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Biochemistry

4.3Carbohydrates&Glycogen

4.3a)Carbohydrates

Prof.Dr.KlausHeese

4. Bio-Energetics

EddytheHorse

Inserthayhere

Afuelcellpoweredvehicle…

Afuelcellpoweredoperator

- energy sources: carbohydrates, proteins, fats, ATP, aerobic-anaerobic systems

- types of energy: mechanical energy, heat energy, chemical energy, electromagnetic energy, nuclear energy, kinetic energy, potential energy, gravitational potential energy, energy conversion

- Law of Conservation of Energy (photosynthesis)

Fuel Summary

• CHO: used anaerobically and aerobically, low amount stored in body

• Fats: used only aerobically: large amounts stored in body, can only be used if CHO is available

• Protein: only used in starvation states

Key words

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Forms of Energy Transfer• The ATP-CP system relying on the “Phosphogens”.

- anaerobic

• The glycolytic (lactic acid) system relying on - anaerobic breakdown of CHO.

• The oxidative system relying on the - aerobic breakdown of CHO, Fat, and Protein depends

on oxygen supply/availability.

• In the ATP-CP system, Pi is separated from CP through the action of creatine kinase (enzyme that adds P).

• Energy yield is 1 mole of ATP per 1 mole of CP.• The Pi can then combine with ADP to reform ATP. • the change in free energy (DG °’ ) (pH 7.0) for the

hydrolysis of PCr is -45.0 kJ/mol compared with -31.8 kJ/mol for ATP,

Creatine Phosphate (CP or PC system)

Creatine synthesis – a 2-step processIntegrated communication between cellular sites of ATP utilization and ATP-generation. Cells utilize enzymatic shuttles to promote ATP delivery and removal of ATPase byproducts, ADP, Pi and H+, to sustain efficient energy utilization. Shuttles comprise near equilibrium enzymes capable of facilitating ligand transfer between cellular compartments by rapidly relaying the displacement of equilibrium. ATP delivery is facilitated through creatinekinase (CK), adenylate kinase (AK), and the glycolytic system, which includes hexokinase (Hex), pyruvate kinase (PK) and 3-phosphoglycerate kinase (PGK). ADP is removed by CK, AK and PGK shuttles. Pi transfer is catalyzed by the near-equilibrium glyceraldehyde 3-phosphate dehydrogenase (GAPDH) shuttle. H+ removal is facilitated by CK and carbonic anhydrase (CA) shuttles. As these shuttle systems operate in parallel, a diminished activity of a single enzyme is rather well tolerated. However, a decrease in the activity of several enzymes could lead to a cumulative impairment in the communication between ATP-generating and ATP consuming sites. Gl, glucose; PEP, phospho-enol pyruvate; Pyr, pyruvate; Cr, creatine.

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Glycolysis• The glycolytic system involves the process of glycolysis,

through which glucose is broken down to pyruvic acid via glycolytic enzymes.

• When conducted without oxygen, the pyruvic acid is converted to lactic acid.

• 1 mole of glucose yields 2 moles of ATP, but 1 mole of glycogen yields 3 moles of ATP.

• Key processes:• Gluconeogenesis: the process by which protein or fat is

converted into glucose.

• Glycogenesis: the process by which glycogen is synthesized from glucose.

• Glycogenolysis: breakdown of glycogen for ATP production.

Kreb’s cycle

Essential Bio-Energy-related Bio-Molecules

Common Structure of NucleotidesDi-Saccharides Lactose and Sucrose

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As for all parts of this lecture:It’s not about human physiology – but, to know the systems in order to be able to make use of it (‘translate’ and apply) from the bioengineering point of view !

Without its knowledge, no innovative and creative ideas for its potential bioengineering applications !

Carbohydrates

Carbohydrates

• glucose providesenergyforthebrain and½ofenergyformusclesandtissues

• glycogen isstoredglucose• glucose isimmediateenergy• glycogen isreserveenergy

Carbohydrates

• allplantfood• milk

• carbohydratesarenotequal– simplecarbohydrates– complexcarbohydrates

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SimpleCarbohydrates

• sugars– monosaccharides – singlesugars– disaccharides – 2monosaccharides

ComplexCarbohydrates

• starches andfibers• polysaccharides

– chainsofmonosaccharides

SimpleCarbohydrates

• monosaccharides– allare6 carbonhexes

• 6carbons• 12hydrogens• 6oxygens• arrangementdiffers

– accountsforvaryingsweetness

– glucose,fructose,galactose

hexose

pentose

Glucose

• mildsweetflavor• knownasbloodsugar• essentialenergysource• foundineverydisaccharideandpolysaccharide

hexose

pentose

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Fructose

• sweetestsugar• foundinfruitsandhoney• addedtosoftdrinks,cereals,deserts

hexose

pentose

Galactose

• hardlytastessweet• rarelyfoundnaturallyasasinglesugar

hexose

pentose

Disaccharides

• =pairsofthemonosaccharides– glucoseisalwayspresent– 2nd ofthepaircouldbefructose,galactose oranotherglucose

– takenapartbyhydrolysis– puttogetherbycondensation– hydrolysisandcondensationoccurwithallenergynutrients

– maltose,sucrose,lactose

Condensation

• makingadisaccharide– chemicalreactionlinking2monosaccharides

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Hydrolysis

• breakingadisaccharide– watermoleculesplits– occursduringdigestion

Maltose• 2glucoseunits• producedwhenstarchbreaksdown• notabundant

• fructoseandglucose• tastessweet

– fruit,vegetables,grains• tablesugarisrefinedsugarcaneandsugarbeets

• brown,white,powdered

Sucrose

Lactose

• glucoseandgalactose• maincarbohydrateinmilk– knownasmilksugar

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Complex(larger/longer)Carbohydrates

• polysaccharides– glycogen andstarch (amylose(linear)andamylopectin(branch))

• builtentirelyofglucose

– fiber• varietyofmonosaccharides andothercarbohydratederivatives

Glycogen• limitedinmeatandnotfoundinplants

– notanimportantdietarysourceofcarbohydrate• BUT

– allglucoseisstoredasglycogen– longchainsallowforhydrolysisandreleaseofenergy

Starches

• storedinplantcells• bodyhydrolyzesplantstarchtoglucose

(amylose(linear,a(1-4)-D-Glucose)andamylopectin(branch, a(1-4)-D-Glucoseanda(1-6)-D-Glucose))Fiber

• structuralpartsofplants– foundinallplant-derivedfood

• bondsoffiberscannotbebrokendownduringthedigestiveprocess– minimalornoenergyavailable

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Fibertypes

• cellulose• pectins• lignins• resistantstarches

– classifiedasfibers– escapedigestionandabsorption

(almostnoenergysupplytohuman,but…)

Buteventuallyusedbyotherorganisms--->

FiberCharacteristics

• solublefibers,viscous,fermentable– easilydigestedbybacteriaincolon– associatedwithprotectionagainstheartdiseaseanddiabetes

• lowercholesterolandglucoselevels

– foundinlegumesandfruits

Fiber

• insolubleandnoteasilyfermented– promotebowelmovements– alleviate/promotesconstipation--->haveinfood– foundingrainsandvegetables

CarbohydrateDigestion

• breakdownintoglucose– bodyisabletoabsorbanduse

• largestarchmolecules– extensivebreakdown

• disaccharides– brokenonce

• monosaccharides– don’tneedtobebrokendown

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CarbohydrateDigestion

• beginsinmouth– chewingreleasessaliva– enzymeamylasehydrolyzesstarchtopolysaccharidesandmaltose

• stomach– noenzymesavailabletobreakdownstarch– aciddoessomebreakdown– fibersinstarchprovidefeelingoffullness

• smallintestine– majorityofcarbohydratedigestiontakesplacehere

– pancreaticamylasereducescarbstoglucosechainsordisaccharides

– specificenzymesfinishthejob• maltase

– maltoseinto2glucose• sucrase

– sucroseintoglucoseandfructose• lactase

– lactoseintoglucoseandgalactose

Biotechnologicalapplications

Enzymes involved in CHO digestion / break down

• largeintestine– 1-4hoursforsugarsandstarchestobedigested

– onlyfibersremain• attractwater,whichsoftensstool

– bacteriafermentsomefibers• water,gas,short-chainfattyacids(usedforenergy)

CHO digestionCarbohydrateAbsorption

• glucosecanbeabsorbedinthemouth• majorityabsorbedinsmallintestine

– activetransport• glucose

– facilitateddiffusion• fructose• smallerriseinbloodglucose

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LactoseIntolerance• morelactoseisconsumedthancanbedigested

– lactosemoleculesattractwater• causefloating,abdominaldiscomfort,diarrhea

– intestinalbacteriafeedonundigestedlactose• produceacidandgas

LactoseIntolerance

• age,damage,medication,diarrhea,malnutrition• managementrequiresdietarychange

– 6grams(1/2cup)usuallytolerable– takeingradually– hardcheeses&cottagecheese– enzymedropsortablets

• lactosefreedietisextremelydifficulttoaccomplish

CarbohydrateMetabolism• 1/3ofbody’sglycogenisstoredinliver

– releasedasglucosetobloodstream1. eat– intakeglucose2. livercondensesextraglucosetoglycogen3. bloodglucosefalls4. liverhydrolyzesglycogentoglucose

Glycogenisbulky,sowestoreonlysomuch:shorttermenergysupply

Fatisthelongtermenergysupply.

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GlucoseforEnergy

• enzymesbreakapartglucose– yieldingenergy• inadequatesupplyofcarbohydrates

– ketonebodies(fatfragments)areanalternateenergysourceduringstarvation

– excessketonescanleadtoketosis:imbalanceofacidsinbody

• minimumof50– 100gramsofcarbs/dayareneededtoavoidketosis

GlucoseHomeostasis

• maintaininganevenbalanceofglucoseiscontrolledbyinsulinandglucagon– insulin

• movesglucoseintotheblood(ensuresproperusage/uptakeofglucosebythevariouscells/tissues/organs– if--->diabetes)

– glucagon• bringsglucoseoutofstorage

• maintainingbalance– balancedmealsatregularintervals

• fiberandsomefatslowthedigestiveprocessdown• glucosegetsintothebloodslowandsteady

3or5,doesn’tmatter--- butenergyinput~energyoutput

MaintainingBloodGlucose

Homeostasis

IntestineWhenapersoneats,bloodglucoserises.

1

2

Insulinstimulatestheuptakeofglucoseintocellsandstorageasglycogenintheliverandmuscles.Insulinalsostimulatestheconversionofexcessglucoseintofatforstorage.

3

4

5

6

7 Bloodglucosebeginstorise.

a Thestresshormoneepinephrineandotherhormonesalsobringglucoseoutofstorage.

GlucoseInsulinGlucagonGlycogen

Glucagonstimulateslivercellstobreakdownglycogenandreleaseglucoseintotheblood.a

Liver

Lowbloodglucosestimulatesthepancreastoreleaseglucagonintothebloodstream.

Asthebody'scellsuseglucose,bloodlevelsdecline.

Glucagon

Pancreas

Fatcell

Liver

Muscle

Highbloodglucosestimulatesthepancreastoreleaseinsulin.

Pancreas

Insulin

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Imbalance

• diabetes– afterfoodintake,bloodglucoserisesandisnotregulatedbecauseinsulinisinadequate(--->useofketobodies)

• hypoglycemia– bloodglucosedropsdramatically

• toomuchinsulin,activity,inadequatefoodintake,illness• dietadjustmentincludesfiber-richcarbsandprotein

Sugar

• ½comesfromnaturalsources,½fromrefinedandadded– sucrose,cornsyrup,honey

• excesscanleadtonutrientdeficienciesandtoothdecay– emptycalories– sugarandstarchbreakdowninthemouth

StarchandFiber

• dietthatincludesstarch,fiberandnaturalsugars– wholegrains,vegetables,legumes,fruits

• mayprotectagainstheartdiseaseandstroke• reducestheriskoftype2diabetes• enhancesthehealthofthelargeintestine• canpromoteweightloss

StarchandFiber

• starchintake– 45-65%– 225– 325grams(DV(dailyvalue)is300grams)

– 900-1300kcal/2000kcal– RDA(RecommendedDailyAllowance)is130grams

• fiberintake– DailyValue(DV)is25grams/2000kcal

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Groceries

• grains:1serving=15grams• vegetables

– ½cupstarchy=15grams– ½cupnonstarchy=5grams

• fruit:1serving=15grams• milk:1cup=12grams• meat:noneorlittle• legumes:½cup=15grams

ArtificialSweeteners

• helpkeepsugarandenergyintakedown• anythingweeathasFDA(foodanddrugadministration)approval– saccharin– aspartame– acesulfame potassium– sucralose– neotame

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SugarReplacers

• sugaralcohols– providebulkandsweetness

• cookies,gum,candy,jelly

– docontainminimalkcal– lowglycemicresponse

• absorbedslowly

– donotcausedentalcaries

Biochemistry

4.3b)GlycogenMetabolism

Prof.Dr.KlausHeese

Glycogen isapolymerofglucose residueslinkedbyw a(1à4)glycosidic bonds,mainlyw a(1à6)glycosidic bonds,atbranchpoints.Glycogenchains&branchesarelongerthanshown.

Glucoseisstoredasglycogenpredominantlyinliver andmuscle cells.

H O

OHH

OHH

OH

CH2OH

HO H

H

OHH

OH

CH2OH

H

O

HH H O

OH

OHH

OH

CH2

HH H O

H

OHH

OH

CH2OH

H

OH

HH O

OH

OHH

OH

CH2OH

H

O

H

O

1 4

6

H O

H

OHH

OH

CH2OH

HH H O

H

OHH

OH

CH2OH

HH

O1

OH

3

4

5

2

glycogen

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GlycogenPhosphorylase catalyzesphosphorolyticcleavageofthea(1à4) glycosidic linkagesofglycogen,releasingglucose-1-phosphateasreactionproduct.

glycogen(n residues) + Pi à

glycogen (n–1 residues) + glucose-1-phosphate

This phosphorolysismay be compared to hydrolysis:Hydrolysis:R-O-R' + HOH à R-OH + R'-OHPhosphorolysis:R-O-R' + HO-PO3

2- à R-OH + R'-O-PO32-

glucose-1-phosphate

H O

OHH

OHH

OH

CH2OH

H

OPO32-

H

Glycogencatabolism(breakdown):

Themajorproductof glycogenbreakdownisglucose-1-phosphate,fromPhosphorylase activity.

Pyridoxal phosphate(PLP),aderivativeofvitaminB6,servesasprostheticgroupforGlycogenPhosphorylase.

pyridoxal phosphate (PLP)

NH

CO

P

O-O

O

OH

CH3

CH O

-

+

H2

Pyridoxalphosphate(PLP)isheldattheactivesitebyaSchiffbase linkage,formedbyreactionofthealdehydeofPLPwiththee-aminogroupofalysine residue.Incontrasttoitsroleinotherenzymes,thephosphate ofPLPisinvolvedinacid/basecatalysisbyPhosphorylase.

NH

CO

P

O-O

O

O-

CH3

HC-

+

H2

N

(CH2)4

Enz

H+

Enzyme (Lys)-PLP Schiff base

H3N+ C COO-

CH2

CH2

CH2

CH2

NH3

H

+

lysine

ThePi substrate bindsbetweenthephosphateofPLPandtheglycosidic Olinkingtheterminalglucoseresidueoftheglycogen.

NH

CO

P

O-O

O

O-

CH3

HC-

+

H2

N

(CH2)4

Enz

H+

Enzyme (Lys)-PLP Schiff base

AfterthePi phosphatesubstratedonatesH+ duringcleavageoftheglycosidic bond,itreceivesH+ fromthephosphatemoietyofPLP.PLPthentakesbacktheH+ asthephosphateOattacksC1ofthecleavedglucosetoyieldglucose-1-phosphate.

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Aglucoseanalog,N-acetylglucosamine(GlcNAc),isadjacenttopyridoxalphosphateattheactivesite inthecrystalstructureshown.

GlycogenPhosphorylase:ahomodimericenzyme,subjecttoallostericcontrol.Ittransitionsbetween“relaxed” (active)&“tense” (inhibited)conformations.

PLP

PLP GlcNAc

GlcNAc

inhibitor

Human Liver Glycogen Phosphorylase PDB 1EM6

Question:WhywouldaninhibitorofGlycogenPhosphorylasebeasuitabletreatmentfordiabetes?

Aclassofdrugsdevelopedfortreatingthehyperglycemiaofdiabetes(chloroindole-carboxamides),inhibitliverPhosphorylaseallosterically.Theseinhibitors bindatthedimerinterface,stabilizingtheinactive(tense)conformation.

PLP

PLP GlcNAc

GlcNAc

inhibitor

Human Liver Glycogen Phosphorylase PDB 1EM6

Phosphoglucomutase catalyzesthereversiblereaction:glucose-1-phosphate ßà glucose-6-phosphate

Aserine OH attheactivesitedonates&acceptsPi.Thebisphosphate isnotreleased.Phosphoglycerate Mutase hasasimilarmechanism,butinsteadusesHisforPi transfer.

glucose-1-phosphate glucose-6-phosphate

H O

OHH

OHH

OH

CH2OH

H

OPO32-

H H O

OHH

OHH

OH

CH2OPO32-

H

OH

HH O

OHH

OHH

OH

CH2OPO32-

H

OPO32-

H

Enzyme-Ser-OPO32- Enzyme-Ser-OPO3

2-Enzyme-Ser-OH

glycolysis Glucose-6-phosphatemayenterGlycolysisor(mainlyinliver)bedephosphorylatedforreleasetotheblood.LiverGlucose-6-phosphatase catalyzesthefollowing,essentialtotheliver'sroleinmaintainingbloodglucose:glucose-6-phosphate+H2O à glucose+Pi

Mostothertissueslackthisenzyme.

Glycogen Glucose Hexokinase or Glucokinase Glucose-6-Pase Glucose-1-P Glucose-6-P Glucose + Pi Glycolysis Pathway Pyruvate Glucose metabolism in liver.

blood

GlycogenPhosphorylase

Phosphoglucomutase

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Uridine diphosphate glucose (UDP-glucose)istheimmediateprecursorforglycogensynthesis.Asglucoseresiduesareaddedtoglycogen,UDP-glucoseisthesubstrateandUDPisreleasedasareactionproduct.Nucleotidediphosphate sugarsareprecursorsalsoforsynthesisofothercomplexcarbohydrates,includingoligosaccharidechainsofglycoproteins,etc.

OO

OHOH

HH

H

CH2

H

HN

N

O

O

OP

O

O-

P

O

O-

H O

OH

H

OHH

OH

CH2OH

H

O

H

UDP-glucose

Glycogensynthesis

Nucleotidediphosphate sugar

OO

OHOH

HH

H

CH2

H

HN

N

O

O

OP

O

O-

P

O

O-

H O

OH

H

OHH

OH

CH2OH

H

O

H

O-P

O

O-

H O

OH

H

OHH

OH

CH2OH

H

O

H

OO

OHOH

HH

H

CH2

H

HN

N

O

O

OP

O

O-

P

O

O-

OP-O

O

O-

PPi

+

UDP-glucose

glucose-1-phosphate UTP

UDP-Glucose Pyrophosphorylase

Nucleotidediphosphate sugar

UDP-glucoseisformedfromglucose-1-phosphate:w glucose-1-phosphate+UTPà UDP-glucose+PPiw PPi +H2Oà 2PiOverall:w glucose-1-phosphate+UTPà UDP-glucose+2PiSpontaneoushydrolysisofthe~P bondinPPi (P~P)drivestheoverallreaction.

CleavageofPPi istheonlyenergycostforglycogensynthesis(one ~Pbondperglucoseresidue).

Glycogenin, theenzymethatinitiatesglycogensynthesis.

Glycogenin isanenzymethatcatalyzestheattachmentofaglucosemoleculetooneofitsowntyrosine residues.

Glycogenin isadimer,andevidenceindicatesthatthe2copiesoftheenzymeglucosylate oneanother.

Tyr- active site

active site -Tyr

Glycogenin dimer

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Aglycosidicbond isformedbetweentheanomericC1oftheglucosemoietyderivedfromUDP-glucoseandthehydroxyloxygenofatyrosine side-chainofGlycogenin.UDPisreleasedasaproduct.

H O

OH

H

OHH

OH

CH2OH

HO H

H

OHH

OH

CH2OH

H

O

HHC

CH

NH

CH2

O

O

H O

OH

H

OHH

OH

CH2OH

HH

C

CH

NH

CH2

O

O1

5

4

3 2

6

H O

OH

H

OHH

OH

CH2OH

HH

O1

5

4

3 2

6

P O P O Uridine

O

O-

O

O-

C

CH

NH

CH2

HO

O

tyrosine residue of Glycogenin

O-linked glucose residue

+ UDP

UDP-glucose

Glycogenin thencatalyzesglucosylationatC4oftheattachedglucose(UDP-glucoseagainthedonor),toyieldanO-linkeddisaccharidewitha(1®4)glycosidiclinkage.Thisisrepeateduntilashortlinearglucosepolymer witha(1®4)glycosidiclinkagesisbuiltuponGlycogenin.

H O

OH

H

OHH

OH

CH2OH

HO H

H

OHH

OH

CH2OH

H

O

HHC

CH

NH

CH2

O

O

H O

OH

H

OHH

OH

CH2OH

HH

C

CH

NH

CH2

O

O1

5

4

3 2

6

H O

OH

H

OHH

OH

CH2OH

HH

O1

5

4

3 2

6

P O P O Uridine

O

O-

O

O-

C

CH

NH

CH2

HO

O

UDP-glucose

O-linked glucose residue

a(1à4) linkage

+ UDP

+ UDP

Answer:MostoftheGlycogeninisfoundassociatedwithglycogenparticles (branchedglycogenchains)inthecytoplasm.

GlycogenSynthase thencatalyzeselongation ofglycogenchainsinitiatedbyGlycogenin.

Question:WherewouldyouexpecttofindGlycogenin withinacell?

GlycogenSynthase catalyzestransferoftheglucosemoietyofUDP-glucosetothehydroxylatC4oftheterminalresidueofaglycogenchaintoformana(1® 4) glycosidiclinkage:

glycogen(nresidues) +UDP-glucoseàglycogen(n+1residues) +UDP

Abranchingenzyme transfersasegmentfromtheendofaglycogenchaintotheC6hydroxylofaglucoseresidueofglycogentoyieldabranchwithana(1®6) linkage.

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Bothsynthesis & breakdown of glycogenarespontaneous.Ifbothpathwayswereactivesimultaneouslyinacell,therewouldbea"futilecycle"withcleavageofone~Pbondpercycle (informingUDP-glucose).Topreventsuchafutilecycle,GlycogenSynthaseandGlycogenPhosphorylasearereciprocallyregulated,byallostericeffectorsandbyphosphorylation.

Glycogen Synthesis

UTP UDP + 2 Pi

glycogen(n) + glucose-1-P glycogen(n + 1) Glycogen Phosphorylase Pi

GlycogenPhosphorylase inmuscle issubjecttoallostericregulationbyAMP,ATP,andglucose-6-phosphate.Aseparateisozyme ofPhosphorylase expressedinliverislesssensitivetotheseallostericcontrols.

w AMP (presentsignificantlywhenATPisdepleted)activates Phosphorylase,promotingtherelaxedconformation.

w ATP &glucose-6-phosphate,whichbothhavebindingsitesthatoverlapthatofAMP,inhibit Phosphorylase,promotingthetenseconformation.

w Thusglycogenbreakdownis inhibitedwhenATPandglucose-6-phosphateareplentiful.

GlycogenSynthaseisallosterically activated byglucose-6-P (oppositeofeffectonPhosphorylase).ThusGlycogenSynthaseisactivewhenhighbloodglucoseleadstoelevatedintracellularglucose-6-P.ItisusefultoacelltostoreglucoseasglycogenwhentheinputtoGlycolysis(glucose-6-P),andthemainproductofGlycolysis(ATP),areadequate.

Glycogen Glucose Hexokinase or Glucokinase Glucose-6-Pase Glucose-1-P Glucose-6-P Glucose + Pi Glycolysis Pathway Pyruvate Glucose metabolism in liver.

Phosphoglucomutase

GlycogenPhosphorylase

Regulationbycovalentmodification(phosphorylation):

Thehormonesglucagon and epinephrine activateG-proteincoupledreceptorstotriggercAMP cascades.

w Bothhormonesareproducedinresponsetolowbloodsugar.

w Glucagon,whichissynthesizedbya-cellsofthepancreas,activatescAMP formationin liver.

w EpinephrineactivatescAMP formationinmuscle.

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ThecAMP cascaderesultsinphosphorylation ofaserinehydroxylofGlycogenPhosphorylase,whichpromotestransitiontotheactive (relaxed)state.

Thephosphorylatedenzymeislesssensitivetoallostericinhibitors.

Thus,evenifcellularATP&glucose-6-phosphatearehigh,Phosphorylase willbeactive.

Theglucose-1-phosphateproducedfromglycogeninlivermaybeconvertedtofreeglucose forreleasetotheblood.

Withthishormone-activatedregulation,theneedsoftheorganismtakeprecedenceoverneedsofthecell.

SignalcascadebywhichGlycogenPhosphorylaseisactivated.

Hormone (epinephrine or glucagon)

via G Protein (Ga-GTP)

Adenylate cyclase Adenylate cyclase (inactive) (active) catalysis

ATP cyclic AMP + PPi

Activation Phosphodiesterase AMP

Protein kinase A Protein kinase A (inactive) (active) ATP

ADP

Phosphorylase kinase Phosphorylase kinase (P) (b-inactive) (a-active) Phosphatase ATP Pi ADP Phosphorylase Phosphorylase (P) (b-allosteric) (a-active) Phosphatase

Pi

Commonly used terminology:

w"a" is the form of the enzyme that tends to be active, and independent of allosteric regulators (in the case of Glycogen Phosphorylase, when phosphorylated).

w"b" is the form of the enzyme that is dependent on local allosteric controls (in the case of Glycogen Phosphorylase when dephosphorylated).

ThecAMP cascade inducedinliverbyglucagonorepinephrinehastheoppositeeffectonglycogensynthesis,itpromotesthebreakdownofglucogen toglucosebyGlycogenPhosphorylase

GlycogenSynthase isphosphorylated byProteinKinaseAaswellasbyPhosphorylase Kinase.

Phosphorylation ofGlycogenSynthasepromotesthe"b"(lessactive)conformation.

ThecAMP cascadethusinhibitsglycogensynthesis.

Insteadofbeingconvertedtoglycogen,glucose-1-Pinlivermaybeconvertedtoglucose-6-P,anddephosphorylatedforreleasetotheblood.

Highcytosolicglucose-6-phosphate,whichwouldresultwhenbloodglucoseishigh,turnsoffthesignalwithregardtoglycogensynthesis.

TheconformationofGlycogenSynthaseinducedbytheallostericactivatorglucose-6-phosphateissusceptibletodephosphorylation byProteinPhosphatase.

Glycogen Glucose Hexokinase or Glucokinase Glucose-6-Pase Glucose-1-P Glucose-6-P Glucose + Pi Glycolysis Pathway Pyruvate Glucose metabolism in liver.

GlycogenPhosphorylase

Phosphoglucomutase

cAMP+ blood/

othercells

cAMP inhibitsglycogensynthesisinducedbyglucagon&epinephrine.

Insulininhibitsglycogenbreakdown

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Insulin,producedinresponsetohighbloodglucose,triggersaseparatesignalcascadethatleadstoactivationofPhosphoprotein Phosphatase.

ThisphosphatasecatalyzesremovalofregulatoryphosphateresiduesfromPhosphorylase,Phosphorylase Kinaseenzymes.

Thus insulinantagonizeseffectsofthecAMP cascadeinducedbyglucagon &epinephrine.

--->

Insulin,inhibitsglycogenbreakdown.

Ca++ alsoregulatesglycogenbreakdowninmuscle.Duringactivationofcontractioninskeletalmuscle,Ca++ isreleasedfromthesarcoplasmicreticulumtopromoteactin/myosininteractions.ThereleasedCa++ alsoactivates Phosphorylase Kinase,whichinmuscleincludescalmodulin asitsd subunit.Phosphorylase KinaseispartlyactivatedbybindingofCa++tothissubunit.

Phosphorylase Kinase inactive

Phosphorylase Kinase-Ca++ partly active

P-Phosphorylase Kinase-Ca++ fully active

Phosphorylation oftheenzyme,viaacAMPcascadeinducedbyepinephrine,resultsinfurtheractivation.Theseregulatoryprocessesensurereleaseofphosphorylatedglucosefromglycogen,forentryintoGlycolysis toprovideATP neededformusclecontraction.Duringextendedexercise,asglycogenstoresbecomedepleted,musclecellsrelymoreonglucoseuptakefromtheblood,andonfattyacidcatabolismasasourceofATP.

Phosphorylase Kinase inactive

Phosphorylase Kinase-Ca++ partly active

P-Phosphorylase Kinase-Ca++ fully active

Ageneticdefect intheisoformofanenzymeexpressedinliver causesthefollowingsymptoms:w Aftereating aCHOmeal,elevated bloodlevelsofglucose,lactate,&lipids.

w Duringfasting,lowbloodglucose &highketonebodies.Which liverenzymeisdefective?

ExplainSymptoms:w Aftereating,bloodglucose ishighbecauselivercannotstoreitasglycogen.SomeexcessglucoseisprocessedviaGlycolysis toproducelactate &fattyacidprecursors.

w Duringfasting,glucose islowbecausetheliverlacksglycogenstoresforgenerationofglucose.Ketonebodies areproducedasanalternativefuel.

GlycogenSynthase

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KetoneBodies

• Useoffattyacidsinthecitricacid(Kreb’s)cyclerequirescarbohydrates(CHO)fortheproductionofoxaloacetate(OAA)– ‘fatburnsinCHOflame’.

• Duringstarvationordiabetes,OAAisusedtomakeglucose

– Fattyacidsarethenusedtomakeketonebodies(acetoacetateandD–3–hydroxybutarate)

‘Ketone-bodies’ as energy substrates for the brain

(3-HB)

[AcAc)](3-HB)1) 2) 3)

Ketone-bodies [2)&3)] oxidized in neurons, astrocytes and oligodendrocytes; but free fatty acids exclusively in astrocytes [ 1)-3) in astrocytes]

1)-3) Products from lipid metabolism: maternal milk, starvation and diabetes

The acetyl-CoA produced by mitochondrial beta-oxidation of fatty acids enters the Kreb's cycle to produce energy, but that is not the only fate of acetyl-CoA. In liver mitochondria, some acetyl-CoA is converted to acetoacetate, beta-hydroxybutyrate, and acetone, collectively called ketone bodies. Ketone bodies are transported to other tissues such as brain, muscle or heart where they are converted back to acetyl-CoA to serve as an energy source. The brain normally uses only glucose for energy, but during starvation ketone bodies can become the main energy source for the brain. In the metabolic condition called ketosis, ketone bodies are produced faster than they are consumed by tissues and the smell of acetone can be detected on a person's breath. The smell of acetone is one indication that a person may have diabetes. The consumption of high-fat/low carbohydrate diets has been used as a weight loss program by many, intentionally inducing ketosis to consume fat stores, but these ketogenic diets can cause unwanted side effects related to increased urea production resulting from protein intake and risk of heart disease from increased cholesterol and fat intake.

Acetyl-CoA alternativepathway AminoAcidsandtheirconnectionstotheKrebsCycle

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Question: HowwouldyounutritionallytreatdeficiencyofliverGlycogenSynthase?

w Frequentmealsofcomplexcarbohydrates(avoidingsimplesugarsthatwouldleadtoarapidriseinbloodglucose)

w Mealshighinprotein toprovidesubstratesforgluconeogenesis.

GlycogenStorageDiseases aregeneticenzymedeficienciesassociatedwithexcessiveglycogenaccumulationwithincells.

Someenzymeswhosedeficiencyleadstoglycogenaccumulationarepartoftheinter-connectedpathwaysshownhere.

glycogen glucose-1-P Glucose-6-Phosphatase glucose-6-P glucose + Pi fructose-6-P Phosphofructokinase fructose-1,6-bisP Glycolysis continued

Symptoms inadditiontoexcessglycogenstorage:

wWhenageneticdefectaffectsmainlyanisoformofanenzymeexpressedinliver,acommonsymptomishypoglycemia,relatingtoimpairedmobilizationofglucoseforreleasetothebloodduringfasting.

wWhenthedefectisinmuscle tissue,weakness&difficultywithexercise resultfrominabilitytoincreaseglucoseentryintoGlycolysisduringexercise.

w Additionalsymptomsdependontheparticularenzymethatisdeficient.

Glycogen Storage Disease Symptoms, in addition to glycogen accumulation

Type I, liver deficiency of Glucose-6-phosphatase (von Gierke's disease)

hypoglycemia (low blood glucose) when fasting, liver enlargement.

Type IV, deficiency of branching enzyme in various organs, including liver (Andersen's disease)

liver dysfunction and early death.

Type V, muscle deficiency of Glycogen Phosphorylase (McArdle's disease)

muscle cramps with exercise.

Type VII, muscle deficiency of Phosphofructokinase.

inability to exercise.

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MaintainingBloodGlucose

Homeostasis

IntestineWhenapersoneats,bloodglucoserises.

1

2

Insulinstimulatestheuptakeofglucoseintocellsandstorageasglycogenintheliverandmuscles.Insulinalsostimulatestheconversionofexcessglucoseintofatforstorage.

3

4

5

6

7 Bloodglucosebeginstorise.

a Thestresshormoneepinephrineandotherhormonesalsobringglucoseoutofstorage.

GlucoseInsulinGlucagonGlycogen

Glucagonstimulateslivercellstobreakdownglycogenandreleaseglucoseintotheblood.a

Liver

Lowbloodglucosestimulatesthepancreastoreleaseglucagonintothebloodstream.

Asthebody'scellsuseglucose,bloodlevelsdecline.

Glucagon

Pancreas

Fatcell

Liver

Muscle

Highbloodglucosestimulatesthepancreastoreleaseinsulin.

Pancreas

Insulin