Section 6: Section 6:

Carbohydrate MetabolismCarbohydrate Metabolism

3. Anaerobic & aerobic 3. Anaerobic & aerobic glycolysisglycolysis

10/21/200510/21/2005

Complete oxidation of glucoseComplete oxidation of glucose

stoichiometry:stoichiometry:glc + 6 Oglc + 6 O22 6 CO 6 CO22 + 6 H + 6 H22OO G'G' º = – 686 kcal/molº = – 686 kcal/mol

ATP yieldATP yield• theoretical: >90theoretical: >90• actual: actual: 30-3230-32

first stage: glycolysis (10-11 steps)first stage: glycolysis (10-11 steps)• location: cytosol of all cells (including microorganisms)location: cytosol of all cells (including microorganisms)• 2 parts2 parts

glc glc 2 glyceraldehyde 3-P (GAP) 2 glyceraldehyde 3-P (GAP) ((steps 1-5steps 1-5))

2 GAP 2 GAP 2 pyruvate/lactate 2 pyruvate/lactate ((steps 6steps 6 -10/11-10/11))

(686/7.3)(686/7.3)

1

GlycolysisGlycolysis

glucose 6-phosphate(glc 6-P)

H

C

OH

OH

CH2

OHOH

HO

H

O

H

H

PO2-

3

C

H

CH2

OH

OH

CH2

OH

OH

HH

O

O

PO2-

3

fructose 6-phosphate(frc 6-P)

C

C H

CH2

O

O

C

C

CH2

OH

OHH

OHH

OPO2-

3

PO2-

3

ADP

fructose 1,6-bis phosphate(1,6FBP)

ATP

3.3. phosphoryl transferphosphoryl transfer phosphofructokinasephosphofructokinase

irreversibleirreversiblecommitted stepcommitted step

2.2. isomerizationisomerizationphosphoglucosephosphoglucoseisomeraseisomerase

glc

1.1. ATP ((see L2sl9 “Phosphorylation of glc”see L2sl9 “Phosphorylation of glc”)) ADP

2

4. aldol cleavage4. aldol cleavagealdolasealdolase

C

C H

CH2

O

O

C

C

CH2

OH

OH

OH

OPO2-

3

PO2-

3 C

CH2

CH2

O

O

OH

PO2-

3

C

C

CH2

OH

OHH

OPO2-

3

+

fructose 1,6-bis phosphate(1,6FBP)

dihydroxyacetonephosphate(DHAP)

glyceraldehyde3-phosphate(GAP)

H

H

3

5. isomerization5. isomerizationtriose phosphatetriose phosphate isomerase isomerase

6. oxidation-driven6. oxidation-driven phosphorylation phosphorylation

GAP DHaseGAP DHase

7. phosphoryl transfer7. phosphoryl transferphosphoglycerate phosphoglycerate

kinase kinase3-phospho-glycerate(3PG)

PO2-

3

C

CH2

CH2

O

OH

OPO2-

3

C

C

CH2

OH

OHH

OPO2-

3

C

C

CH2

OO

OHH

OPO2-

3

C

C

CH2

OO

OHH

OPO2-

3

ADP

ATP + H+

NAD+ + Pi

NADH + H+

1,3-bis phospho-glycerate(1,3BPG)

GAP DHAP

–

4

8. 8. phosphoryl shiftphosphoryl shift phosphoglyceratephosphoglycerate mutase mutase

9. dehydration9. dehydration enolaseenolase

10. phosphoryl transfer10. phosphoryl transfer irreversibleirreversible pyruvate kinasepyruvate kinase

C

C

CH2

OO

OHH

OPO2-

3

C

C

CH2

OO

OH

OH

PO2-

3

C

C

CH2

OO

OPO2-

3

C

C

CH3

OO

O

ADP

ATP

H2O

2PG 3PG

phosphoenolpyruvate(PEP)

pyruvate(pyr)

–

–

–

–

5

Regeneration of NADRegeneration of NAD++: 1. electron shuttles: 1. electron shuttles stoichiometry of steps 1-10:stoichiometry of steps 1-10:

glc + 2 NADglc + 2 NAD++ →→ 2 pyruvate + 2 NADH + 4 H 2 pyruvate + 2 NADH + 4 H++

NAD present in cells in only catalytic amounts, so NAD present in cells in only catalytic amounts, so regeneration of NADregeneration of NAD++ is necessary is necessary

cytosolic NADH cannot enter mitochondriacytosolic NADH cannot enter mitochondria solutionsolution: : ee–– pair carried to mitochondrial pair carried to mitochondrial ee–– transport chain transport chain

via a shuttle (short linking pathway)via a shuttle (short linking pathway) net reaction:net reaction:

NADHNADHcytcyt + oxid + oxid ee– – carriercarriermitomito → NAD → NAD++cytcyt + red. + red. ee– – carriercarriermitomito

22 e e––cytcyt →→ 22 e e––mitomito malate-aspartate shuttlemalate-aspartate shuttle

• main shuttle in heart & liver cellsmain shuttle in heart & liver cells• ee–– pair eventually transferred to mitochondrial pair eventually transferred to mitochondrial

matrix NADmatrix NAD++, so ATP yield is 2.5/, so ATP yield is 2.5/ ee–– pairpair6

GOP-DHAP shuttleGOP-DHAP shuttle main shuttle in brain & skeletal musclemain shuttle in brain & skeletal muscle net reactionnet reaction

NADHNADHcytcyt + +

HH++ + + EE-FAD-FAD

↓↓

NADNAD++cytcyt + +EE-FADH-FADH22

yieldsyields1.5 ATP 1.5 ATP per eper e–– pair pair

Fig. 18.37

‚

7 e–s from complex II, others

Regeneration of NADRegeneration of NAD++: 2. reduction of pyruvate: 2. reduction of pyruvate conditions limiting electron shuttles:conditions limiting electron shuttles:

• mitochondria scarce (“fast” muscle) or absent (RBC)mitochondria scarce (“fast” muscle) or absent (RBC)• limited Olimited O22 supply (ischemia) supply (ischemia)• high demand for ATP causes glycolysis rate > shuttle ratehigh demand for ATP causes glycolysis rate > shuttle rate

ee–– pair is transferred to pyruvate: pair is transferred to pyruvate:

as a result, glycolysis can occur without net oxidation: as a result, glycolysis can occur without net oxidation: anaerobicallyanaerobicallyfermentationfermentation: any anaerobic process: any anaerobic process

NAD + H+ +H NAD+ +

pyruvate L-lactate

lactateDHase

C HOHCH3

CO O

COCH3

CO O

_ _

11. oxidation-11. oxidation- reduction reduction

8

Glycolysis stoichiometriesGlycolysis stoichiometriesAerobic glycolysis:Aerobic glycolysis: ATPATP

yield yieldsteps 1-10 steps 1-10 glc + 2 NADglc + 2 NAD++ →→ 2 pyruvate + 2 NADH + 4H 2 pyruvate + 2 NADH + 4H++ 2 2

Regen. of NADRegen. of NAD++: GOP shuttle + ox phos: GOP shuttle + ox phos2 H2 H++ + 2 NADH + O + 2 NADH + O22 →→ 2 NAD 2 NAD++ + 2 H + 2 H22OO 3 3**

glc + Oglc + O22 → 2 pyruvate + 2 H → 2 pyruvate + 2 H++ + 2 H + 2 H22O O 55

Anaerobic glycolysis:Anaerobic glycolysis:steps 1-10steps 1-10 glc + 2 NAD glc + 2 NAD++ →→ 2 pyruvate + 2 NADH + 4 H 2 pyruvate + 2 NADH + 4 H++ 2 2

step 11step 11 2 pyruvate +2 NADH + 2H 2 pyruvate +2 NADH + 2H++ →→ 2 lactate +2 NAD 2 lactate +2 NAD++

(steps 1-11)(steps 1-11) glc → 2 lactate + 2 H glc → 2 lactate + 2 H++ 22

** 5 if malate-aspartate shuttle used 5 if malate-aspartate shuttle used

9

Effect of glycolysis products (pyruvate/lactate):Effect of glycolysis products (pyruvate/lactate):acidificationacidification stoichiometry of both aerobic & anaerobic glycolysis stoichiometry of both aerobic & anaerobic glycolysis

shows production of 2 Hshows production of 2 H++// glcglc unlike phosphate-containing metabolites,unlike phosphate-containing metabolites,

lactate & pyruvate permeant to most cell membraneslactate & pyruvate permeant to most cell membranes(as protonated forms: lactic acid & pyruvic acid)(as protonated forms: lactic acid & pyruvic acid)• microorganismsmicroorganisms: :

their environment becomes acidictheir environment becomes acidic e.g.e.g., plaque bacteria on enamel surface ferment carbs, plaque bacteria on enamel surface ferment carbs

low pH increases solubility of Ca phosphate mineralslow pH increases solubility of Ca phosphate minerals repeated acid attacks produce carious lesionrepeated acid attacks produce carious lesion

• skeletal muscle during exerciseskeletal muscle during exercise::[lactate], [pyruvate] & [H[lactate], [pyruvate] & [H++] rise] rise10

Fate of pyruvate/lactateFate of pyruvate/lactatepyruvatepyruvate has a number of alternative fates has a number of alternative fates

•e.g.e.g., oxidized further in mitochondria (, oxidized further in mitochondria (next lecturenext lecture))•diffusion out of cell (efflux)diffusion out of cell (efflux)

lactatelactate has only 1 metabolic fate: oxidation back to pyruvate has only 1 metabolic fate: oxidation back to pyruvate•if oxidation limited, efflux occursif oxidation limited, efflux occurs

blood distributes theseblood distributes theseliver converts them liver converts them

back to glc by back to glc by gluconeogenesis gluconeogenesis ((next lecturenext lecture))

combination of muscle combination of muscle glycolysis & liver glycolysis & liver gluconeogenesis: gluconeogenesis:

Cori cycleCori cycle

11

LIVERLIVER MUSCLEMUSCLE

glucose glucose glucose glucose6 ATP 6 ATP 2 ATP 2 ATP outout

pyruvate pyruvate bloodblood pyruvatepyruvate lactate lactate lactate lactate

gluconeogenesisgluconeogenesis glycolysisglycolysis

Net effect is transfer of energy Net effect is transfer of energy from liver to musclefrom liver to muscle

The Cori cycle

stepstep enzymeenzyme inhibitorinhibitor activatoractivator

1 1 hexokinasehexokinase glc 6-Pglc 6-P 33 phosphofructokinase phosphofructokinase ATP, ATP, AMP,AMP,

citrate* citrate* ADPADP

mechanism of control:mechanism of control:

both kinases have allosteric sites to whichboth kinases have allosteric sites to whichactivators/inhibitors bind activators/inhibitors bind

Control of glycolysisControl of glycolysis

* provides coordination with Krebs (citric acid) cycle* provides coordination with Krebs (citric acid) cycle12

Next time:Next time:

4. 4. GluconeogenesisGluconeogenesis Pyruvate oxidationPyruvate oxidation