Glycogen Synthesis and Mobilisation Flashcards
what is role of liver glycogen and muscle glycogen? which is more?
Liver glycogen (~100g):
- is used to maintain glucose levels in the blood
- a temporary buffer that the liver can release in the blood to allow you to even out during mealtimes.
Muscle glycogen (~400g):
- is used to maintain muscle during bursts of activity: an instant burst of rapid movement so our cells can gain energy instantly.
- does not release glucose into blood ! nothing to do with maintaining blood glucose, yet allows you to undergo rapid exercise.
If the muscle had no store of energy in moments of exercise, it would have to gain it from circulation which is not as fast.
why store glucose as glycogen instead of glucose? (2)
glycogen:
- less osmotically active than glucose (if stored glucose - would cause water to come into cell & burst)
-
quick to transition or mobilise
to either:
a) maintain blood glucose: to / from liver
b) produce ATP via anaerobic or aerobic resp glycolysis: muscle
describe structure of glycogen:
- made from which monomer?
- what type of bonds connect the monomer?
-
- polymer of glucose residues
- attached by alpha 1-4 glycosidic bonds (but forms branches by using 1-6 links)
what do two starting materials do you need before glycogen synthesis?
what do you convert one of ^ into for glycogen synthesis (and how)?
what is the mechanism of glycogen synthesis? (3)
- *glycogen synthesis needs:**
- a primer (protein that glucose will attach to): glycogenin.
- glucose-6-phosphate (G6P)
BUT: NEED TO CONVERT G6P -> UDP-glucose before can be added to glycogen:
a) G6P –> G1P
b) G1P –> UDP-glucose
- *glycogen synthesis:**
- UDP-glucose added to glycogenin primer initially, and then non-reducing ends of glucose by enzyme glycogen synthase to create a glycosidic-1-4 bonds
- the UDP is lost and one glucose is added onto the glycogen
- branches are made by branching enzyme: creates a 1-6 glycosidic bonds
what is name of large macromolecule of glycogen?
why is it advantageous for glycogen to be stored as branched macromolecule?
large macromolecule of glycogen: macroglycogen
advantage as branched macroglycogen:
- more enzymes can work on to build / reduce quickly
what is glycogen breakdown aka?
explain how this occurs (4)
glycogenolysis:
- debranching enzyme: breaks down the a-1,6 glycosidic bonds (the branches of glucose)
- glycogen phosphorylase: breaks down a-1,4 glycosidic bonds: free G1Ps
- phosphoglucomutase: converts G1P to G6P
- in the liver: glucose-6-phosphatase removes the P group = free glucose
(but step 4 does not occur in the muscle - instead, it is immediately used in glycolysis)
- in the liver glycogenolysis is mediated by which hormone?
^^ does WHAT to glycogen phosphorylase?
liver glycogenolysis is mediated by **glucagon:
- glucagon**
phosphorylates glycogen phosphorylase = its active form during periods of fasting or overnight: starts the glycogenolysis pathways (see previous slide)
- in muscle - how can glucose-6-phosphate produce ATP (aerobically and anaerobically?
- why does muscle not have a role in raising blood glucose levels?
in muscle, glucose-6-phosphate can produce ATP:
- **aerobically (via Acetyl coA, krebs cycle & oxidative phosphorylation) - anaerobically (via lactate)
- free glucose cannot**
be produced / released from skeletal muscle bc it doesnt have glucose-6-phosphatase (to convert G6P -> glucose)
from which - liver or muscle is free glucose released into blood?
free glucose is released from liver because only the liver contains glucose-6-phosphatase, which converts glucose-6-phosphate into glucose. muscle doesnt have this enzyme
glycogen production and breakdown is carried out by which hormone signalling molecules (4) and which do they act on - liver or muscle?
- insulin: muscle and liver - builds glycogen stores
2. glucagon: only liver - breaks down glyocgen stores to release glucose
3. adrenaline: muscles via a & b adrergic receptors - release glucose
4 calcium: muscles via a & b adrergic receptors - release glucose
when is insulin / glucagon released?
what do insulin and glucacon to do: & how?
a) glycogen synthase
b) glycogen phosphorylase
* key - learn this *
- *insulin: released after meal.** insulin works via protein phosphatase (removes Ps):
- activates glycogen synthase - by removing P
- inhibits glycogen phosphorylase - by removing P
- *glucagon & adrenaline: released between meals / when fasting:** works via cAMP, protein kinase A and phosphorylase kinase: adds P
- inhibits glycogen synthase - adds P
- activates glycogen phosphorylase - adds P
in the production / break down of glucagon, glycogen synthase and glycogen phosphorylase are made active / inactive by the addition of what?
glycogen synthase is activated by removing P, inactivated by adding P
glycogen phosphorylase is activated by adding P, inactivated by removing P
therefore work antagonistically !
what are the two pathways that insulin causes glycogen synthase to be activated and cause glucose -> glycogen?
insulin:
- activates phosphodiesterase
- activates protein phosphastase
which are two different pathways that both end up in the glucose –> glycogen
whats the MoA of insulin
- activating phosphodiesterase?
- activating protein phosphastase?
1. - insulin activates phosphodiesterase
- phosphodiesterase breaks down cAMP -> AMP
- this means protein kinase A is not activated
- this means that glycogen synthase is not phosphorylated and not switched off
- glycogen synthase: glucose -> glycogen
AND
- phosphorlase kinase is not active: glycogen phosphorylase not activated = glycogen break down inhibited
2. insuline activates protein phosphatase -> downstream consequences also: - glycogen synthase: glucose -> glycogen
what is the mechanism of adrenaline, glucagon and calcium activating glycogen -> glucose
- adrenaline, glucagon and calcium activate adenylate cyclase
- adenylate cyclase causes: ATP –> cAMP
- cAMP inhbits glycogen synthase by phosphorylasing it AND cAMP promotes phosphorylation of phosphorylase kinase
- phosphorylase kinase promotes phosphorylation of glycogen phosphorylase
- glycogen phosphorylase breaks down glycogen into glucose
