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Glycolytic Pathway - Stage Three
May 21, 2024
Glycolytic Pathway: Stage Three
Overview
Focus on steps in stage three of glycolysis
Harvest energy from glyceraldehyde-3-phosphates (GAP)
Formation of ATP, NADH, and pyruvate
NADH and pyruvate can be further processed under aerobic conditions for more ATP
Step 1: Conversion of GAP to 1,3-Bisphosphoglycerate (1,3-BPG)
Enzyme:
Glyceraldehyde-3-Phosphate Dehydrogenase (GAPDH)
Reaction:
GAP is converted to 1,3-BPG
NAD+ is reduced to NADH
Addition of orthophosphate (Pi) to GAP
Mechanism
Hydride transfer from GAP to NAD+
Formation of a thioester intermediate
NADH leaves the active site, replaced by NAD+
NAD+ helps depolarize and weaken thioester bond
Orthophosphate enters and forms 1,3-BPG
Enzyme catalytic residues:
Cys 149
and
His 176
Enzyme active site restores catalytic residues
Uncatalyzed Reaction
Divides into two reactions: Oxidation-reduction and dehydration
Energy release and consumption balance out
Without enzyme, carboxylic acid intermediate is too stable to proceed
Catalyzed Reaction
Formation of a less stable thioester intermediate instead of carboxylic acid
Thioester intermediate is higher in energy
Enzyme couples favorable oxidation-reduction with the unfavorable dehydration
Step 2: Phosphate Transfer from 1,3-BPG to ADP
Enzyme:
Phosphoglycerate Kinase
Reaction: Substrate-Level Phosphorylation
1,3-BPG transfers a phosphate group to ADP, forming ATP
Energy Consideration:
1,3-BPG has a higher phosphoryl transfer potential than ATP
Formation of 3-phosphoglycerate
Net Result:
Formation of 2 ATP molecules
2 molecules of 1,3-BPG form 2 ATP and 2 3-phosphoglycerates
Summary
Step 1 forms 2 NADH and 2 H+ and 2 1,3-BPG
Step 2 forms 2 ATP from ADP and 2 3-phosphoglycerate
Net ATP at this stage is zero (2 used in stage one and 2 produced in stage three)
Key Terminology:
Glyceraldehyde-3-Phosphate (GAP):
Substrate molecule
1,3-Bisphosphoglycerate (1,3-BPG):
Product of GAP conversion
NAD+:
Nicotinamide adenine dinucleotide, oxidized form
NADH:
Reduced form of NAD+
Orthophosphate (Pi):
Inorganic phosphate
Substrate-Level Phosphorylation:
Direct transfer of a phosphate group to ADP to form ATP
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