Storage Of Glucose Notes

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 10^th 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)

Glycogen synthesis

-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 (UDP-glucose) – 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-1,6-glycosidic bond. As phoshporolysis doesn’t occur, a free glucose molecule is released and the phosphorylated by hexokinase.

-the glycogen transferase and the debranching enzyme converts the branched structure into a linear one and allows further cleavage by glycogen phosphorylase

-In eukaryotes the transferase and the debranching activites are present in a single polypeptide chain-bifuctional enzyme.

Mobilisation of glycogen

Glycogen phosphorylase exists in an inactive form ‘b’ and an active form ‘a’. Each of these two interconvertible forms has an active relaxed state (R) and a much less active tense state (T). The equilibrium for the active form favours the R form whereas the inactive form favours the T state.

-in resting muscle most of the glycogen phosphorylase is in the b (inactive) form- activated by the presence of high AMP concentrations and inhibited by ATP and glucose-6-P. The transition of the phosphoyrlase b between the active R state and the less active T state is controlled by the energy charge of the muscle cell.

-phosphorylase b is converted into phosphorylase a by the phosphorylation which is caused by hormones or increased calcium concentration due to muscle contraction. The enzyme that catalyses the phosphorylation of glycogen phoshporylase is glycogen phosphorylase kinase.

-phosphorylase a is fully active, regardless of the levels of AMP, ATP and glucose-6-P.

-so muscle glycogen mobilisation involves

-allosteric activation of the b form by increasing concentrations of AMP, but decreasing concentrations of ATP and glucose-6-P

-conversion of the b form to a form by- hormonal signalling/ electrical activity, increase in calcium concentration and this results in the intensity of excercise.

Control of glycogen synthesis and breakdown

Glycogen targeting proteins

Family of proteins that tether enzymes of glycogen metabolism to glycogen molecules and control their activity

G_M is a major form in skeletal muscle G_L is a major form in liver

Functions are determined by phosphorylation of the protein at different sites in response to hormonal signalling

Glycogen metabolism in muscle

Store of glycogen is used by muscle cells themselves to provide substrate for ATP synthesis. This is very important in Type IIb muscle as that are anaerobic glycolytic muscle fibres where the blood can’t supply o2/nutrients to the tissue fast enough so the muscle uses the glycogen store as its substrate

-glycogen stores are replenished after meals or after excercise

-mobilised in response to increased energy demand during excercise

Storage

-after exercise carbohydrate rich food is eaten to restock glycogen stores- when blood glucose levels are high insulin is...

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