Chemotrophic Enegy Metabolism - Chapter 9

Chapter 9: Chemotrophic Energy Metabolism: Glycolysis and Fermentation

ATP: The Primary Energy Molecule in Cells

Structure of ATP:
- Phosphate groups linked by phosphoanhydride bonds.
- Linked to ribose via a phosphoester bond.
- Adenine linked to ribose forms adenosine.

The Importance of ATP over AMP

Reasons for ATP Preference:
- Enzyme compatibility: ATP fits better with enzymes compared to AMP.
- Energy release: Hydrolysis of ATP releases more energy than that of AMP.
- Storage longevity: ATP has a longer storage duration than AMP.
- Phosphate availability: Cells have sufficient phosphate available.

Exergonic Nature of ATP Hydrolysis

Hydrolysis Process: ATP hydrolysis to ADP and inorganic phosphate (Pi) is exergonic due to several factors:
- Charge repulsion: Negative charges on phosphate groups repel each other.
- Resonance stabilization: Stabilization of hydrolysis products.
- Increased entropy: The products have higher freedom of movement, contributing to energy release.

Charge Repulsion in ATP

The three phosphate groups possess negative charges that repel each other, contributing to strain on the covalent bonds linking them.

Resonance Stabilization

Concept:
- Electron delocalization leads to lower energy configurations in certain bonds, known as resonance hybrids.
- Phosphate resonance stabilization: Formation of phosphoester bonds leads to less resonance stabilization and higher energy.

Entropy Increase in ATP Hydrolysis

Removing a phosphate from ATP leads to increased spatial randomization of ADP and Pi, thus decreasing their free energy.

Importance of ATP in Energy Metabolism

ATP's Role: Occupies a crucial position among energy-rich phosphorylated compounds.
- Can phosphorylate less energy-rich compounds but not more energy-rich ones.

Free Energy Values of Phosphorylated Compounds

Phosphorylated Compounds:
- Phosphoenolpyruvate (PEP): ΔG°′ = -14.8 kcal/mol
- 1,3-Bisphosphoglycerate: ΔG°′ = -11.8 kcal/mol
- ATP: ΔG°′ = -7.3 kcal/mol
- Other compounds yield progressively less energy.

Exergonic Transfer of Phosphate Groups

Hydrolysis Examples: Demonstrates the energy transfer achieved through exergonic reactions.

The ATP/ADP Cycle

Represents a reversible process for conserving, transferring, and releasing energy within cells.

Oxidation in Energy Sources

Definition: Removal of electrons, often associated with energy sources.
- In biological systems, this may involve the removal of protons (H+).
- oxidation of Glucose is highly exergonic (that burst of energy is what makes us feel better)

The Role of Dehydrogenases

Enzymes that catalyze oxidation reactions, transferring electrons during reduction processes, classifying them as endergonic.
overall reaction is a hydrogenation. This process is crucial for energy production, as it allows cells to harness the energy released from the oxidation of substrates.

Key Process in Glycolysis

Glycolysis Overview: Key pathway for glucose metabolism.
- Produces 2 net ATP, 2 NADH, and 2 pyruvate.
- Is essential for both aerobic and anaerobic pathways.

Phases of Glycolysis

Phase 1: Energy Investment: Consumes 2 ATP molecules per glucose.

- covert glucose to fructose-1, 6-bisphosphate —> 2 glyceraldehyde-3-phosphate + 2ADP

Phase 2: Oxidation and ATP Generation: Involves NAD+ to NADH (gly-6) and ATP generation. (gly-7) (ADP + Pi —> ATP) (get the 2 ATP back from stage 1 = 0 ATP actually formed)

- glyceraldehyde-3-phosphate —> 3-phosphoglycerate is highly exergonic

Phase 3: Pyruvate Formation: Converts 3-phosphoglycerate to pyruvate with additional ATP generation. (make 2 atp)

- 3-3-phospaoglycerate + ATP —> pyruvate + ATP

Enzymes Involved in Glycolysis

Phase 1 Reactions: Hexokinase, phosphoglucoisomerase, phosphofructokinase, aldolase, triose phosphate isomerase.
Energy Yield Summary: Negative net yield (-2 ATP) in Phase 1, make 2 ATP so 0 ATP total (phase 2), and a net gain of 2 ATP in Phase 3, resulting in a total yield of 2 ATP from glycolysis. Furthermore, glycolysis also produces two molecules of NADH, which can be used in the electron transport chain to generate additional ATP during oxidative phosphorylation.

Summary of Glycolysis Phases

Phase 1 (Gly-1 to Gly-5): Preparation and cleavage; negative energy yield.
Phase 2 (Gly-6 to Gly-7): Oxidation and ATP generation; familiarize with NAD+ and NADH.
Phase 3 (Gly-8 to Gly-10): Leads to pyruvate formation with ATP generation.

Glycolysis Outputs

Net Outputs:
- 4 ATP (2 used during phase 1) resulting in 2 ATP net gain, 2 pyruvate molecules, and 2 NADH.
- Pathway is highly exergonic toward pyruvate formation.

Pyruvate Under Oxygen Conditions

Aerobic and Anaerobic Fate: Depends on oxygen availability post-glycolysis, leading either to aerobic respiration or fermentation.

Fermentation Processes

does not produce ATP. Oxidize NADH to NAD+ (to keep powering glycolysis).
does not directly produce Atp. indirectly allows glycolysis to produce atp.

Ethanol Fermentation: Allows conversion of glucose to ethanol with limited ATP yield.
Lactic Acid Fermentation: Involves conversion to lactic acid, common in muscles., enabling ATP production without oxygen.

Cancer Cells and Aerobic Glycolysis

Warburg Effect: Cancer cells favor glycolysis even when oxygen is present to support rapid growth and nutrient demands. (Aerobic Glycolysis)

Alternative Glycolytic Substrates

Disaccharides: Lactose [glucose + galactose], sucrose [glucose + fructose], and maltose [glucose + glucose] can also be processed by glycolysis.
Monosaccharides and Conversion: Mannose and fructose enter glycolysis through specific enzymatic steps.

Regulation of Glycolysis and Gluconeogenesis

Reciprocal Regulation: Glycolysis and gluconeogenesis are independently regulated based on cellular conditions to avoid counterproductive simultaneous activity.

Key Regulators in Metabolism

Fructose-2,6-Bisphosphate: Vital regulator activating glycolysis and inhibiting gluconeogenesis.
- Synthesis of F2,6 B P is catalyzed by phosphofructokinase-2 (P F K-
  2).