Enzymes and Biological Reactions part 6/yt video

Enzymes: Catalysts of Life

Introduction

• Thousands of chemical reactions occur every second inside living cells.
• These reactions are catalyzed by enzymes.
• An enzyme is a protein that catalyzes a chemical reaction.

Enzyme Function

• Catalysis: Initiate, accelerate, and ensure consistent outcomes in chemical reactions.
• Enzymes often work together in pathways like the citric acid cycle.
• The citric acid cycle is a series of chemical reactions used by cells to generate energy from carbohydrates.
• Enzymes regulate essential life tasks: metabolism, protein synthesis, cell renewal, and growth.

Life Sustaining Power of Enzymes

• Enzymes catalyze reactions under mild conditions (pH, temperature, atmospheric pressure).
• Catalyzed reactions are $10^6$ to $10^{12}$ times faster than uncatalyzed reactions.
• Without enzymes, reactions would require additional energy as heat.
• Heat occasionally provides enough energy to trigger a reaction.

Mechanism of Enzyme Action

• Transition State: In most reactions, an unstable, high-energy transition state forms as substrates transform into products.
• Template Role: Enzymes act as templates, binding to substrates and positioning them for product formation.
• Enzymes stabilize the transition state with reactive groups, facilitating the reaction.

Aconitase Example: Citric Acid Cycle

• Aconitase catalyzes a reaction in the citric acid cycle.
• Aconitase binds to citrate (substrate), removes a hydroxyl group and a hydrogen atom to form cis-aconitate (intermediate).
• Aconitase then adds the hydrogen and hydroxyl back in slightly different positions to form isocitrate (product).

Active Site

• Amino acids in the active site recognize and hold the substrate in the optimal position.
• Some amino acids recognize and hold the substrate; others are directly involved in catalysis.

Role of Specific Amino Acids

• Histidine 101: Acts as an acid by donating a proton.
• Serine 642: Acts as a base by accepting a proton from the substrate.

Iron-Sulfur Cluster

• The active site of aconitase contains an iron-sulfur cluster.
• The iron-sulfur cluster stabilizes the substrate electrostatically and positions it relative to catalytic residues.

Reaction Steps: Dehydration

• Histidine acts as an acid, protonating the hydroxyl on the substrate, allowing it to leave as a water molecule.
• Serine acts as a base, extracting a hydrogen atom from the opposite side of the substrate, forming cis-aconitate.

Reaction Steps: Hydration

• Cis-aconitate flips upside down for the hydration reaction.
• Histidine grabs a hydrogen atom from a water molecule, placing the resulting hydroxyl group back onto the substrate.
• Serine returns its hydrogen atom to complete the product, isocitrate.

Enzyme as Catalyst

• Enzymes are not changed by the reaction; they extract and replace atoms.
• Enzymes start and end in the same state, which is a hallmark of a catalyst.
• Enzymes can perform thousands of reactions consecutively.

Flexible Active Site

• The shape of the active site is often flexible.
• Enzymes surround their substrates, creating an ideal reaction environment.

Conclusion

• Enzymes are fundamental to life on Earth.
• Enzymes maintain life processes in every cell of every living creature.