Glycolysis - Creightonchemistry.creighton.edu/~jksoukup/lec16gly2007.pdf · glc-6-phosphate is not a substrate for transporter HK in all cells Glucokinase (isozyme of HK) in hepatocytes,

Glycolysis

Glucose utilization in cells of higher plants and animals

GlycolysisWhat is it?10-step metabolic pathwayStarts the breakdown of glucose to get energy

GlycolysisGlycolysis consists of reactions that convert glc to pyruvateUnder aerobic conditions: pyruvate → acetyl CoAUnder anaerobic conditions: pyruvate → ethanol + CO2 orpyruvate → lactate

Glycolysis

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

GlycolysisPhosphorylated IntermediatesEach of the 9 intermediates between glc and pyr are phosphorylated

Functions of intermediates

1. Role of charged phosphate

2. Energy storage

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)

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

Reaction 2 - Glucose 6-phosphate isomeraseMove carbonyl to carbon 2ΔG˚ = 1.7 kJ/mol (reversible)

Glycolysis - Prep

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)

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

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

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

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

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

Glycolysis - PayoffReaction 10: Pyruvate kinaseHarvest the high phosphoryl group transfer potential of PEP tomake ATPΔG˚ = -31.4 kJ/mol (irreversible)

Koelle, lec15, p11

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

Glycolysis - Enzyme mechanisms

Glycolysis - Enzyme mechanisms

Koelle, lec15, p15

Glycolysis - Enzyme mechanisms3. Energy coupling: Glyceraldehyde 3-phosphate dehydrogenase

His - general baseCys - nucleophile

Glycolysis - Enzyme mechanisms3. Energy coupling: Glyceraldehyde 3-phosphate dehydrogenase

Koelle, lec15, p16

His - general baseCys - nucleophile