Term
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Definition
•Glycogen serves as storage carbohydrate in animals, insects and fungi
•Glc residues in a-1,4 linkage- tens of thousands per molecule (up to 100 million Da molecular weight)
•Heavily branched (a-1,6 branchpoints 8-12 residues apart) to allow rapid mobilization of Glc by sequential removal of monomers from many non-reducing ends
•In animals, glycogen accumulates in most tissues but major accumulation in liver and muscle- 1-2% wet weight resting muscle, 6-8% wet weight “fed” liver
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Term
Why store fuel as a glucose polymer rather than fat? |
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Definition
-Muscle cannot mobilize fat as rapidly as glycogen.
-Fat cannot be metabolized anaerobically.
-Animals cannot use carbons from most fatty acids for Glc synthesis, so fat metabolism alone is not adequate to maintain blood Glc levels.
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Term
Why store glucose as a polymer? |
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Definition
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1. Reserve in [glycogen]intracellular is only ~ 0.01 uM; same amount of reserve if stored as Glc monomers would require 0.4 M Glc in cytosol, which would cause a significant osmotic pressure problem
2. Retaining Glc in cells requires phosphorylation to Glc-6-P; if stored as Glc monomers would cause serious ionic strength problem
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Term
UDP-Glc formation is catalyzed by ? |
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Definition
Glc-1-P uridylyl transferase
Reaction requires 1 UTP
UTP is energetically equivalent to ATP due to nucleoside diphosphate kinase reaction:
UDP + ATP <-> UTP + ADP
=> 1 ATP is required for each Glc residue added to glycogen
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Term
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Definition
-catalyzes addition of Glc from UDP-Glc to growing chain
-can form a-1->4 but not 1->6 (branch) linkages
-cannot catalyze de novo glycogen synthesis
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Term
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Definition
-a self glucosylating enzyme that generates a glycogen “primer”
•Glycogenin catalyzes covalent linkage of Glc to its own Tyr194
•Up to seven additional Glc residues are linked to the first.
Glycogenin-Glc 8 primes glycogen synthesis
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Term
Glycogenin and glycogen synthase act together to form the main chain |
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Definition
1.1st Glc is transferred to glycogenin
2.Glycogen synthase joins the complex
3.Linear polymer (n~8 Glc) elongated on glycogenin
4.Glycogen synthase extends and dissociates from glycogenin
5.Branching occurs; glycogen synthase may dissociate and act elsewhere on the molecule, glycogenin remains covalently bound
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Term
How does branching occur? |
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Definition
Glycosyl-(1,4->1,6)-transferase, or branching enzyme, moves a terminal segment of ~ 7 residues to a branch position
Energy of a-1,4 bond hydrolysis drives a-1,6 bond formation
Branching: Pros
Increases number of non-reducing ends for digestion
Increases density of polymer
Increases hydration/solubility of polymer
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Term
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Definition
stores allow maintenance of blood Glc
Glycogen is stored in the fed state, and Glc from glycogen is exported in fasting state to maintain blood Glc
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Term
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Definition
-are used to fuel activity
-Glycogen is stored after meals and during inactivity
-Glycogen is used as a major source of Glc during activity
Remember, fasting state does NOT deplete muscle glycogen for weeks, whereas exercise depletes it in hours
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Term
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Definition
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-is an allosteric activator of glycogen synthase
-Activates glycogen synthase ONLY in the phosphorylated (less active) state
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Term
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Hormonal signals determine phosphorylation state of glycogen synthase.....
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Definition
-Insulin and glucagon are peptide hormones secreted by pancreatic b and a cells, respectively, in response to blood Glc levels
-When blood Glc is high, pancreatic cells secrete insulin, whereas secretion of glucagon is inhibited
-When blood Glc falls, pancreatic cells no longer release insulin, and secretion of glucagon increases
-Liver cells have cell surface receptors for insulin and glucagon that have different signaling mechanisms to provoke opposite responses
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Term
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Definition
-turns glucose-6-phosphate to glucose-1-phosphate
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