Exam 3 Lecture 10

Overview of Reaction Mechanisms and Glycolysis

Key Concepts

  • Discussion on the balance of atoms in the reaction.

    • Oxygen: 16 oxygens on the product side.

    • Hydrogen: 14 hydrogens counted on both sides, indicating no oxidation reduction reaction.

  • Importance of stable carbanion and resonance stabilization.

    • Cleaving Carbon-Carbon Bond: Electrons in the bond move to form an acyl anion which is not resonance stabilized.

    • Stabilization by TPP (Thiamine Pyrophosphate): This mechanism involves TPP to stabilize acyl anion.

Glycolysis: Overview and Importance

Introduction to Glycolysis

  • Preparation to discuss Chapter 13 on glycolysis reactions.

  • Reference to flowchart illustrating glycolysis.

  • Importance of memorizing reactions and enzymes involved in glycolysis.

  • Recommended strategy: Use chemical logic for understanding, not just memorization.

Metabolic Pathway of Glycolysis

  • Overall Goal: Oxidation of glucose (6-carbons) to pyruvate (3-carbons).

  • Break down of glucose involves carbon-carbon bond cleavage and production of metabolites.

  • NAD+ Reduction: Two NAD+ molecules are reduced to two NADH.

  • Overall Free Energy Change: Approximately 90extkJ/mol90 ext{ kJ/mol} under cellular conditions.

Catabolic Pathways

  • Catabolic pathways have two main purposes:

    1. Generate Energy (ATP)

    2. Intermediates for Biosynthetic Recycling (e.g., pyruvate).

Phases of Glycolysis

  • Glycolysis is divided into two sequential stages:

    1. Preparatory Phase (Energy Investment Phase): Initial steps requiring ATP input.

    2. Energy Payoff Phase: Subsequent steps generating ATP and NADH.

ATP Synthesis Methods

  • Three ways to synthesize ATP:

    1. Photophosphorylation: Energy from the sun (not applicable to humans).

    2. Oxidative Phosphorylation: Requires oxygen and generates substantial ATP.

    3. Substrate-Level Phosphorylation: Enables ATP synthesis without oxygen, occurs in glycolysis.

Substrate-Level Phosphorylation

  • Definition: Involves transferring phosphate from a high-energy compound to ADP to form ATP.

  • Key substrates for substrate-level phosphorylation in glycolysis include:

    • 1,3-Bisphosphoglycerate

    • Phosphoenolpyruvate

  • Both substrates have higher phosphoryl transfer potentials than ATP, which is necessary for the reaction to occur.

The Preparatory Phase of Glycolysis

  • Begins with glucose entry into the pathway and concludes with the formation of two glyceraldehyde-3-phosphates (GAP or G3P).

  • Reactions: First five reactions consist of:

    1. Two phosphorylations

    2. Two isomerizations

    3. One carbon-carbon bond cleavage

  • Energy Investment: Input of 2 ATPs is required in the preparatory phase.

Flowchart of Reactions and Regulatory Elements

  • Use flowchart to visualize glycolysis mechanism:

    • Red squares: Represent energy investments.

    • Green squares: Energy production.

    • Regulatory points highlighted to show controlling steps:

    • Step 1, Step 3, Step 10 are metabolically irreversible under cellular conditions.

Reaction Specifics in the Preparatory Phase

  • Reaction 1:

    • Substrate: Glucose (6 carbons)

    • Products: Glucose-6-phosphate and ADP.

    • Catalyzed by: Hexokinase.

    • Key Point: Hexokinase undergoes a conformational change when glucose binds; ATP acts as the electrophile.

    • Thermodynamics: Coupled reaction becomes favorable by the hydrolysis of ATP, maintaining a thermodynamically favorable phosphoryl transfer.

  • Reaction 2 (Isomerization):

    • Converts glucose-6-phosphate (aldose) to fructose-6-phosphate (ketose).

    • Catalyzed by: Phosphoglucose isomerase.

  • Reaction 3 (Phosphoryl Transfer):

    • Fructose-6-phosphate is phosphorylated to fructose-1,6-bisphosphate.

    • Key Points: This reaction is highly regulated (committed step).

    • Catalyzed by: Phosphofructokinase-1 (PFK-1).

  • Reaction 4 (Carbon-Carbon Bond Cleavage):

    • Cleavage of the bond between carbon 3 and carbon 4 produces

    • Glyceraldehyde-3-phosphate

    • Dihydroxyacetone phosphate.

    • Catalyzed by: Aldolase.

    • Importance of Enolate and Resonance Stabilization:

    • Enolates require stabilization (using cation) and formation of shift base in the aldolase mechanism.

Specific Mechanism of Aldolase

  • Active site of aldolase involves catalytic residues:

    • Lysine and Aspartate are pivotal.

  • Mechanism involves:

    • Ring opening under neutral conditions.

    • Nucleophilic attack on ketone.

    • Formation of tetrahedral intermediate followed by proton transfers to yield an imine (shift base) which stabilizes the enolate.

  • Resulting products: Glyceraldehyde-3-phosphate and dihydroxyacetone phosphate.

Conclusion and Recap

  • Key points encapsulated from glycolysis covered in class will be continued and reinforced in the next session.

  • A reminder for the due discussion worksheet prior to next class.