GLYCOLYSIS

Glycolysis: oxidative breakdown of glucose to pyruvate with capture of some energy as ATP, NADH (first step in respiration) Themes:

• sequenti , al simple enzymatic reactions • investm ent of energy; then recapture • control of pathway at key step :s <G <0 • production of useful intermediates

Glycolysis Subsequent reactions glucose pyruvate lactate (or) CO2+ethanol (or)

malate

CO2 + H2O --

Compare respiration to combustion: C6H12O6 + 6 O2 6 CO2 + 6 H2O

G ’o = -2840 k /J mol In respiratio ,n including glycolysis, som e G ’o

(~ 35%) is retained in formation of ATP

O2 absent

O2 present

Glycolysis can be divided into two phases: priming and payoff

Phase 1: five priming reactions

1. glucose + ATP G-6-P + ADP

2. G-6-P F-6-P

3. F-6-P + ATP F-1,6-bisP + ADP

4. F-1,6,bisP DHAP + G-3-P

5. DHAP G-3-P

Glycolysis can be divided into two phases: priming and payoff

Phase 2: five payoff reactions

6. G-3-P + PI + NAD+ 1,3-bisPGA + NADH

7. 1,3-bisPGA + ADP 3-PGA + ATP

8. 3-PGA 2-PGA

9. 2-PGA PEP

10. PEP + ADP pyruvate + ATP

Reaction 1 • kinase: adds Pi using ATP

as donor • Note the need for Mg2+

with ATP (comm )on • G ’o =+13.8 /kJmol = -30.5 -16.7 ’ =G G ’o + RT [ ]/[ ]ln P R = -33 .9 • Reactions with <<0G can

be regulat :ed • Hexokinase is allosterically

inhibited b yG-6- , P Km ~ 0.1 m M ([gluco ]se in bl ~ood 4 mM)

• Glucokinase in liver not allostericall y inhibited: Km ~ 10 mM

• Phosphorylat ion of glucose traps it in the cell

Reaction 2 • isomera : se catalyzes the

structural rearrangement of isomers (aldose

ketose)

• Go’ =+1.67 kJ/mol G’ = -2.92 kJ/mol

• Near equilibrium:

not regulated

Reaction 3 • G ’o =+16.3 /kJmol = -30.5 -14.2

’ = G -18.89 • Regulated • ATP is an alloster ic inhibitor

AMP reverses AT P inhibition (ATP down 8% resu lts in

AMP up 4-fo )ld

Reaction 4 Equilibrium:

Go’ =+23.8 kJ/mol G’ = -0.23 kJ/mol

Reaction 5 Equilibrium:

Go’ =+7.56 kJ/mol G’ = +2.41 kJ/mol

Another isomerase: ketose aldose

Reaction 6

Go’ =+6.3 kJ/mol G’ = -1.29 kJ/mol

Near equilibrium:not regulated

Note that the C in-C-O-PO3 has beenoxidized (from aldehydeto acid)

The coupling of acyl-phosphate formation to oxidation avoidsan energy hump that would drastically slow the reaction.

Reaction 7

Go’ =-18.9 kJ/mol G’ = +0.1 kJ/mol

Near equilibrium: not regulated Note “substrate level phosphorylation” of ADP (but enzyme named for the back reaction)

Reaction 8 mutase: an enzyme that catalyzes the shifting of a functional group from one position to another within the same molecule

Go’ =+4.4 kJ/mol G’ = +0.83 kJ/mol

Near equilibrium: not regulated

Reaction 9

Go’ =+1.8 kJ/mol G’ = +1.1 kJ/mol

Near equilibrium: not regulated

Reaction 10

Go’ =-31.4 kJ/mol G’ = -23.0 kJ/mol

Not near equilibrium, so controllable: activated by AMP, fructose-1,6-bisP; inhibited by ATP (product of this reaction), acetyl-CoA (product of a following reaction and substrate of citric acid cycle), alanine (trans- amination analog of pyruvate)

Summary of glycolysis: glucose + 2 ADP + 2 Pi + 2 NAD+ 2 pyruvate + 2 ATP + 2 H2O + 2 NADH + 2 H+

What happens next? It depends on the environment: aerobic or anaerobic. Regeneration of NAD+ is essential to keep the process going.

including oxidationof NADH

In fungi and plants in the absence of O2, pyruvate is decarboxylated to acetaldehyde; acetaldehyde is reduced to ethanol; NADH is oxidized to NAD+.

The NAD+ is recycled to oxidize more glyceraldehyde-3-P.

In animals and some bacteria, pyruvate is reduced to lactate asNADH is oxidized to NAD+.

Energy efficiency glucose 2 lactate Go’ = -196 kJ/mol 2 ADP 2 ATP Go’ = + 61 (31%) glucose 2 ethanol + 2 CO2 Go’ = -235 kJ/mol 2 ADP 2 ATP Go’ = + 61 (26%)

Control: activity of glycolysis depends on allosteric enzymes andresponds to energy requirement.

In muscle:

Control: activity of glycolysis depends on allosteric enzymes andresponds to energy requirement.

In liver, fructose-2,6-bisP is a important stimulator of PFK:

enhances substrate activation relieves allosteric inhibition

Cancer cells produce most of their ATP by glycolysis (Warburg effect) Why do proliferating cells switch to a less efficient metabolism? Probable answer: growth requires more C-compounds and reduction power (NADPH), than ATP energy. (see Science 324:1029 May 22, 2009)

Summary

•Glucose is metabolized to pyruvate in a series of 10 reactions.

•Glycolysis provides the cell (cytoplasm) with 2 mol ATP/glucose.

•Glycolysis also provides cytoplasm with 2 mol NADH/glucose.

•In the absence of O2, NADH is oxidized by reduction of pyruvate.

•In the presence of O2, NADH is oxidized in the mitochondria.

•Rate of glycolysis is controlled at 3 key allosteric enzymes.