GLYCOGEN METABOLISM Flashcards
(11 cards)
What is glycogenesis vs glycogenolysis
glycogenesis = making glycogen
glycogenolysis = breaking down glycogen into glucose
where is glycogen stored
- Cytosol of all tissues, mainly in liver (up to 6% of weight) and muscle (1-2% of weight)
- STORED IN LIVER and some in muscle
what is the finction of glycogen stores in liver and muscle
Liver:
- storage and export of glucose; maintains blood [glucose]
- Depleted after 12-18 hours fasting
- Essential for glucose homeostasis
Muscle:
- Fuels glycolysis within muscle only. Depleted through prolonged vigorous exercise
- Not essential for glucose homeostasis (muscle can mobilise its glycogen but can not release it into the blood stream- because no glucose 6-phosphatase)
- Used for high intensity exercise
- No glucagon receptors in muscle -> don’t mobilise glycogen when starving
what happens in glycogenolysis in the muscle
- In muscle, the glucose 6-P enters glycolysis producing ATP.
- This process of anaerobic glycogenolysis in muscle 3 moles of ATPs are produced per mole glucose 6-P since there is no need to for the first ATP requiring step, ie hexokinase, since the glucose is already phosphorylated.
Thus, more ATP is produced in anaerobic glycogenolysis than anaerobic glycolysis, and the speed ie rate per minute exceeds aerobic glycolysis.
summarised steps glycogenesis
- Glycogen synthase adds glucose residues to the 2 ends of the chain of glycogen
- When glycogen chain is long enough, branching enzyme ‘snips’ the left chain and reattaches it to the right chain (in diagram) to form 1—>6 glucosidic bonds (making 3 ends to branched glycogen structure)
- Each end of chain represents active site where glucose molecules can be attached or metabolised again, which speeds up process of storage and mobilisation
3 summary steps og glycogenolysis
- Branching points removed through Glucan transferase debranching enzyme
- Glycogen phosphorylase removes one glucose residue at a time from each of the ends of the glycogen ends of the branches, released glucose 1-P
- Glucose 1-P mutated back to glucose 6-P to be used in glycolysis or, in the case of the liver, to be released into blood stream to maintain glucose homeostasis
- Once these chains of glucose residues get short enough, transferase enzyme shifts 3 of those glucose residues onto the end, then debranching enzyme snips off the branched point
- Glycogen phosphorylase continues to remove glucose residues one glucose at a time
- Usually only 2 ATP produced anaerobically, but when glucose is provided from the store of glycogen, you yield 3 ATP per molecule of glucose
This is because hexokinase step is not required so investment = -1 ATP, produced = 4 ATP
Therefore net ATP = 4-1 = 3 per molecule of glucose
what is the net ATP gain from glycogenolysis
Usually only 2 ATP produced anaerobically, but when glucose is provided from the store of glycogen, you yield 3 ATP per molecule of glucose
This is because hexokinase step is not required so investment = -1 ATP, produced = 4 ATP
Therefore net ATP = 4-1 = 3 per molecule of glucose
what are the limiting steps of glycogenolysis and glycogenesis
Glycogen breakdown slowest step = catalysed by glycogen phosphorylase
Glycogen synthesis slowest step = catalysed by glycogen synthase
When starving, what do glucagon and adrenlin do to regulate glycogenesis and glycogenolysis
- stimulates cAMP -> phosphorylates phosphrylase kinase = activated -> phosphorylates glycogen phosphorylase which converts glycogen to UDP-glucose (glycogen breakdown)
- at same time, cAMP kinase phosphorylates glycogen synthase so it is inactive (inhibits glycogen production)
- inhibits removal of phosphate groups by protein phosphatase 1 -> inhibit enzyme by activating cAMP kinase which will phophorylate inhibitor 1 = activated -> inhibitor 1 inhibits protein phosphatase 1 so it cannot remove phosphates (keeps glycogen production inhibited)
what occurs when muscles need to break down glycogen?
Ca2+:
- activates phosphorylase kinase (via calmodium component) which activates glycogen phosphrylase which breaks down glycogen
- also activates calmodium dependent protein kinase which phosphorylates glycogen synthase (inhbited) to inhibit glycogen formation
how does insulin regulate glycogen metabolism -> wants to stop using glucose
- activates phospodiesterase = breakdown of cAMP
- no longer phsophrylating phosphorylase kinase -> doesn’t phosphorylate glycogen phosphorylase so no glycogen breakdown -> and no phosphorylation of glycogen synthase so it remains active and turns UDP-glucose to glycogen
- inhibitor 1 not stimulated, therefore protein phosphatase 1 removes all phosphate groups and activates glycogen synthase
