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Glycolysis
Glucose utilization in cells of higher plants and animals
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GlycolysisWhat is it?10-step metabolic pathwayStarts the breakdown of glucose to get energy
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GlycolysisGlycolysis consists of reactions that convert glc to pyruvateUnder aerobic conditions: pyruvate → acetyl CoAUnder anaerobic conditions: pyruvate → ethanol + CO2 orpyruvate → lactate
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Glycolysis
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GlycolysisATP formation coupled to glycolysis
Glucose + 2NAD+ → 2pyruvate + 2NADH + 2H+ ΔG˚ = -146 kJ/mol
2ADP + 2Pi → 2ATP + 2H2OΔG˚ = 2 x 30.5 kJ/mol = 61 kJ/mol
Net reaction:Glc + 2NAD+ + 2ADP + 2Pi → 2pyr + 2NADH + 2H+ + 2ATP + 2H2O ΔΔΔG˚ = -146 kJ/mol + 61 kJ/mol = -85 kJ/mol
After glycolysis, pyruvate goes on to be oxidized completely toCO2 and H2O which has a total standard ΔG˚ = -2840 kJ/mol
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GlycolysisPhosphorylated IntermediatesEach of the 9 intermediates between glc and pyr are phosphorylated
Functions of intermediates
1. Role of charged phosphate
2. Energy storage
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GlycolysisPhosphorylated IntermediatesEach of the 9 intermediates between glc and pyr are phosphorylated
Functions of intermediates
1. They are ionized (-) at pH 7, because membrane is impermeableto charged species, intermediates cannot diffuse out of cell
2. Conserve metabolic energy, energy released in breakingphosphoanhydride bonds (ATP) is partially conserved in phosphateester (glc-6-phosphate)
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Glycolysis - PrepReaction 1 - HexokinaseKinases transfer phosphoryl group from ATP to other molecules
Koelle, lec15, p7
Glc enters cell - how?glc-6-phosphate is not a substrate for transporterHK in all cellsGlucokinase (isozyme of HK) in hepatocytes, more specific for glc
irreversible
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Reaction 2 - Glucose 6-phosphate isomeraseMove carbonyl to carbon 2ΔG˚ = 1.7 kJ/mol (reversible)
Glycolysis - Prep
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Reaction 3 - Phosphofructokinase-1Adds another phosphoryl group to sugarRegulatory enzyme, major point of regulation in glycolysisActivity of PFK-1 regulated how?
by ATP levelsby other allosteric effectors - F2,6BP
ΔG˚ = -14.2 kJ/mol (irreversible)
Koelle, lec15, p8
Glycolysis - Prep
(PFK-1)
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Reaction 4 - AldolaseBreaks the 6-carbon compound into two 3-carbon compoundsAn aldol cleavage (an aldehyde and an alcohol are generated)
ΔG˚ = 23.8 kJ/mol (irreversible or reversible - why??)
During glycolysis the products are removed quickly by next twosteps, pulling the rxn in the direction of cleavage even though +ΔG˚
Koelle, lec15, p8
Glycolysis - Prep
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Reaction 5 - Triose phosphate isomeraseGet two identical 3-carbon unitsNeed glyceraldehyde 3-phosphate to go to next stepEnd of prep phase
ΔG˚ = 7.5 kJ/mol (reversible)
Koelle, lec15, p9
Glycolysis - Prep
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Glycolysis - PayoffReaction 6: Glyceraldehyde 3-phosphate dehydrogenaseLoss of 2e- and 2H+
First of 2 energy-conserving rxns that form ATPΔG˚ = 6.3 kJ/mol (reversible)
Biochemists use “~” to indicate bonds with high phosphoryl grouptransfer potentials
Principle 3 - few important molecules carry the “currencies” ofmetabolism, NAD+ = Reducing packet
Koelle, lec15, p10
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Glycolysis - PayoffReaction 7: Phosphoglycerate kinase (named for reverse rxn)Harvest the high phosphoryl group transfer potential of 1,3-bisphosphoglycerate to make ATPΔG˚ = -18.5 kJ/mol
Sum of last 2 rxns (steps 6/7)Glyceraldehyde 3-phosphate + ADP + Pi + NAD+ ⇔3-phosphoglycerate + ATP + NADH + H+
ΔG˚ = -12.5 kJ/mol
Prin. 4 coupled rxns drive energy-requiring processes
Koelle, lec15, p10
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Glycolysis - PayoffReaction 8 and 9: Phosphoglycerate mutase and EnolaseRearrange the molecule to produce a form with a high phosphorylgroup transfer potential (“~”)Step 8: ΔG˚ = 4.4 kJ/molStep 9: ΔG˚ = 7.5 kJ/mol
Large differences in free energies of hydrolysis:2-phosphoglycerate → -17.6 kJ/molPhosphoenolpyruvate → -61.9 kJ/mol
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Glycolysis - PayoffReaction 10: Pyruvate kinaseHarvest the high phosphoryl group transfer potential of PEP tomake ATPΔG˚ = -31.4 kJ/mol (irreversible)
Koelle, lec15, p11
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Glycolysis - Enzyme mechanisms1. Same few chemical transformations get used over & overGlycolysis has:
2 isomerases (+ 1 mutase)4 kinasesdehydrogenase
2. Breaking a carbon-carbon bond: aldolase
Koelle, lec15, p14
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Glycolysis - Enzyme mechanisms
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Glycolysis - Enzyme mechanisms
Koelle, lec15, p15
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Glycolysis - Enzyme mechanisms3. Energy coupling: Glyceraldehyde 3-phosphate dehydrogenase
His - general baseCys - nucleophile
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Glycolysis - Enzyme mechanisms3. Energy coupling: Glyceraldehyde 3-phosphate dehydrogenase
Koelle, lec15, p16
His - general baseCys - nucleophile