01:694:301 Lecture Notes - Lecture 16: Triosephosphate Isomerase, Glucose-6-Phosphate Isomerase, Dihydroxyacetone Phosphate

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Chapter 16 – ‘Glycolysis and Gluconeogenesis’ PART I
Glycolysis is the sequence of reactions that metabolizes 1 molecule of glucose to 2
molecules of pyruvate with net production of 2 molecules ATP
o It is an
pathway (very ancient)
o Parts of it are essentially ‘universal’; takes place in cytoplasm
! Glucose is common and important fuel source. Why?
Availability in primitive conditions
Strong tendency to exist in ring formation and little tendency to
modify proteins (ring conformation – equatorial " high stability)
o Takes place in cytoplasm in eukaryotes
o Stage I – the cleavage of the fructose 1,6-bisphosphate into two three-
carbon fragments
The strategy of initial steps of glycolysis is trap glucose and form
compound that can be cleaved by phosphorylated 3-C unit
o Stage II – ATP harvested when 3-C fragments are oxidized by pyruvate
**You need to KNOW all intermediated with structures, all enzyme, and all
cofactors as shown in Fig. 16.2 (Refer to handout)
o Note in that figure that 3 steps (all ‘kinase rxn’) are irreversible. KNOW which
ones they are
o Step 1Hexokinase traps glucose in cell and begins glycolysis
! It phosphorylated by ATP "
glucose 6-phosphate
! This step is important for two reasons:
1) Glucose 6-phosphate can’t pass membrane through glu-transporters
2) Addition P-group destabilizes glucose
! Hexokinase – catalyzes transfer of P-group from ATP to hexoses; requires
Mg2+ for activity; substrate induced cleft closing around glucose
o Step 2Glucose 6-phosphate converted to fructose 6-phosphate
! Isomerization converts aldose (glucose) to ketose (fructose)
! Catalyzed by phosphoglucose isomerase; this rxn takes time due to opening
of cyclic forms of sugars
o Step 3 Fructose 6-phosphate is then phosphorylated to fructose-1,6-BP
! Uses ATP
! Catalyzed by phosphofructokinase (PFK), an allosteric enzyme that sets
pace of glycolysis
o Step 4 F-1,6-BP is cleaved into 2 3-C fragments (GAP and DHAP)
! Fructose 1,6-BP is cleaved into
glyceraldehyde 3-phosphate
dihydroxyacetone phosphate
! Catalyzed aldolase (reverse rxn is aldol condensation)*
! GAP is on direct pathway while DHAP is not
There needs to be means for converted DHAP to GAP (or else lose
o Step 5 DHAP isomerized to GAP
! GAP (aldose) and DHAP (ketose) are isomers that readily interconvert
! Catalyzed by triose phosphate isomerase (TPI); rxn rapid and reversible
TPI catalyzes transfer of H-atom from C-1 to C-2 intramolecular redox
STAGE II (each step occurs 2x)
o Step 6Conversion GAP into 1,3-biphosphoglycerate (1,3-BPG)
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