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Chapter 16

BCM 475 Chapter Notes - Chapter 16: Dihydroxyacetone Phosphate, Hexokinase, Ketose


Department
Biochemistry
Course Code
BCM 475
Professor
Welch
Chapter
16

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STAGE 1
STEP 1 PHOSPHORYLATION | Hexokinase traps glucose in the cell GLYCOLYSIS BEGINS
- Glucose enters cell through transport proteins
- Phosphorylated by ATP to form glucose-6-phosphate
o Cannot pass through membrane due to the negative charges on the phosphoryl groups
o Addition of phosphoryl facilitates eventual metabolism of glucose into 3-carbon molecules
(high phosphoryl transfer potential)
- Catalyzed by hexokinase
Importance of glucose-induced structural change of hexokinase
1. Environment around glucose becomes more nonpolar
o Favors reaction between hydrophilic OH groups of glucose and terminal phosphoryl group of
ATP
2. Enables kinase to discriminate against H2O as a substrate
o Keeps water out of active site
STEP 2 ISOMERIZATION | Glucose-6-phosphate Fructose 1,6-biphosphate
- Conversion of aldose into ketose
- Catalyzed by phosphoglucose isomerase
- Multistep process including ring opening, isomerization, ring closing into 5 membered ring
STEP 3 SECOND PHOSPHORYLATION | Fructose-6-phosphate to Fructose 1,6-biphosphate
- Phosphorylation uses ATP
- Catalyzed by phosphofructokinase (PFK)
- prevents re-formation of glucose 6-phosphate (F-1,6 BP)
STEP 4 CLEAVAGE | F-1,6 BP Two 3-Carbon Fragments
- F-1,6 BP is cleaved into:
o glyceraldehyde 3-phosphate (GAP) aldose, on direct pathway of glycolysis
o dihydroxyacetone phosphate (DHAP) ketose, NOT on direct pathway
- Reversible (reverse reaction = aldol condensation)
- Catalyzed by aldolase
STEP 5 ISOMERIZATION | GAP and DHAP
- isomers; can be easily interconverted
o Reaction proceeds readily from DHAP to GAP, which can continue on the glycolysis pathway
GAP is constantly being removed (Le Chatelier’s principle)
- Catalyzed by TPI (triose phosphate isomerase)
- Rapid, reversible reaction
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