is a water-soluble vitamin and is part of the vitamin B complex group. Several forms of the vitamin are known, but pyridoxal phosphate is the active form and is a cofactor in many reactions of amino acid metabolism, including transamination, deamination, and decarboxylation...
) at each catalytic site. Pyridoxal phosphate links with basic residues (in this case Lys680) and covalently forms a Schiff base
Schiff base
A Schiff base, named after Hugo Schiff, is a compound with a functional group that contains a carbon-nitrogen double bond with the nitrogen atom connected to an aryl or alkyl group, not hydrogen....
. Once the Schiff base linkage is formed, holding the PLP molecule in the active site, the phosphate group on the PLP readily donates a proton to an inorganic phosphate molecule, allowing the inorganic phosphate to in turn be deprotonated by the oxygen forming the α-1,4 glycosidic linkage. PLP is readily deprotonated because its negative charge is not only stabilized within the phosphate group, but also in the pyridine ring, thus the conjugate base resulting from the deprotonation of PLP is quite stable. The protonated oxygen now represents a good leaving group
Leaving group
In chemistry, a leaving group is a molecular fragment that departs with a pair of electrons in heterolytic bond cleavage. Leaving groups can be anions or neutral molecules. Common anionic leaving groups are halides such as Cl−, Br−, and I−, and sulfonate esters, such as para-toluenesulfonate...
, and the glycogen chain is separated from the terminal glycogen in an SN1
SN1 reaction
The SN1 reaction is a substitution reaction in organic chemistry. "SN" stands for nucleophilic substitution and the "1" represents the fact that the rate-determining step is unimolecular...
fashion, resulting in the formation of a glucose molecule with a secondary carbocation at the 1 position. Finally, the deprotonated inorganic phosphate acts as a nucleophile
Nucleophile
A nucleophile is a species that donates an electron-pair to an electrophile to form a chemical bond in a reaction. All molecules or ions with a free pair of electrons can act as nucleophiles. Because nucleophiles donate electrons, they are by definition Lewis bases.Nucleophilic describes the...
and bonds with the carbocation, resulting in the formation of glucose-1-phosphate and a glycogen chain shortened by one glucose molecule.
There is also an alternative proposed mechanism involving a positively charged oxygen in a half-chair conformation.
Structure
The glycogen phosphorylase monomer is a large protein, composed of 842 amino acids with a mass of 97.434 kDaKDA
KDA may refer to:* Karachi Development Authority* Kongsberg Defence & Aerospace* Kotelawala Defence Academy* Kramer Design Associates* Lithium diisopropylamide, KDA is the potassium analogue of lithium diisopropylamideOr kDa may refer to:...
in muscle cells. While the enzyme can exist as an inactive monomer or tetramer, it is biologically active as a dimer
Protein dimer
In biochemistry, a dimer is a macromolecular complex formed by two, usually non-covalently bound, macromolecules like proteins or nucleic acids...
of two identical subunits.
The glycogen phosphorylase dimer has many regions of biological significance, including catalytic sites, glycogen binding sites, allosteric sites, and a reversibly phosphorylated serine residue. First, the catalytic sites are relatively buried, 15Å from the surface of the protein and from the subunit interface. This lack of easy access of the catalytic site to the surface is significant in that it makes the protein activity highly susceptible to regulation, as small allosteric effects could greatly increase the relative access of glycogen to the site.
Perhaps the most important regulatory site
Regulatory site
A regulatory site is a site on an allosteric protein to which a modulator molecule binds. A ligand-binding site on a receptor or enzyme distinct from the active site. Allosteric modulators alter enzyme activity by binding to the regulatory site. Also known as an "allosteric site"....
is Ser14, the site of reversible phosphorylation
Phosphorylation
Phosphorylation is the addition of a phosphate group to a protein or other organic molecule. Phosphorylation activates or deactivates many protein enzymes....
very close to the subunit interface. The structural change associated with phosphorylation, and with the conversion of phosphorylase b to phosphorylase a, is the arrangement of the originally disordered residues 10 to 22 into α helices. This change increases phosphorylase activity up to 25% even in the absence of AMP, and enhances AMP activation further.
The allosteric site of AMP
Adenosine monophosphate
Adenosine monophosphate , also known as 5'-adenylic acid, is a nucleotide that is used as a monomer in RNA. It is an ester of phosphoric acid and the nucleoside adenosine. AMP consists of a phosphate group, the sugar ribose, and the nucleobase adenine...
binding on muscle isoforms of glycogen phosphorylase are close to the subunit interface just like Ser14. Binding of AMP at this site, corresponding in a change from the T state of the enzyme to the R state, results in small changes in tertiary structure at the subunit interface leading to large changes in quaternary structure. AMP binding rotates the tower helices (residues 262-278) of the two subunits 50˚ relative to one another through greater organization and intersubunit interactions. This rotation of the tower helices leads to a rotation of the two subunits by 10˚ relative to one another, and more importantly disorders residues 282-286 (the 280s loop) that block access to the catalytic site in the T state but do not in the R state.
The final, perhaps most curious site on the glycogen phosphorylase protein is the so-called glycogen storage site. Residues 397-437 form this structure, which allows the protein to covalently bind to the glycogen chain a full 30 Å from the catalytic site . This site is most likely the site at which the enzyme binds to glycogen granules before initiating cleavage of terminal glucose molecules. In fact, 70% of dimeric phosphorylase in the cell exists as bound to glycogen granules rather than free floating.
In mammals, the major isozyme
Isozyme
Isozymes are enzymes that differ in amino acid sequence but catalyze the same chemical reaction. These enzymes usually display different kinetic parameters Isozymes (also known as isoenzymes) are enzymes that differ in amino acid sequence but catalyze the same chemical reaction. These enzymes...
s of glycogen phosphorylase are found in muscle, liver, and brain. The brain type is predominant in adult brain and embryonic tissues, whereas the liver and muscle types are predominant in adult liver and skeletal muscle, respectively.
Clinical significance
The inhibition of glycogen phosphorylase has been proposed as one method for treating type 2 diabetes. Since glucose production in the liver has been shown to increase in type 2 diabetes patients, inhibiting the release of glucose from the liver’s glycogen’s supplies appears to be a valid approach. The cloning of the human liver glycogen phosphorylase (HLGP) revealed a new allosteric binding site near the subunit interface that is not present in the rabbit muscle glycogen phosphorylase (RMGP) normally used in studies. This site was not sensitive to the same inhibitors as those at the AMP allosteric site, and most success has been had synthesizing new inhibitors that mimic the structure of glucose, since glucose-6-phosphateGlucose-6-phosphate
Glucose 6-phosphate is glucose sugar phosphorylated on carbon 6. This compound is very common in cells as the vast majority of glucose entering a cell will become phosphorylated in this way....
is a known inhibitor of HLPG and stabilizes the less active T-state. These glucose derivatives have had some success in inhibiting HLPG, with predicted Ki values as low as 0.016 mM.
Mutations in the muscle isoform of glycogen phosphorylase (PYGM) are associated with McArdle's Disease (glycogen storage disease type V
Glycogen storage disease type V
Glycogen storage disease type V is a metabolic disorder, more specifically a glycogen storage disease, caused by a deficiency of myophosphorylase. Its incidence is reported as 1 in 100,000, approximately the same as glycogen storage disease type I....
). More than 65 mutations in the PYGM gene that lead to McArdle disease have been identified to date. Symptoms of McArdle disease include muscle weakness, myalgia
Myalgia
Myalgia means "muscle pain" and is a symptom of many diseases and disorders. The most common causes are the overuse or over-stretching of a muscle or group of muscles. Myalgia without a traumatic history is often due to viral infections...
, and lack of endurance, all stemming from low glucose levels in muscle tissue.
Mutations in the liver isoform of glycogen phosphorylase (PYGL) are associated with Hers' Disease (glycogen storage disease type VI
Glycogen storage disease type VI
Glycogen storage disease type VI is a type of glycogen storage disease caused by a deficiency in liver glycogen phosphorylase or other components of the associated phosphorylase cascade system.It is also known as "Hers' disease", after Henri G...
). Hers' disease is often associated with mild symptoms normally limited to hypoglycemia
Hypoglycemia
Hypoglycemia or hypoglycæmia is the medical term for a state produced by a lower than normal level of blood glucose. The term literally means "under-sweet blood"...
, and is sometimes difficult to diagnose due to residual enzyme activity.
The brain isoform of glycogen phosphorylase (PYGLB) has been proposed as a biomarker for gastric cancer
Stomach cancer
Gastric cancer, commonly referred to as stomach cancer, can develop in any part of the stomach and may spread throughout the stomach and to other organs; particularly the esophagus, lungs, lymph nodes, and the liver...
.
Regulation
Glycogen phosphorylase is regulated by both allosteric control and by phosphorylationPhosphorylation
Phosphorylation is the addition of a phosphate group to a protein or other organic molecule. Phosphorylation activates or deactivates many protein enzymes....
.
Hormones such as epinephrine
Epinephrine
Epinephrine is a hormone and a neurotransmitter. It increases heart rate, constricts blood vessels, dilates air passages and participates in the fight-or-flight response of the sympathetic nervous system. In chemical terms, adrenaline is one of a group of monoamines called the catecholamines...
, insulin
Insulin
Insulin is a hormone central to regulating carbohydrate and fat metabolism in the body. Insulin causes cells in the liver, muscle, and fat tissue to take up glucose from the blood, storing it as glycogen in the liver and muscle....
and glucagon
Glucagon
Glucagon, a hormone secreted by the pancreas, raises blood glucose levels. Its effect is opposite that of insulin, which lowers blood glucose levels. The pancreas releases glucagon when blood sugar levels fall too low. Glucagon causes the liver to convert stored glycogen into glucose, which is...
regulate glycogen phosphorylase using second messenger amplification systems that are linked to G proteins. Epinephrine activates adenylate cyclase through a seven transmembrane receptor
G protein-coupled receptor
G protein-coupled receptors , also known as seven-transmembrane domain receptors, 7TM receptors, heptahelical receptors, serpentine receptor, and G protein-linked receptors , comprise a large protein family of transmembrane receptors that sense molecules outside the cell and activate inside signal...
coupled to Gs
Heterotrimeric G protein
"G protein" usually refers to the membrane-associated heterotrimeric G proteins, sometimes referred to as the "large" G proteins. These proteins are activated by G protein-coupled receptors and are made up of alpha , beta and gamma subunits, the latter two referred to as the beta-gamma...
which, in turn, activates adenylate cyclase
Adenylate cyclase
Adenylate cyclase is part of the G protein signalling cascade, which transmits chemical signals from outside the cell across the membrane to the inside of the cell ....
to increase intracellular concentrations of cAMP. cAMP binds to and releases an active form of protein kinase A (PKA). Next, PKA phosphorylates phosphorylase kinase
Phosphorylase kinase
Phosphorylase kinase is a serine/threonine-specific protein kinase which activates glycogen phosphorylase to release glucose-1-phosphate from glycogen...
, which, in turn, phosphorylates glycogen phosphorylase b, transforming it into the active glycogen phosphorylase a. This phosphorylation is added onto the glycogen phosphorylase b serine 14. In the liver, glucagon
Glucagon
Glucagon, a hormone secreted by the pancreas, raises blood glucose levels. Its effect is opposite that of insulin, which lowers blood glucose levels. The pancreas releases glucagon when blood sugar levels fall too low. Glucagon causes the liver to convert stored glycogen into glucose, which is...
activates another G-protein-linked receptor that triggers a different cascade, resulting in the activation of Phospholipase C (PLC). PLC indirectly causes the release of calcium from the hepatocytes' endoplasmic reticulum into the cytosol. The increased calcium availability binds to the calmodulin
Calmodulin
Calmodulin is a calcium-binding protein expressed in all eukaryotic cells...
subunit and activates glycogen phosphorylase kinase. Glycogen phosphorylase kinase activates glycogen phosphorylase in the same manner mentioned previously.
Glycogen phosphorylase b is not always inactive in muscle, as it can be activated allosterically by AMP. An increase in AMP concentration, which occurs during strenuous exercise, signals energy demand. AMP activates glycogen phosphorylase b by changing its conformation from a tense to a relaxed form. This relaxed form has similar enzymatic properties as the phosphorylated enzyme. An increase in ATP concentration opposes this activation by displacing AMP from the nucleotide binding site, indicating sufficient energy stores.
Upon eating a meal, there is a release of insulin
Insulin
Insulin is a hormone central to regulating carbohydrate and fat metabolism in the body. Insulin causes cells in the liver, muscle, and fat tissue to take up glucose from the blood, storing it as glycogen in the liver and muscle....
, signaling glucose availability in the blood. Insulin indirectly activates PP-1 and phosphodiesterase. The PP-1 directly dephosphorylates glycogen phosphorylase a, reforming the inactive glycogen phosphorylase b. The phosphodiesterase converts cAMP to AMP. This activity removes the second messenger (generated by glucagon and epinephrine) and inhibits PKA. In this manner, PKA can no longer cause the phosphorylation cascade that ends with formation of (active) glycogen phosphorylase a. These modifications initiated by insulin end glycogenolysis in order to preserve what glycogen stores are left in the cell and trigger glycogenesis (rebuilding of glycogen).
Phosphorylase a and phosphorylase b each exist in two forms a T (tense) inactive state and R (relaxed) state. Phosphorylase b is normally in the T state, inactive due to the physiological presence of ATP and Glucose 6 phosphate, and Phosphorylase a is normally in the R state (active).
An isoenzyme of glycogen phosphorylase exists in the liver sensitive to glucose concentration, as the liver acts as a glucose exporter. In essence, liver phosphorylase is responsive to glucose, which causes a very responsive transition from the R to T form, inactivating it; furthermore, liver phosphorylase is insensitive to AMP.
Historical significance
Glycogen phosphorylase was the first allosteric enzyme to be discovered. This accomplishment was one of many landmark achievements made by Carl and Gerty CoriGerty Cori
Gerty Theresa Cori was an American biochemist who became the third woman—and first American woman—to win a Nobel Prize in science, and the first woman to be awarded the Nobel Prize in Physiology or Medicine.Cori was born in Prague...
. In 1943, with the help of Arda Green, the pair illustrated that glycogen phosphorylase existed in either the a or b forms depending on its phosphorylation state, as well as in the R or T states based on the presences of AMP.