Glycolysis Steps 5-7

Step 5: Isomerization

• Dihydroxyacetone phosphate (DHAP) converts to glyceraldehyde-3-phosphate (GAP) via triose phosphate isomerase.
• This ensures both molecules are in the same form.
• This results in two molecules of GAP.
• It is a reversible step, preferring the forward direction due to equilibrium.

Step 6: Oxidation-Reduction

• Glyceraldehyde 3-phosphate (GAP) is oxidized and phosphorylated to 1,3-bisphosphoglycerate (1,3-BPG) with glyceraldehyde-3-phosphate dehydrogenase (GAPDH).
• This generates NADH capturing energy in the form of reducing equivalents.
• The source of Pi (inorganic phosphate) is free, not from ATP because the reaction has a negative change in Gibbs free energy.
• This step is reversible.

Step 7: Phosphorylation

• 1,3-BPG is converted to 3-phosphoglycerate (3-PG) by glycerate kinase (GK).
• ATP is generated through substrate-level phosphorylation.
• Step is reversible.

Steps 8-10

• Steps 1, 3, and 10 are the irreversible steps, and Kinases are used.
• Includes hexokinase, phospho frucho kinase and pyruvate kinase.

Step 8: Phosphoryl-Shift Isomerization

• 3-PG is isomerized to 2-phosphoglycerate (2-PG), by phosphoglycerate mutase, which involves transferring a phosphate group.
• The phosphate group is moved from carbon 3 to carbon 2.
• Reversible Step.

Step 9: Dehydration

• 2-PG loses water to form phosphoenolpyruvate (PEP), catalyzed by enolase.
• Phosphorus is on carbon 2
• Reversible Step

Step 10: Phosphorylation

• PEP is converted to pyruvate by pyruvate kinase (PK), which generates ATP via substrate-level phosphorylation
• A final step producing ATP and pyruvate, a crucial intermediate in various metabolic pathways
• The last irreversible step in the pathways

Energy Needs and Production in Glycolysis

• Glucose to Glucose 6-Phosphate requires -1 ATP
• Fructose 6-Phosphate to Fructose 1,6-bisphosphate requires -1 ATP
• Glyceraldehyde 3-Phosphate to 1,3-bisphosphoglycerate yields 2 NADH
• 1,3-bisphosphoglycerate to 3-Phosphoglycerate yields 2 ATP
• Phosphoenolpyruvate to Pyruvate yields 2 ATP
• NET: 2 ATP AND 2 NADH

Other Information

• The net ATP production in glycolysis is 2 because NADH produced in the cytoplasm may not enter the mitochondria for further energy production
• NADH in glycolysis is not an energy molecule, but an electron carrier, playing a role in redox reactions.

Synthesis of 2,3-Bisphosphoglycerate in Red Blood Cells

• An alternate pathway in red blood cells from 1,3-bisphosphoglycerate to 3-phosphoglycerate.
• Oxygen delivery to tissues is increased by binding to deoxyhemoglobin, decreasing its affinity for O2 and increasing O2 release.
• It doesn't need oxygen and happens in the cytoplasm in RBCs.
• RBCs depend on glycolysis.
• Net ATP production is zero in this case.
• The molecule, 2,3-BPG, binds to hemoglobin reducing the affinity of hemoglobin to rebind oxygen.

Fates of Pyruvate

• Pyruvate loses one carbon to become acetyl CoA and enter the Krebs cycle
• Pyruvate can gain CO2 to become oxaloacetate to supply the tricarboxylic acid cycle or create amino acids as aspartate
• Under anaerobic conditions, pyruvate can be converted to lactate
• In non-human cells (yeast), pyruvate can be converted into acetaldehyde then ethanol.

Pyruvate to Ethanol

• The conversion of pyruvate to ethanol happens in yeast.
• The decarboxylation of pyruvate removes CO2 and becomes acetaldehyde, reduced to ethanol by alcohol dehydrogenase (NADH is oxidized).
• CO2 diffuses away, which makes food fluffy during baking.

Pyruvate to Lactate

• Occurs under anaerobic conditions
• Pyruvate is reduced to lactate by accepting a proton.
• The carbonyl (ketone) converts to hydroxyl (secondary alcohol), using NADH as a coenzyme and the enzyme lactate dehydrogenase.
• The enzyme can reconvert lactate to pyruvate; it works in both directions.
• NADH and NAD+ alternate.
• The point of the forward reaction is to regenerate NAD+ to use it again.

When is Lactate Produced?

• In cells with low energy demand
• In rigorously exercising muscle needing increased energy, lactate level is increased 5 to 10 folds
• During Hypoxia to survive brief episodes of hypoxia

Clinical Hint: Lactic Acidosis

• Lactic acidosis is when there is an increase of plasma acidity
• Increased production or decreased utilization of lactic acid
• The most common cause is the impairment of oxidative metabolism due to collapse of the circulatory system.
• Also, due to impaired O2 transport, respiratory failure, or uncontrolled hemorrhage.
• Causes: Direct inhibition of oxidative phosphorylation, hypoxia in any tissue, alcohol intoxication with high NADH/NAD+
• Decreased gluconeogenesis, pyruvate dehydrogenase, TCA cycle activity or pyruvate carboxylase.