22-GLYCOGEN METABOLISM – Dr. Venk Flashcards
What is the advantage of glycogen breakdown by phosphorolysis compared to hydrolysis?
Phosphorolysis will yield glucose-1-phosphate without using any ATP.
This is important when energy needs are very short in supply. Also, the body’s concentration of P_i is high enough to drive the reaction towards the right direction towards glycogen breakdown. One glucose molecule is broken off at a time to give glucose-1-phosphate, which can be readily converted to glucose-6-phosphate.
Describe the significance of glycogenolysis in liver versus muscle.
In muscle glycogenolysis is used to breakdown glycogen, convert glucose-1-phosphate to glucose-6-phosphate and directly channel it into glycolysis for the muscle’s ATP needs. In muscle, glucose-6-phosphatase is ABSENT to insure that glucose directly enters glycolysis and not another pathway after glycogenolysis.
In the liver, glucose-6-phosphatase will use the end product of glycogenolysis and cleave off the phosphate to release free glucose in the blood for the body’s needs.
Describe how glucose-1-phosphate is converted to glucose-6-phoshphate.
Phosphoglucomutase converts G-1-Phosphate to G-6-Phosphate. The enzyme contains a phosphorylated Serine in the active site. G-1-Phosphate binds to the active site and the enzyme’s phosphate is attached to C-6. A transient intermediate, G-1,6-bisphosphate is generated and stays in the active site. The enzyme will then remove the phosphate on C-1, regenerating the phosphoenzyme. G-6-Phosphate is released from the active site.
Describe the phosphorylation mediated regulation of enzyme activity.
Phosphorylation of many enzymes is a simple way of avoiding the futile cycle of having synthesis and breakdown occurring at the same time.
Phorphorylation of PKA will lead to activation and breakdown of glycogen. But, phosphorylation of Glycogen Synthase will lead to its inactivation, preventing synthesis in the middle of breakdown.
Name the two hormones that are involved in triggering glycogenolysis and describe their specificity.
Epinephrine will bind to alpha-adrenergic receptors and activate the Phospholipase C pathway. G-proteins will activate PLC which will hydrolize PIP2 to IP3 and DAG. IP3 will bind to the endoplasmic reticulum and release Ca2+. Ca2+ will in turn bind to calmodulin-like subunit of Phosphrylase Kinase. At the same time, DAG will active Protein Kinase C, which will phosphorylate glycogen synthase, making it inactive (eliminating synthesis and breakdown at the same time).
Glucagon, released by beta cells of the pancrease will bind to the G-protein receptor of hepatic cells. This activates adenylate cyclase to release the second messenger cAMP. CAMP will bind to the regulatory subunit of Protein Kinase A to activate it. Once PKA is activated it will phosphorylate phosphorylase kinase.
Describe the mechanism by which PKA is activated.
Glucagon binds the G-protein receptor of hepatic cells. This activates adenylate cyclase to release the second messenger cAMP. cAMP will bind to the regulatory (R) subunit of Protein Kinase A, causing the R subunit to separate from the catalytic (C) subunit, which is now an active enzyme. Once PKA is activated it will phosphorylate phosphorylase kinase.
Describe the glycogenolysis cascade, and discuss the kinases that are involved in the pathway.
Glycogenolysis regulation is done through a complex cascade of phosphorylations. Alpha cells of the pancreas sense low blood glucose and will release glucagon. Glucagon will bind to the G-protein receptor of hepatic cells. This activates adenylate cyclase to release the second messenger cAMP. CAMP will bind to the regulatory subunit of Protein Kinase A to activate it. Once PKA is activated it will phosphorylate phosphorylase kinase. The phosphorylated Phosphorylase kinase will further phosphorylate Phosphorylase-b to Phosphorylase-a. Phosphorylase-b can generate G-1-phosphate under resting conditions. But, Phosphorylase-a can have a larger increase in glycogen breakdown under higher energy demands.
Glycogenolysis is stopped by the enzyme Phosphoprotein Phosphatase-1 (PP-1). It will remove the phosphate from Phosphorylase Kinase, so that it can no longer activate Phosphorylase-b to Phosphorylase-a. And it will also remove the phosphate from Phosphorylase-a, making it the less active Phosphorylase-b. The actions of PP-1 are regulated by Phosphoprotein Phosphatase Inhibitor (PPI). PPI will inhibit PP-1 by binding to it. PPI will only bind to PP-1 when phosphorylated by PKA. By doing this, the cells can insure that the phosphate attached by PKA to Phosphorylase Kinase and Phosphorylase-a are not immediately removed by PP-1.
To summarize,:
PKA will phosphorylate Phosphorylase Kinase to make it active
Phosphorylase Kinase will phosphorylate phosphorylase-b to phosphorylase-a
PKA will also phosphorylate PPI, which will inhibit PP-1 (PP-1 removes phosphates from phosphorylase-a and Phosphorylase Kinase)
Describe the role of calcium in glycogenolysis, especially with reference to the neuromuscular stimulus.
When the muscle is stimulated, Ca2+ is released to initiate muscular contractions. In addition to stimulating muscle cells, Ca2+ can bind to a calmodulin-like subunit of Phosphorylase Kinase and activate it. This way, the muscles are stimulated and Phosphorylase Kinase is activated to initiate glycogen breakdown for the muscles’ energy needs.
Describe the activation of the alpha-adrenergic receptor and the associated cascade of biochemical events.
Epinephrine will bind to alpha-adrenergic receptors and activate the Phospholipase C pathway. G-proteins will activate PLC which will hydrolyze PIP_2 to IP3 and DAG. IP_3 will bind to the endoplasmic reticulum and release Ca2+. Ca2+ will in turn bind to calmodulin-like subunits of Phosphrylase Kinase. At the same time, DAG will activate Protein Kinase C, which will phosphorylate glycogen synthase, making it inactive (eliminating synthesis and breakdown at the same time).
Describe the role of protein phosphatase inhibitor (PPI) during glycogenolysis.
Glycogenolysis is stopped by the enzyme Phosphoprotein Phosphatase-1 (PP-1). It will remove the phosphate from Phosphorylase Kinase, so that it can no longer activate Phosphorylase-b to Phosphorylase-a. And it will also remove the phosphate from Phosphorylase-a, making it the less active Phosphorylase-b. The actions of PP-1 are regulated by Phosphoprotein Phosphatase Inhibitor (PPI). PPI will inhibit PP-1 by binding to it. PPI will only bind to PP-1 when phosphorylated by PKA. By doing this, the cells can insure that the phosphate attached by PKA to Phosphorylase Kinase and Phosphorylase-a are not immediately removed by PP-1.
Describe the fight of flight response.
Epinephrine is a hormone released from the adrenal medulla under stressful conditions. It will prep the body for a burst of high activity. The hormone will bind to alpha-adrenergic receptors and start the glycogenolysis cascade to provide the necessary ATP and it will shut off glycogen synthesis.
Describe the role of AMP and Calcium in muscle.
AMP is the “spent energy” of muscles. AMP is a positive effector for glycogen phosphorylase. AMP stabilizes glycogen phosphorylase in the Relaxed conformation, allowing it to cleave glycogen for more ATP synthesis.
Calcium binds to phosphorylase kinase and activates it and continues glycolgenolysis. Ca2+ also directly activates protein Kinase C, similar to DAG, which also starts the glycogenolysis cascade.
Describe how UDP-glucose is formed.
Before glucose is incorporated into glycogen, it must be activated into UDP-glucose. Uridylyl trasferase will join G-1-Phosphate and UTP into UDP-glucose. Now, glycogen synthase can use it to produce glycogen.
How are alpha 1-6 branches in glycogen formed?
Glucan transferase/branching enzyme will transfer a 6-7 glucose fragment from the end of the growing glycogen polymer (at least 11 glucose long) and relocate it to an internal glucose reside at the C-6 position.
What is glycogenin?
Glycogenin has a tyrosine reside that can attach to the C-1 of UDP-glucose. This acts as a primer for glycogen synthase. All glucose residues will added from this first glucose on the primer.
Describe the three major allosteric effectors and its effect on glycogen synthase-b.
Glycogen synthase-b is the phosphorylated form of glycogen synthase. Glycogen synthase will have:
Decreased affinity for UDP-glucose (substrate)
Increased dependency on glycogen synthase for allosteric activator G-6-Phosphate
Increased affinity for ATP and Pi
Describe the properties and role of glucose-6-phosphatase in glycogen metabolism.
Glucose-6-Phosphatase is located on the cisternal surface of the ER. In order for the ER to package glucose for excretion, G-6-P must transverse the ER membrane by a translocase. Then, the G-6-Phosphatase can cleave the phosphate off of G-6-P and yield free glucose which enters the bloodstream. OR, in muscle, where G-6-Phosphatase is absent, G-6-P can be directly channeled into glycolysis for ATP production.
Describe how a futile cycle of glycogenolysis and glycogen synthesis (i.e. operating at the same time) is prevented.
Phosphorylation of many enzymes is a simple way of avoiding the futile cycle of having synthesis and breakdown occuring at the same time. Phorphorylation of PKA will lead to activation and breakdown of glycogen. But, phosphorylation of Glycogen Synthase will lead to its inactivation, preventing synthesis in the middle of breakdown.
Describe very briefly the mode of action of alpha-andrenergic and beta-andrenergic hormones.
The action of the hormones at alpha-adrenergic receptors are coupled through G-proteins that activate phospholipase C (PLC), leading to the hydrolysis of PIP2 into IP3 and DAG.
IP3 releases DAG and Ca2+ activates PKC, which phosphorylates and Inactivates glycogen synthase, either directly or via Ca-Calmodulin.
