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Bavidra kulendrarajah Storage of glucose
-glucose can't be stored as high concentrations disrupt the osmotic balance of the cell- cell death/damage
-in animals storage form of glucose is glycogen- nonosmotically active polymer and can be readily be mobilised into glucose
-most of the glucose residues in glycogen are linked by a-1,4-glycosidic bonds. Branches at about every 10th residue are created by a-1,6-glycosidic bonds Glycogen: large, highly hydrated, branched polymer of glucose and occupies a large volume of the cell so storage is very limited Present in all tissues but significant amounts in liver and muscle- granules in the cytoplasm Reserves Liver: 70g 280Kcal - store helps to maintain constant blood glucose concentrations Muscle 225g 900Kcal -local source of glucose for concentration-glycogen is an endogenous substrate Blood glucose 15g 60kcal Total: 310grams
1240kcal (less than 5% of body energy stores)
-takes place when glucose is abundant Glucose-6-P ? glucose-1-P ? UDP-Glucose ? glycogen and other complex sugars
-substrate: activated from of glucose uridine diphosphate glucose (UDPglucose) - glucose1-P reacts with Uridine triphosphate (UTP) to form UDP-glucose and PPi
-Glycogen synthase: catalyses the reaction where UDP-glucose is added to the C4 hydroxyl group of a terminal glucose residue to form an a-1,4 glycosidic bonds. The UDP group is displaced by the hydroxyl group. This forms a growing chain of glucose molecules. UDP-Glucose + (glycogen)n ? (Glycogen)n+1 + UDP
-Glycogen synthase can only add glucose residues to a polysaccharide chain that has more than 4 residues. Glycogen synthesis requires a primer. Glycogenin , is a protein, that acts as a primer that can autoglycosylation and forms the nucleus for glycogen synthesis and controls the size of the product.
-Glycogen synthase catalyses only the formation of alpha 1,4 linkages. Another enzyme forms alpha-1,6 linkages.
-branching: increases the solubility and increases the number of ends- the number of terminal residues increased which are the sites of action of glycogen phosphorylase and synthase, so increases the rate of glycogen synthesis/degradation Energetic cost of synthesis
-one ATP is consumed per residue of glucose added
-as ATP is needed to synthesise UTP from UDP ATP + UDP ? UTP + ADP Glycogen mobilisation Phosphorolysis (cleavage of the bond by addition of orthophosphate): Glycogen phosphorylase cleaves the glycogen by adding an orthophosphate (Pi) to form glucose-1-phosphate (Glycogen)n + Pi ? (Glycogen)n-1 + Glucose-1-P
-glycogen phosphorylase acts on the terminal ends of the glycogen as the ends have a free 0H group on C4. Orthophosphate splits the glycosidic linkaged between C1 of terminal residue and C4 of the adjacent one
-Glucose-1-P is converted into Glucose-6-P by isomerase enzyme, phosphoglucomutase
-There is sequential removal of glucose residues from free terminals until 4 th residue from a branch is reached. At this point the glycogen phosphorylase cannot remove residues
-Glycogen transferase shifts a block of three glucose residues from one outerbranch to another. This transfer results in a single glucose residue attached by an alpha-1,6-glycosidic linkages
-alpha-1,6-glucosidase (debranching enzyme) hydrolyses the alpha-1,6glycosidic bond. As phoshporolysis doesn't occur, a free glucose molecule is released and the phosphorylated by hexokinase.
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