Glycogen Metabolism

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WHAT IS THE PREFFERED ENERGY SOURCE OF THE BRAIN

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Glucose

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IT IS THE SUBSTRATE FOR ANAEROBIC GLYCOLYSIS

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Glucose

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THE BLOOD GLUCOSE CAN BE OBTAINED FROM THREE PRIMARY SOURCES

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Diet

degradation of glycogen

gluconeogenesis

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IT CAN PROVIDE SUSTAINED SYNTHESIS OF GLUCOSE, BUT IN SLOW PROCESS IN RESPONDING TO A FALLING BLOOD GLUCOSE LEVEL.

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Gluconeogenesis

*when glycogen stores are depleted, specific tissues synthesise gluxose de novo, using amino acids from the body's proteins as primary source of carbons for the gluconeogenic pathway.

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STORAGE FORM OF GLUCOSE

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Glycogen

* this sugar is rapidly released from the LIHVER AND KIDNEY when the dietary source glucose is absent

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THE MAIN STORES OF GLYCOGEN IN THE BODY ARE FOUND

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Skeletal muscle and Liver

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MUSCLE GLYCOGEN

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Serve as a Fuel Reserve for the synthesis of ATP during MUSCLE CONTRACTION

*is Not affected by SHORT PERIODS OF FASTING (a few days) and Only moderately decreased in PROLONGED FASTING (WEEKS)

*This is SYNTHESIZED to replenish muscle stores after they have been depleted during strenuous exercise

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LIVER GLYCOGEN

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Maintain the blood glucose concentration (EARLY STAGES OF A FAST)

*stores increase during the well-fed state (DEPLETED DURING A FAST)

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WHAT IS THE PRIMARY GLYCOSIDIC BOND OF GLYCOGEN?

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ALPHA (1--->4) LINKAGE

*Glycogen - branched polysaccharide (alpha - D- Glucose)

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THE GLYCOGENESIS requires

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ATP AND UTP

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THIS IS THE SOURCE OF ALL THE GLUCOSYL RESIDUES THAT ARE ADDED TO THE GROWING GLYCOGEN MOLECULE

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UDP-Glucose

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Review

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*GLUC-1,6-BIPHOSPHATE-an obligatory intermediate (phosphoglucomutasE)

*Gluc-1-phos + UTP(--->UDP) (UDP-GLUC-PYROPHOSPHORYLASE) = UDP GLUCOSE AND INORGANIC PHOSPHATES

*PPi (PYROPHOSPHATASE) = 2 INORGANIC PHOSPHATASE

NOTE: PYROPHOSPHATASE = ensures that the UDP-GLUCOSE-PYROPHOSPHORYLASE reaction proceeds in the direction of UDP-GLUC production.

*Glycogen synthase - FOR MAKING ALPHA (1---->4) linkages in the glycogen, it can only elongate already existing chains of gluc

*GLYCOGENIN (tyrosine)= a primer; glycogen fragment that serve as an acceptor of glucose residues from UDP-GLUC (AUTOGLUCOSYLATION)

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IT FORMS THE CORE OF A GLYCOGEN GRANULE

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Glycogen in (an enzyme)

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ELONGATION OF GLYCOGEN CHAINS

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by transfer of GLUC from UDP-GLUC to the NON-REDUCING END OF THE GROWING CHAIN

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AMYLOSE

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the product if no other synthetic enzyme acted on the chain

the linear ( unbranched) molecule of GLUCOSYL RESIDUES attached by alpha (1---->4 linkages)

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BRANCHING ENZYME- removes a chain of 6-8 GLUCOSYL RESIDUES from the nonreducing end of the glycogen chain, breaking an alpha (1--->4) bond to another residue on the chain and attaches it to a NON-TERMINAL GLUCOSYL RESIDUES by an alpha (1--->6) linkage

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amylo-alpha (1--->4) ---> alpha(1--->6)- transglucosidase (4:6 transferase)

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WHAT IS THE PRIMARY PRODUCT WHEN GLYCOGEN IS DEGRADED (GLYCOGENOLYSIS)

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Gluc-1-phos (from alpha (1--->4) linkage)

another note:

FREE GLUC (from alpha (1--->6) Linkage by AMYLO-ALPHA (1--->6)-GLUCOSIDASE)

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WHEN THERE IS PHOSPHORYLATION...

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GLYCOGENOLYSIS will be activated (GLYCOGEN PHOSPHORYLASE BY simple phosphorolysis)

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IN THE LIVER, THE GLUCOSE-6-PHOS IS TRANSPORTED IN THE ER BY:

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Gluc-6-phosphate translocase

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READ THIS

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In the MUSCLE, Gluc-6-phosphate cannot be dephosphorylated because of a lack og glucose-6-phosphatase. INSTEAD, it enters GLYCOLYSIS, providing energy needed for MUSCLE CONTRATION.IT

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IT IS THE ONLY GSD THAT IS A LYSOSOMAL STORAGE DISEASE

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TYPE II-POMPE disease

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REMEMBER THIS

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*accelerates during a periods of fasting DURING ACTIVE EXERCISE = GLYCOGENOLYSIS

*occurs when the muscle is at rest = GLYCOGENESIS (Fed state)

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WHAT IS THE ENZYME THAT PHOSPHORYLATES THE UDP INTO UTP

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nucleoside disphosphate kinase

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THIS ENZYME CATALYZES THE SYNTHESIS OF cAMP

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Adenylyl cyclase

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IT ACTIVATES CAMP-DEPENDENT PROTEIN KINASE A

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C-AMP (bind to the regulatory subunit barrier of PKA and release catalytic subnits)an

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AN ENZYME THAT DEGRADES C-AMP INTO 5'AMP

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phosphodiesterase (activated by the INSULIN)

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ACTIVE FORMS:

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GLYCOGEN SYNTHASE = dephosphorylated (by PROTEIN PHOSPHATASE -1 which removes the phosphate groups hydrolytically)

GLYCOGEN PHOSPHORYLASE = PHOSPHORYLATED (GLYCOGEN PHOSPHORYLASE KINASE)

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WHAT IS THE REGULATED ENZYME IN GLYCOGENESIS

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Glycogen synthase

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another differentiation

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GLYCOGENESIS = stimulated WHEN THE SUBSTRATE AVAILABILITY AND ENERGY LEVELS ARE HIGH

GLYCOGENOLYSIS = increased when GLUCOSE AND ENERGY LEVELS ARE LOW

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IN THE WELL FED STATE,

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the GLYCOGEN SYNTHASE (ACTIVE) in both LIVER AND MUSCLE is ALLOSTERICALLY activated by GLUC-6-PHOSPHATE

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THE activated CALCIUM-CALMODULIN COMPLEX activates the (MUSCLE)

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MUSCLE PHOSPHORYLASE KINASE B ------> PKA

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DURING THE FIGHT OR FLIGHT SITUATIONS, ____________________ IS RELEASED FROM THE ADRENAL MEDULLA AND SIGNALS THE NEED FOR BLOOD GLUCOSE WHICH CAUSES HEPATIC GLYCOGENOLYSIS

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epinephrine

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IN THE MUSCLE, THE PRESENCE OF HIGH AMP WILL

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activates the GLYCOGEN PHOSPHORYLASE without phosphorylation

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WHAT IS THE BUILDING BLOCK OF GLYCOGEN?

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UDP-GLUCOSE

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AN ELEVATED INSULIN LEVEL RESULTS

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Overall INCREASED GLYCOGENESIS and DECREASED GLYCOGENOLYSIS

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AN ELEVATED GLUCAGON (EPINEPHRINE) WILL RESULT

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INCREASED GLYCOGENESIS AND DECREASED GLYCOGENOLYSIS

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FASTING STATE:

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DEC INGESTION OF FOOD = DEC BLOOD GLUCOSE = DEC RELEASE OF OF P INSULIN & INC RELEASE OF GLUCAGON = INC PROTEIN KINASE = PHOSPHORYLATION OF Glycogen Phosphorylase (activated) and Glycogen Synthase (inactivated) = GLYCOGENOLYSIS

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WELL - FED STATE

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INC INGESTION OF GLUCOSE = INC BLOOD GLUCOSE =INC RELEASE OF INSULIN & DEC RELEASE OF GLUCAGON = INC PROTEIN PHOSPHATASE ACTIVITY = DEPHOSPHORYLATION OF Glycogen Phosphorylase (INactivated) and Glycogen Synthase (activated) = GLYCOGENESIS

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VON GIERKE DISEASE

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Prevents the liver from releasing free GLUCOSE (FROM GLYCOGENESIS and gluconeogenesis) into the blood, causing SEVERE FASTING HYPOGLYCEMIA, HYPERLACTICACIDEMIA AND HYPERURICEMIA.