Microbial Metabolism and Enzyme Function: Study Notes

Microbial Metabolism and Enzyme Function

Enzymes: Mechanism of Action

Enzymes are biological catalysts that accelerate chemical reactions in living organisms. They function by lowering the activation energy required for reactions, allowing processes to occur rapidly and efficiently under physiological conditions.

• Substrate: The molecule upon which an enzyme acts.

• Active Site: The specific region of the enzyme where the substrate binds.

• Substrate-Enzyme Complex: The temporary association formed when a substrate binds to the enzyme's active site.

• Activation Energy: The minimum energy required to initiate a chemical reaction.

• Transition State: A high-energy intermediate state during the reaction.

• Cofactors and Coenzymes: Non-protein molecules that assist enzymes in catalysis. Cofactors are often metal ions, while coenzymes are organic molecules (e.g., NAD+).

Example: Amylase catalyzes the breakdown of starch into sugars.

Illustration: Enzyme + Substrate → Enzyme-Substrate Complex → Enzyme + Product

Thermodynamics in Biology

Thermodynamics governs the energy changes in biological systems. The first law states that energy cannot be created or destroyed, only transformed.

• First Law of Thermodynamics: $\Delta E = q + w$ (where $\Delta E$ is the change in internal energy, $q$ is heat, and $w$ is work).

• Application: In cells, chemical energy from nutrients is converted into ATP and heat.

Metabolic Reactions: Anabolism and Catabolism

Metabolism consists of all chemical reactions in a cell, divided into two main types:

• Anabolism: Biosynthetic reactions that build complex molecules from simpler ones; require energy (endergonic).

• Catabolism: Degradative reactions that break down molecules to release energy (exergonic).

• Endergonic Reactions: Require energy input ($\Delta G > 0$).

• Exergonic Reactions: Release energy ($\Delta G < 0$).

Example: Synthesis of proteins (anabolism); breakdown of glucose (catabolism).

ATP Cycle and Energy Transfer

ATP (adenosine triphosphate) is the primary energy currency in cells. The breakdown of ATP releases energy for cellular processes.

• ATP Hydrolysis: $\text{ATP} + \text{H}_2\text{O} \rightarrow \text{ADP} + \text{P}_i + \text{Energy}$

• Energy Coupling: Exergonic breakdown of ATP is coupled to endergonic cellular reactions.

Oxidation-Reduction (Redox) Reactions

Redox reactions involve the transfer of electrons between molecules, crucial for energy production in cells.

• Oxidation: Loss of electrons.

• Reduction: Gain of electrons.

• Example: NAD+ + 2e- + 2H+ → NADH + H+

Phototrophs and Chemotrophs

Organisms obtain energy through different mechanisms:

• Phototrophs: Use light energy (e.g., cyanobacteria).

• Chemotrophs: Use chemical energy from organic or inorganic compounds.

• Aerobic Chemotrophs: Use oxygen as the final electron acceptor (e.g., Escherichia coli).

• Anaerobic Chemotrophs: Use other molecules (e.g., nitrate, sulfate) as electron acceptors.

Example: Thiobacillus oxidizes sulfur compounds for energy.

Aerobic Respiration: Stages and Substrates

Aerobic respiration is a multi-step process that converts glucose into ATP using oxygen.

• Glycolysis: Glucose → Pyruvate

• Krebs Cycle (Citric Acid Cycle): Pyruvate → CO2 + NADH + FADH2

• Electron Transport Chain: NADH/FADH2 → ATP (via oxidative phosphorylation)

Substrates: Glucose, pyruvate, NAD+, FAD

Products: ATP, CO2, H2O

Oxidative Phosphorylation and Chemiosmosis

Oxidative phosphorylation uses the electron transport chain and chemiosmosis to generate ATP.

• Electron Transport Chain: Transfers electrons, pumps protons across membrane.

• Chemiosmosis: Proton gradient drives ATP synthesis via ATP synthase.

• Equation: $\text{ADP} + \text{P}_i \rightarrow \text{ATP}$

Location: Prokaryotes (plasma membrane); Eukaryotes (mitochondrial inner membrane)

Fermentation vs. Anaerobic Respiration

Both processes occur in the absence of oxygen but differ in their mechanisms and ATP yield.

Process	Final Electron Acceptor	ATP Yield
Fermentation	Organic molecule (e.g., pyruvate)	Low (2 ATP per glucose)
Anaerobic Respiration	Inorganic molecule (e.g., nitrate, sulfate)	Moderate (varies, but more than fermentation)
Aerobic Respiration	Oxygen	High (up to 38 ATP per glucose)

ATP Production: Aerobic > Anaerobic > Fermentation

Fermentation: Types and Examples

Fermentation is a metabolic process that regenerates NAD+ by transferring electrons to organic molecules.

• Lactic Acid Fermentation: Pyruvate → Lactic acid (e.g., Lactobacillus)

• Alcoholic Fermentation: Pyruvate → Ethanol + CO2 (e.g., Yeast)

• Mixed Acid Fermentation: Produces a variety of acids (e.g., Escherichia coli)

Example: Yogurt production by Lactobacillus (lactic acid fermentation)

Exoenzymes

Exoenzymes are enzymes secreted by cells to function outside the cell, aiding in the breakdown of large molecules.

• Function: Degrade polymers (e.g., proteins, starch) into smaller units for uptake.

• Example: Amylase (breaks down starch), Protease (breaks down proteins)

Additional info: Academic context and examples have been expanded for clarity and completeness.